Nicolas Surdyk
email: n.surdyk@brgm.fr

Alexis Gutierrez

Nicole Baran

Dominique Thiéry

A lumped model to simulate nitrate concentration evolution in groundwater at catchment scale

Keywords: Groundwater quality, Lumped model, Nitrate, Agricultural programs of measures, Impact, Lag time

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

In Europe, the Nitrate Directive (91/676/EEC), adopted in 1991, concerns the protection of surface and ground waters against nitrate contamination by preventing nitrate from agricultural sources and by promoting the use of good farming practices. This directive is closely relative to the European water framework directive (2000/60/EC), which aims to achieve the objective of good chemical status by defining and implementing the necessary measures and to the groundwater directive (2006/118/EC) which confirms the 50 mg/l quality standard as an European standard for nitrate concentration in groundwater.

The implementation of the measures can be difficult to accept by local actors, especially if we lack concrete elements to quantify the expected impact of these measures. This is especially true for groundwater where the time lag between the change of agricultural practices and the impact on groundwater quality can be several years or even decades [START_REF] Donald | Lag Time in Water Quality Response to Best Management Practices: A Review[END_REF][START_REF] Gutierrez | Long-term transfer of diffuse pollution at catchment scale: Respective roles of soil, and the unsaturated and saturated zones (Brévilles, France)[END_REF].

Thus, the use of models appears as an alternative to the a priori evaluation of the effects of measures.

Dynamic, process-based catchment models are designed to represent the processes governing catchment hydrology and water quality, and may therefore be useful tools for catchment management. But, the complexity of these types of models brings a number of issues related to parameter uncertainty [START_REF] Jackson-Blake | Are our dynamic water quality models too complex? A comparison of a new parsimonious phosphorus model, SimplyP, and INCA-P[END_REF] and parameterization [START_REF] Woodward | Predicting nitrate discharge dynamics in mesoscale catchments using the lumped StreamGEM model and Bayesian parameter inference[END_REF][START_REF] Knowling | Role of model parameterization in risk-based decision support: An empirical exploration[END_REF]. To produce reliable predictions, these models require large quantities of detailed data to characterize the physical properties of the model domain and parameters [START_REF] Hong | Simplified Lumped Groundwater Model to simulate Nitrate Concentration Dynamics[END_REF]. These issues are generally due to low information from geological and hydrological data combined with the complexity and uncertainty of representing the subsurface processes [START_REF] Woodward | Uncertainty in the modelling of spatial and temporal patterns of shallow groundwater flow paths: the role of geological and hydrological site information[END_REF]. Despite their complexity, their performance could be poor [START_REF] Refsgaard | A good-looking catchment can turn into a modeller's nightmare[END_REF]. If the use of complex model might be useful in terms of knowledge of the processes, it has limitations when applied in an operational context [START_REF] Perrin | Does a large number of parameters enhance model performance? Comparative assessment of common catchment model structures on 429 catchments[END_REF]. Besides, it was demonstrated that very simple models can achieve a level of performance almost as high as models with more parameters [START_REF] Perrin | Does a large number of parameters enhance model performance? Comparative assessment of common catchment model structures on 429 catchments[END_REF].

So, simpler approaches are needed for decision making based on readily available data [START_REF] Jackson-Blake | Are our dynamic water quality models too complex? A comparison of a new parsimonious phosphorus model, SimplyP, and INCA-P[END_REF]. As a consequence, there is a renewed interest for lumped conceptual models [START_REF] Woodward | Predicting nitrate discharge dynamics in mesoscale catchments using the lumped StreamGEM model and Bayesian parameter inference[END_REF][START_REF] Charizopoulos | A lumped conceptual approach for modeling hydrological processes: the case of Scopia catchment area, Central Greece[END_REF]. Some previous works have already shown the effectiveness of lumped model to provide a simple approach to the simulation of groundwater nitrate concentrations that requires little data [START_REF] Hong | Simplified Lumped Groundwater Model to simulate Nitrate Concentration Dynamics[END_REF].

Nevertheless, codes and applications to different case studies remain scarce. BICHE (Bilan CHimique des Eaux -Water chemistry balance), developed by BRGM [START_REF] Thiéry | Modélisation globale des transferts de nitrates dans un bassin hydrogéologique pour prévoir l'évolution des concentrations dans les eaux souterraines. Description du modèle BICHE[END_REF], is a lumped model, which simulates nitrate transfer together with the hydrological functioning at the scale of a catchment. The model has a conceptual structure represented by a cascade of linear tanks interacting through mathematical functions to describe the fluxes (water or nitrate) between the tanks. BICHE considers the catchment as an undivided entity and uses lumped values of input variables and parameters. Crop diversity is taken into account but not its location within the catchment. Therefore, input variables need to be aggregated and measured parameters are not representative of the whole. As lumped parameters cannot be measured on the field, such parameters can only be determined by trial-and-error calibration within physical bounds. Moreover, as several combination of physical parameters may produce the same simulation, semi-empirical parameters, such as recession constants, are used. Although, the model does not take into account plot location within a catchment, it becomes particularly useful when using prospective scenarios or comparison of different scenarios. In the model, nitrate concentration which depends on the fertilizer applications, consumption by crops and soil interaction is transferred towards surface or groundwater according to characteristic of the catchment and climatic conditions [START_REF] Thiéry | Forecast of changes in piezometric levels by a lumped hydrological model[END_REF][START_REF] Thiéry | Forecasting the evolution of nitrate contents in spring water using a lumped hydrologic model IFAC Proceedings Volumes[END_REF]. Parameters are calibrated automatically using optimization routines. This calibration process seek the best fit of simulated data to measured time series (observed groundwater levels and nitrate concentrations). This methodology is classical with lumped model and is improved when optimization is framed and constrained by expert evaluation [START_REF] Gharari | Using expert knowledge to increase realism in environmental system models can dramatically reduce the need for calibration[END_REF]. The possibility is given to separate the calibration process in two phases: first establish the hydrological fitting and then calibrate the nitrate transfer. This may be useful in the first stage of calibration.

Thanks to a relatively limited number of parameters, automatic calibration, and a low computation time, the application of the BICHE model is relatively easy and straightforward. This reinforces its operational purpose. However the strengths and limitations of this tool should be well understood prior to its use beyond research projects.

Thus, the main objective of this paper is to demonstrate that BICHE is a tool for stakeholders and decision makers to set up or assess agricultural and environmental programs of measures.

To achieve this objective, we carried out a three-folded evaluation of the model's potential. The first step of this evaluation was to assess the ability of the lumped model BICHE to simulate the observed time series nitrate concentrations in groundwater for various hydrogeological and agricultural contexts over long time periods (minimum 10 years). The second step was to evaluate the robustness of the calibration by using/simulating an extended set of data (i.e by simulating additional years using the initial calibration). The last step was to assess model interest for decision makers thanks to a retrospective analysis of the implemented agricultural measures (i.e by simulating an alternative set of input data considering continuity of practices as they were before the initiation of environmental measures).

The dataset for this evaluation is composed of groundwater level and nitrate concentration time series on three agricultural catchments, where surveys of agricultural practices were available over a long period. To better highlight the ability of the model to simulate water and nitrate transfers in different hydrogeological and agronomic contexts, the three catchments have been selected to serve as representative combinations of some aquifers, soils and climate of France and Western Europe.

Materials and methods

Model description

The chemical transfer model BICHE is derived from the lumped hydrologic tank model Gardenia [START_REF] Thiéry | Forecast of changes in piezometric levels by a lumped hydrological model[END_REF][START_REF] Thiéry | Validation du code de calcul GARDÉNIA par modélisations physiques comparatives[END_REF]. The hydrological model is a tank model that simulates the water cycle from the rain falling on the basin to the outlet discharge (a river, a spring or a pumping borehole). The input data are the atmospheric precipitation (rainfall and snow) on the catchment area and the potential evapotranspiration (PET) and when necessary water pumping for irrigation. A calibration is done on a series of discharges at the basin outlet or of groundwater levels at one spot, or both. On the studied sites which are exclusively groundwater catchments, the calibration only used groundwater fluctuations. The water cycle is represented by a system of three compartments (tanks): 1) the first one considers water exchanges between soil and atmosphere (precipitation and evapotranspiration by computing at each time step a water balance generating actual evapotranspiration and "effective rainfall", 2) an intermediate tank simulates rapid flow towards the outlet river and groundwater recharge. This rapid flow stands for runoff or karst flow which generates a rapid impact on water flow, and 3) the third one simulates slow flow representing slow groundwater motion and contribution to the observed system (river or water table). Flow between the three tanks are governed by a maximum of six hydrological parameters described in appendix A.

Nitrate transfer is based on the same structure as water transfer: a nitrate content is associated to each flow in each compartment so that the computed mass fluxes at the output of the system allows the comparison of simulated and observed nitrate concentration (Fig. 1) at the outlet of the basin. To compute nitrate content in the first compartment, one must input into the model the time evolution of four series over the simulation duration (and even starting before for model initialization): 1) the nitrogen spread as fertilizer over the catchment area during the growing season; 2) crop nitrogen needs during the various seasons; 3) natural soil mineralization; and 4) the crop nitrogen residual mineralization after harvest.

The model computes at a chosen time step (from hourly to yearly, but usually monthly for nitrate computation on the long run), the balance between the quantities of nitrogen which are added (by fertilizers) or transformed (by mineralization), withdrawn by the plants or which flows out of the soil compartment. Exchanges with immobile water or adsorption effects are taken into account through exchange coefficients thus simulating retardation effects. The complete list and description of parameters is given in Appendix A.

Calibration consists in comparing observed values with simulated data. The model has an automatic calibration process based on a non-linear optimization algorithm adapted from the Rosenbrock method [START_REF] Rosenbrock | An automatic method for finding the greatest or least value of a function[END_REF]. Calibration is done in a semi-automatic way: the user provides an initial value and a range of variation for each parameter, to keep the parameters within plausible values and ensure that secondary results (as recharge computation) remain consistent. The user may also fix values of parameters he knows best, thus reducing the number of optimized parameters. Previous work showed that imposing constraints prevents lumped model from overfitting during calibration and improve reliability outside the calibration period [START_REF] Gharari | Using expert knowledge to increase realism in environmental system models can dramatically reduce the need for calibration[END_REF]. As hydrological parameters have an influence on mass transfer, but mass transfer parameters have no influence on water flows, we chose to first calibrate the hydrologic parameters in order to reproduce the water table level fluctuations at one observation well, and then, the mass transfer parameters to simulate the nitrate content in the outflow (or in the aquifer). Alternatively, the hydrological parameters and the mass transfer parameters can be calibrated simultaneously. This sometimes leads to better fitting albeit less controlled parametrization justifying our choice to proceed in two steps.

The water level fluctuation and the concentration level in the aquifer at the beginning of the simulation depend on historical data that is usually unknown. This problem is solved by using a "warm-up period" to initialize the model. In our simulations, the warming-up process consisted in starting the simulation a long period before the beginning of the water levels and concentration monitoring, repeating existing known input series.

The calibration process is based on maximizing the Nash criterion [START_REF] Nash | River flow forecasting through conceptual models part I -A discussion of principles[END_REF] between the observed and simulated series. An overall goodness of fit criterion, "F", combines the Nash criterion of each simulation (groundwater level and nitrate content) (appendix A). Its maximal possible value for a perfect fit is 1.

A potential disadvantage of automatic calibration based on typical objective functions is that they do not account for temporal offsets between predicted values and observed data. A timing difference of several days does not imply a bad fit when the modelling period encompasses several years, but can have a major effect on the goodness of fit criterion. To minimize this limitation, in addition to the evaluation using the goodness of fit criterion, the visual agreement between the curve of simulated chronicle and observed chronicle was also given consideration in the calibration process.

Studied catchments

The three studied groundwater catchments were selected because they presented three different hydrogeological, climatic and agricultural contexts (Table 1). Their sizes varied from 4 to 50 sq. km.

The studied aquifers correspond to basement formations, karstified limestone and fissured limestone (Fig. 2). The low water period generally occurs in November, September and August, respectively. Groundwater levels have intra-annual fluctuations of less than 1m (limestone site) to around 8 m (karstic site). Based on the water level series, the basement, karstic and limestone sites can be described as not very reactive, very reactive or mixed, respectively.

The mean rainfall and the mean potential evapotranspiration are slightly contrasted: rainfall is highest in the basement site (766 mm/yr) and lowest in the limestone site (699 mm/yr) and Potential Evapotranspiration varies according to the sites between 688 mm/yr (basement site) and 846 mm/yr (karstic site).

The three sites, located in agricultural contexts, exhibit different initial agricultural pressure and different temporal evolution. Indeed, for each of the sites, in response to the increase of water resource contamination by nitrate in the last 30 to 50 years, the stakeholders have implemented different measures i.e. both regulatory measures linked, for example to the Nitrate Directive and applied throughout the territory and more local measures. For the basement site, their strategy was based on land use modification, initiated in 1995, resulting in a replacement of vegetables by meadow (the surface in meadow rose from 1% in 1995 to 43% in 2013 of the total utilized agricultural area). For the karstified limestone, land use has a significant proportion of straw cereals (wheat, barley). Noticeably rapeseed surfaces have increased the most over the period studied to the detriment of grass. The Agricultural Chamber led the major action of management by providing a very close agronomic advices from 2003-2004 onwards on around 80% of the cultivated surfaces. For the limestone site, land use shows that straw cereals (in rotation with rapeseed) are by far the most important crops. Grass surfaces have decreased, while rapeseed areas are increasingly from the 1990's. The first action plan on this catchment start in 1991 leading to the conversion of about 1% of the cultivated surface into poplars plantation. From 2003-2004 onwards, 80% of the plots of the catchment are monitored; advices were provided to the farmers to improve fertilization by the agricultural chamber. Land uses and practices are known for several years thanks to detailed interviews of farmers.

Existing and estimated input data

For the basement site, surveys were performed between 1991 and 1995, and between 2008 and 2013. For the karstic site, data were collect between 1992 and 2009. For the limestone site, surveys were performed in 1991, 1999 and between 2007 and 2015. Others sources of information were used to fill the gaps. The Ministry of Agriculture feed a national database since 1970 with data (Utilized Agricultural Area -UAA and distribution of this area between crops) at communal scale. The statistics are available for five dates 1970, 1979, 1988, 2000 and 2010. For years after 2010 i.e. for the period 2011-2015, surface occupations were collected at the plot scale using a geographical information system application (recently developed and used as a reference for the instruction of aid of the common agricultural policy in France) (Agreste, 2018).

Extrapolation (to extend the data set) or interpolation between input data were used when no value were available.

The model has four input series representing crop management: 1) Fertilization 2) Crop nitrogen need 3) Soil mineralization 4) Crop residue mineralization.

The N consumption of each crop was computed using nitrogen theoretical absorption curves (e.g. Justes, et al., 1994 for wheat). Then, nitrogen absorption curves were adjusted according to the local crop yield data when available or departmental yield data otherwise. Local data were available between 2001 and 2009 for karstic site and between 2007 and 2015 for limestone site.

Departmental yields were available from 1989 to present. Those data are available on a French Agriculture Ministry database (Agreste, 2018). The N fertilization for each crop was computed using the N balance model proposed by COMIFER (COMIFER, 2013) for France and parameters were modified according to the local survey. In each catchment, data from the agricultural chamber were available to adjust the doses at different crop growth stages.

The mineralization of soil was computed from soil organic matter with an adapted equation from the LIXIM model [START_REF] Mary | Calculation of nitrogen mineralization and leaching in fallow soil using a simple dynamic model[END_REF]. The adaptation consists to use only one pool of nitrogen, and some improvements to calculate the rate of degradation [START_REF] Valé | Quantification et prédiction de la minéralisation nette de l'azote du sol in situ, sous divers pédoclimats et systèmes de culture français[END_REF]. The crop residuals mineralization was calculated by using flat-rate value for each crop [START_REF] Comifer | Calcul de la fertilisation azotée -Guide méthodologique pour l'établissement des prescriptions locales -Cultures annuelles et prairies[END_REF].

For each model time step, the value of fertilization, N consumption, mineralization and crop residue mineralization of one specific crop is multiplied by its surface to represent the relative value. The relative values of each crop are summed up to represent the fertilization, N consumption, mineralization and crop residue mineralization for the whole catchment. The input files were compiled by weighting the influence of each crop according to its surface in the catchment, for each year.

Three-folded evaluation of the model potential 2.4.1 "Assessment of the model performances"

A first dataset ending in 2010 was prepared for each selected catchment. The starting date for the different simulations is dependent on the first available nitrate concentration record in each selected basin i.e., 1985 for the basement site, 1975 for the karstic site and 1988 for the limestone one. The simulations used a monthly time step.

"Assessment of the model robustness"

The second step aimed to assess the robustness of the model by extending the initial data set for five years (input data and observed data). For this second set of simulations, the previously used data was completed by series of new data (between 2011 and 2015), acquired and collected with the same methodology. This second set of simulation was performed in a second phase of the project that was executed five year after the first one. The simulation was performed without any new calibration procedure. The aim is to assess whether the quality of the simulation without recalibration is acceptable or not or whether a new calibration is necessary.

"Assessment of the interest of the model retrospective approach"

The last step was set up as a demonstrative use of BICHE by decision makers in an evaluation of an environmental and agricultural policy. A retrospective analysis was performed to evaluate the impact of the implemented measures several years ago by comparing simulations with and without this specific action. This last step is only possible if the two former ones have proved valuable. Then, considering that the model correctly simulates and predict reality, the retrospective analysis is a "what if" situation where events are assumed not to have happened and measures not to have been taken. A theoretical scenario "no agro-environmental measure", was built, irrespective of the national and local measures implementation, by considering that the land use and farming practices observed before the 1991 (i.e. date of Nitrate directive implementation) were still in practice in 2015, i.e. by repeating the 1989/1990 agronomic data from 1990 to 2015. On two of the sites, the local measures having been implemented immediately after the application of the Nitrate Directive, this scenario starting date makes it possible to evaluate the cumulative effect of the two types of measures.

Results and discussion

"Assessment of the model performances"

For the 3 studied sites, the goodness of fit criteria (the Nash criteria, see appendix A) are satisfactory [START_REF] Moriasi | Hydrologic and water quality models: performance measures and evaluation criteria[END_REF] whatever the type of agronomic and hydrogeological situations.

The Nash criteria are satisfactory for the hydrological modelling (values of 0.76, 0.71 and 0.50 for the basement, karstic and limestone sites respectively-Fig. 3a, 4a, 5a) because the simulations match seasonal fluctuations as well as interannual variations. The Nash criterion is lower for nitrate simulations in the karstic (0.61-Fig. 4b) and limestone (0.50-Fig. 5b) sites where concentration variation (which can be up to 10 mg/L between two adjacent measurements in the karstic site and even 20 mg/L in the limestone site) cannot be reproduced. For this last site, the Nash criterion computed on the average of the 12 consecutive measurements increases from 0.50 to 0.75. In the basement site which shows no short-term fluctuation and seems to have much more inertia, the concentration simulated has a better Nash criterion (0.94-Fig. 3b).

For all sites a visual inspection of the graphs shows that the simulations reproduce correctly the smoothed concentration evolutions. These results highlight the good performance of BICHE as a lumped model in the simulation of groundwater level and nitrate concentration. Groundwater level are better calibrated than nitrate concentrations except when the aquifer system provides considerable damping of the input signal, to the point of masking its short-term temporal variability and only retaining its trend. This situation is however less common. Previous work has already shown that simple models can simulate hydrological phenomena [START_REF] Orth | Does model performance improve with complexity? A case studywith three hydrological models[END_REF], but that phenomena related to water quality may require additional complexity [START_REF] Medici | Does increased hydrochemical model complexity decrease robustness?[END_REF].

It may seem paradoxical, but the quality of the calibration may not be well translated by the Nash criterion. As discussed above, the Nash criterion is very dependent on the smoothness of the input data, which tend to better illustrate the trends of the series. In a noisy dataset, the Nash criterion is lower but the trend can be properly simulated just as well or even better. Pluri-annual trends are indeed what matters in such simulations, aiming at orientating policies rather than looking for the detailed evolution of concentration in the aquifer.

If reaching a better goodness of fit for nitrate concentration can be obtained with a more complex model, it is however not in line with a need for simplification, guided by the following reasons.

The increase in the number of parameters may improve the calibration but over parametrization also result in increasing the number of equivalent solutions. Refining the time step (e.g. from monthly to daily) may increase short time variability, but the accuracy and quality of data input is not fine enough to be used at such a scale. For instance, it is impossible to know the exact date of fertilization when no record was taken. Additionally, many aquifer systems, as the karstic and limestone sites of this study, show variable vertical transfer velocities resulting in erratic variations of the observed concentration. At the end, the objective being to support decision making in environmental management, the goal of the model should be to promote simulation of major trends in nitrate concentration rather than to focus on unnecessary details which have no impact on the input -output relationship and the predictive capabilities of the model [START_REF] Kirchner | Getting the right answers for the right reasons: Linking measurements, analyses, and models to advance the science of hydrology[END_REF]Siebert, 2003). Therefore, the tested lumped model is assumed to be highly valuable for stakeholders and decision makers as it is a relatively simple, computationally efficient method for predicting nitrate time-series concentration in groundwater in contrasted hydrogeological and agricultural contexts.

Assessing the model robustness

The purpose of this second series of simulations is to evaluate whether the model behaves like the real soil and aquifer system not only within the calibration period but also in prediction, that is, when no observed value can sustain the calibration process. Running the model with the extended set of five years (2011)(2012)(2013)(2014)(2015) without any new calibration process (keeping the exact parameters of the simulations presented in 3.1.1), it will show its robustness and ability to be used in predictive mode.

For the 3 studied sites, for both water level (Fig. 3a,4a,5a) and nitrate concentration (Fig. 3b,4b,5b), the values simulated between 2011 and 2015 by using the parameters calibrated with shorter series are consistent with the observed values. The Nash criterion of hydrological simulations is still satisfactory [START_REF] Moriasi | Hydrologic and water quality models: performance measures and evaluation criteria[END_REF] and even improved in the three sites (0.79, 0.79, 0.53 for basement, karstic and limestone site respectively). The five years are not exceptional ones on the climatic point of view and correspond to the normal years already simulated. Since the calibration reproduce these normal years correctly (as opposed to exceptional years), the extension of the data improves the adjustment criterion.

The matching of the nitrate concentration simulations is slightly degraded by the extension of data. However the Nash criterion remain satisfactory (0.90, 0.56, 0.44 for basement, karstic and limestone site respectively). Paradoxically, it is on the basement site, where the fit criterion is higher and its decrease is the smallest, that the fit seems the most degraded visually. This visual effect is linked to the smoothness of the observed time series, from which any small deviation is easily spotted. The discrepancy is however less than 10 mg/L and the trend is still respected.

The simulation for the karstic site is particularly interesting as the added years 2011-2015 induce an inversion of the trend that is well represented by the model. It is also the case for the limestone site, although the simulation of the last 15 years seems slightly over estimated. In this case, the decrease of the goodness of fit criterion is related to the extension of a slight bias in the initial calibration.

Overall, the robustness of the model is demonstrated in the three agronomic and geological contexts. The model was able to simulate in a satisfactory way a 5 years hydrological and nitrate simulation extension for several systems with different inertia and trends. This point is important to gain the confidence of the decision makers. These simulations validate the use of the three models for predictive purposes, and by extension, show that BICHE is suitable for such uses. Decision makers can rely on such predictions, at least on short terms, without needing to recalibrate the model. However, the model is so easy and fast running (some seconds for 5000 iterations during automatic iteration), that in a normal use, it should be regularly recalibrated to take the best of the full set of data.

Although robust, this type of modelling based on calibration remains dependent on the characteristics of the input series used during the calibration phase: the input time series must be sufficiently long and contrasted to obtain a reliable and robust calibration.

Retrospective analysis on the effect of agronomic measure implementation

The last step illustrates a potential operational use of the model to evaluate the impact of the measures and programs. This evaluation is essential for decision maker who sometimes question whether the measures taken should be maintained and doubt on their actual effectiveness. Thus, the model will answer the question "What would have happened if the environmental measures had not been applied?"

The new set of simulations considers continuity of practices as they were before the initiation of environmental measures (national and local) in those sites. By comparing them to the present situation, we aim to illustrate and quantify the effectiveness of agronomic measures implementation on groundwater quality. This scenario has obviously no impact on groundwater levels and only nitrate comparison will be discussed here (Fig. 3b,4b,5b). N balance comparisons at watershed scale between scenarios have been carried out to help understanding the different results obtained with the two simulations. The N balance is the comparison between N input and N output [START_REF] Oenema | Approaches and uncertainties in nutrient budgets: implications for nutrient management and environmental policies[END_REF]. It was calculated at the catchment scale.

For the basement site, the new simulation shows a further increase in concentrations. There is no trend reversal. We can therefore conclude that the observed reversal is due to the change in practice. The two scenarios are really contrasted. At the end of the simulations, the difference in nitrate concentration is almost 37 mg/L, the "no measures" simulations reaching 106 mg/L in December 2015 vs. 62 mg/L. The difference between the 2 scenarios originates from an important land use change from 1989/90, with 40% of the cultivated area converted in meadows before 2007, that yields to a sharp decrease of fertilization (grass is not fertilized) modifying the total N balance. For these two scenarios, N balances are almost similar from 1985 to 1997 whereas N balances for actual practices is 45 kg/ha lower in 1997 and around 200 kg/ha lower in 2015 than for "no measures" scenarios. As the land use transformation was probably progressive, it is difficult to evaluate the time lag between the action and its effect on groundwater, but the order of magnitude is of 5 years, considering that the first drift between the two curves appears in year 2000.

For the karstic site, in the "no measures" scenario the increase in concentrations continues and is confirmed above the threshold of 50 mg/L, whereas the previous simulation showed a stabilization just below 50 mg/L. Even if the difference between the two scenarios is small in terms of final concentration (about 10 mg/L), the position of the curve on one side or the other of the 50 mg/L threshold is of paramount importance, since 50 mg/L corresponds to the drinkability standard in Europe. The difference between the two scenarios is linked to fertilization efficiency. Between 2003 and 2010, farmers in the catchment were accompanied by the Agricultural Chamber to adjust the fertilization to each plot (over 80% of the catchment) considering field measurement and nitrogen modelling use. This use of integrated crop management techniques to optimize the use of soil and nutrient inputs can be considered the main difference between the two scenarios and therefore, the major factor affecting the observed nitrate reduction in groundwater (compared to what it would have been without this effort). The end of the support provided by the Agricultural Chamber may explain the modification of the N balance and the increase in concentration observed during the ultimate years. The karstic site, as one could expect, appears as the most reactive site of the studied sites, due to fast transfer in the vadose zone and extreme vulnerability of the water table. It may be difficult to exactly define when the measure taken has a significant impact on groundwater, but the order of magnitude, in this karstic example, is after 1 to 2 years.

In this karstic site, it is important to notice that the "no measure" scenario does not simulate a continuous increasing trend as in the other sites, but seems to stabilize on a plateau, except for seasonal variations. It is therefore conceivable to consider that an equilibrium is reached in the aquifer system; a permanent mass flow regime which is set up thanks to the short transit times in the aquifer. Since agronomic input are equal each year, this scenario also evidences the low influence of the climate on nitrate evolution: despite a succession of contrasted years (640 mm/yr in the hydrologic year 2011-2012, 899 mm/yr in the hydrologic year 2013-2014), the interannual variation is not visible.

For the limestone site, the "no measure" simulation drops out the actual simulation as soon as 1995. Unlike in the basement site, the simulation shows an irregular evolution of nitrate concentration, with a global increasing trend leading to a significant difference at the end of the simulation of almost 30 mg/L, the "no measures" simulation reaching 121 mg/L in 2015 vs.

93 mg/L.

The N balances for "no measure" scenario and the second scenario are almost similar from 1988 to 1992. Since 1992, the "second scenario" N balance is inferior for two reasons. First, poplars were planted in the vicinity of the well but the area only represents 1% of UAA and second, the yield increased whereas fertilization was almost stable on the catchment leading to a more efficient N balance. Therefore, it seems that the improvement of the situation originates in fertilization efficiency. Over the period 1990-1995, the two scenario nitrate series are almost identical although voluntary and obligatory environmental and agricultural action plans were implemented (in 1991 and 1992). A delay of about five years seems necessary before the impact of actions can be visible. From 1995, the two curves are different although the same tendencies are preserved.

The no measure simulation, repeating the same agronomic input, clearly evidence the climate impact. In this site, the climatic conditions shape the nitrate evolution in groundwater. The impact of wet years (as 2000-2001) is probably responsible on the decrease of concentration observed in [2005][2006]. Between 2010 and 2015, the N balance increases because the wheat yield decreases rapidly but fertilization decreases more slowly (in adaptation of yield decrease)

For the 3 studied sites, although it may seem theoretical because it probably would have never happen this way, the retrospective analysis of the "no measure" scenario not only highlights the impact of the taken measures, but also quantifies the gain in terms of concentration reduction, estimates delay between measure and impact, and evidence the role of the climate on nitrate concentration variation in groundwater. In addition, the scenario applied on the three sites demonstrated that the implementation of programs of measures makes it possible to reduce nitrate concentrations of groundwater.

For 2 of the 3 sites, the measures taken do not yet make it possible to go below the 50 mg/L threshold, but they have made it possible to reverse the trend of increasing levels with a more or less significant delay effect depending on the inertia of the system. The key role of hydrodynamic on groundwater quality trend is also highlighted. These results are critical for environment managers to decide whether to maintain, strengthen or abandon the measures.

Conclusion

This work conclusively shows that the lumped model BICHE was able to correctly simulate the long term trends of both water and nitrate evolutions in three different hydrogeological and agricultural contexts. By contrast, due to the monthly time step of input data (adapted to input data uncertainty -e.g. real fertilization) and simplifications inherent to lumped models, short-term variations were less correctly reproduced. The lack of detailed agronomic data often leads to the use of rough approximations or input of data from communal or departmental statistics, which can span over larger areas than the catchment. However, in a perspective of environment (2011)(2012)(2013)(2014)(2015) without any new calibration process compared to "calibration"), no measure (continuity of practices as they were before the initiation of environmental program of measures)

Figure 4: Time-series of piezometry (A) and nitrate concentration (B) for the limestone site: observation (real data), calibration (dataset until 2010), validation (extended set of five years (2011)(2012)(2013)(2014)(2015) without any new calibration process compared to "calibration"), no measure (continuity of practices as they were before the initiation of environmental program of measures)

Figure 5: Time-series of piezometry (A) and nitrate concentration (B) for the karstic site: observation (real data), calibration (dataset until 2010), validation (extended set of five years (2011)(2012)(2013)(2014)(2015) without any new calibration process compared to "calibration"), no measure (continuity of practices as they were before the initiation of environmental program of measures) 1998 to 2003 1987; 1993; 1999-2000 1992-93, 1998-2000, 2006 Particularly dry years 1988, 1990, 1995-96, 2004 1988-1990, 2004 2004 Average

  Monthly rainfall and evapotranspiration, available during the whole duration of the simulation at three meteorological stations of Météo-France (national meteorological and climate operator) close to the studied sites were used. Water level time-series are available for 11, 21 and 23 years while nitrate concentrations time-series are available for 31, 41 and 35 years in the basement, karstic and limestone sites, respectively.

  The "No measures scenario" N balance and the "Second scenario" N balance are almost similar from 1985 to 2003 and from 2011 to 2015. But from 2004 to 2010, the "actual practice scenario" N balance is inferior, with a difference of 64 kg/ha in 2006.

Figure 1 :

 1 Figure 1: Diagram of the BICHE model

Figure 3 :

 3 Figure 3: Time-series of piezometry (A) and nitrate concentration (B) for the basement site: observation (real data), calibration (dataset until 2010), validation (extended set of five years(2011)(2012)(2013)(2014)(2015) without any new calibration process compared to "calibration"), no measure (continuity of practices as they were before the initiation of environmental program of measures)

Table caption Table 1 :

 caption1 Studied catchments presentation

						Fertilizers mineralization Soil	Crop Needs	ETP	Rainfall/ Snow
								Crop residues
	U : Upper tank Basement site Basement site						mineralization
		Effective rainfall Limestone site Limestone site		MW Mobile Water	Exchange	RW Retained Water	RUMAX
	H: Hypodermic
	tank Karstic site Karstic site							NO3 Flux in Intermediate
	level in								tank
	tank H	RUIPER	RW			MW
									Rapid Flow	Total flow
			Percolation
	G: Groundwater		THG						NO3 Flux in	NO3 Concentration
	water table level	RW	90	0	MW 90	km 180	TG	Deep tank Slow Flow
			1

Table 1 :

 1 Studied catchment presentation

		BASEMENT SITE	KARSTIC SITE	LIMESTONE SITE
	Catchment municipality
	(NUTS3)	Plourhan	Déols	Pentvert
	Region (NUTS1)	Brittany	Centre-Val de Loire Pays de la Loire
	Station coordinates (WGS84) 48.6400 ; -2.8970	46.8287 ; 1.7235	48.3279; 0.2660
	Closest major town	Saint-Brieuc	Chateauroux	Le-Mans/Alençon
	Catchment area surface	0.6 km 2	50 km 2	9 km 2
	Altitude mean	100 m	150 m	125 m
	average slope	1%	0.25%	0.25%
	Geologic formation	Basement formation karstified limestone fissured limestone
	Soil average thickness	40-80 cm	20-80 cm	45 cm
	Water Holding Capacity (mm) 100	100	50
	Average yearly rainfall (mm/yr) 767	729	699
	Particularly wet years

Acknowledgments

This project POLDIF was funded by the Agence de l'Eau Loire Bretagne (Loire-Brittany water agency) and BRGM.

management, the need is to get simple tools to simulate and extrapolate trends and trend reversal, under agronomic and climatic scenarios. The BICHE lumped model is therefore well suited for such a purpose.

For the three sites, a validation exercise conducted using an extension of 5 years of the dataset proved successful. The hydrological simulation was improved and the nitrate concentration simulation was barely altered proving the robustness of the calibration process. Besides, thanks to the relatively limited number of parameters and the automatic calibration, the use of the BICHE model is very fast (it runs within seconds on standard computers). Lastly, the retrospective analysis of effects of implementation of agricultural practices highlighted the ability of this tool to support the evaluation of programs of environmental measures. The "what if" scenarios have an educational power useful to make stakeholders (particularly farmers) understand that their efforts to reduce agricultural inputs are not in vain and politician that environmental measures are worth trying.

The results of the "what if" scenario undertaken in this work show that the agricultural policies enforced in France had an impact in three contrasted climate and agricultural sites, that can be more or less delayed in time.

Unquestionably, BICHE can be used operationally for decision maker purposes, outside a research framework. Beyond retrospective analysis, this tool could be used in a prospective purpose to support discussion between farmers and stakeholders in order to implement new programs of measures. The model will be useful to determine the extent of efforts to be made and to estimate the time lag between the implementation and the effect on groundwater quality.


Nadjia Kachenoura
email: nadjia.kachenoura@inserm.fr

Khaoula Bouazizi-Verdier

Fabrizio Andreelli

Alban Redheuil

Imagerie par résonance magnétique cardiovasculaire et ses applications dans la cardiomyopathie diabétique Cardiac MRI and its application in diabetic cardiomyopathy

Keywords: IRM -Diabètes -Déformation myocardique -Fibrose interstitielle -Rigidité aortique, ----- MRI -Diabetes -Myocardial strain -Interstitial fibrosis -Aortic stiffness, -----------------

L'imagerie par résonance magnétique (IRM) cardiovasculaire permet, grâce à ses incessantes innovations technologiques, de réaliser une imagerie in vivo d'un coeur battant et d'une artère pulsatile avec une couverture anatomique et un contraste inégalables. Cette exploration peut être réalisée à l'état de repos, mais également sous stress pharmacologique afin de détecter une maladie coronaire, souvent infraclinique, chez les patients diabétiques. Combinée à des méthodes de traitement d'images avancées, elle permet également une analyse quantitative et multiparamétrique du système cardiocirculatoire, en donnant accès à une mesure précise de sa géométrie et de sa fonction, ainsi qu'à ses caractéristiques tissulaires et métaboliques. L'altération de ces biomarqueurs d'IRM dans la cardiomyopathie diabétique à un stade très précoce, avant même l'apparition des symptômes, en fait un examen déterminant dans l'exploration de ces patients. En effet, la capacité de l'IRM à explorer finement l'altération fonctionnelle du coeur et des vaisseaux associant une dysfonction contractile et une rigidité cardiaque à une rigidité aortique ou une atteinte microvasculaire et liées à des processus de fibrose et de surcharge lipidique permettent de découvrir des atteintes restées méconnues et peuvent permettre de préciser le risque cardiovasculaire individuel chez les patients diabétiques.

1. Imagerie par résonance magnétique : une modalité multiparamétrique L'intérêt décisif de l'imagerie par résonance magnétique (IRM) réside dans sa capacité à fournir de façon non invasive et non ionisante une imagerie in vivo au contraste tissulaire inégalé couplé avec une excellente couverture anatomique qui permet une imagerie intrinsèquement multiparamétrique du système cardiocirculatoire. En effet, l'IRM cardiovasculaire est une technique d'imagerie de référence pour l'évaluation clinique du remodelage et de la fonction cardiaque et artérielle, ainsi que l'imagerie myocardique des processus de nécrose et de fibrose d'origine ischémique ou non. Son champ d'application en recherche clinique a également été considérablement élargi vers l'exploration 3D+t des flux sanguins, la caractérisation tissulaire hautement sensible et quantitative par les techniques de cartographie T1 et T2, la perfusion quantitative et la caractérisation métabolique et moléculaire permises par la spectroscopie proton, phosphore et, dernièrement, au carbone 13 hyperpolarisé. Ces techniques d'IRM paraissent donc potentiellement centrales dans l'exploration de la cardiomyopathie diabétique.

Remodelage et fonction cardiaque 2.1.

Remodelage myocardique et volumes ventriculaires L'IRM est la méthode de référence pour l'évaluation des volumes cardiaques et de la masse myocardique grâce à des séquences dynamiques hautement résolues qui permettent d'acquérir, avec un excellent contraste entre le sang et les parois ventriculaires, des coupes jointives selon le petit et le grand axe du coeur et couvrant le coeur entier. Une qualité optimale peut être obtenue indépendamment de la morphologie thoracique, en particulier de l'obésité, qui rendent une exploration ultrasonore aléatoire. De nombreuses études IRM focalisées sur le ventricule gauche (VG) du patient diabétique de type 2 (DT2) ont montré un remodelage concentrique à un stade précoce alors même que la fraction d'éjection est préservée [START_REF] Gulsin | Differential left ventricular and left atrial remodelling in heart failure with preserved ejection fraction patients with and without diabetes[END_REF]. Ce remodelage concentrique du VG a été associé à un mauvais pronostic chez le patient diabétique, et en particulier une incidence plus élevée d'insuffisance cardiaque [START_REF] Gilca | Diabetic cardiomyopathy: current approach and potential diagnostic and therapeutic targets[END_REF]. L'IRM a également été utilisée pour étudier le remodelage et les modifications géométriques de l'oreillette gauche (OG) ans le DT2 et qui ont été associées à une incidence accrue d'événements cardiovasculaires [START_REF] Markman | Association of left atrial structure and function and incident cardiovascular disease in patients with diabetes mellitus: results from multi-ethnic study of atherosclerosis (MESA)[END_REF]. Ces études montrent souvent des variations minimes des volumes OG chez les patients diabétiques, avec cependant une altération de la fonction triphasique de l'oreillette (1 -phase de réservoir : dilatation passive de l'oreillette, induite par la pression du flux sanguin en provenance des poumons via les veines pulmonaires pendant la systole ventriculaire ; 2 -phase de conduit : contraction passive provoquée par l'ouverture de la valve mitrale (ou tricuspide pour le coeur droit), entre l'oreillette et le ventricule, induite par le gradient de pression entre les deux cavités pendant la proto-diastole ventriculaire ; 3 -phase de contraction atriale : contraction active de l'oreillette pendant la télé-diastole ventriculaire. Elle a pour rôle d'achever la vidange de l'oreillette et le remplissage du ventricule). En effet, Shang et al. [START_REF] Shang | Left atrium passive ejection fraction is the most sensitive index of type 2 diabetes mellitus-related cardiac changes[END_REF] montrent que la fraction d'éjection atriale passive était la plus représentative de l'atteinte atriale chez le patient atteint d'un DT2. Graça et al. [START_REF] Graça | Left atrial dysfunction in type 2 diabetes mellitus: insights from cardiac MRI[END_REF] montrent une réduction des fonctions conduit et réservoir de l'OG chez le patient diabétique asymptomatique. Dans la population générale de sujets adultes de plus de 45 ans de l'étude nord-américaine « Multi-Ethnic Study of Atherosclerosis » (MESA), il a été également montré que le volume minimal de l'OG, ainsi que les paramètres de fonction passive et active de l'OG, étaient plus fortement associés à la survenue de maladies cardiovasculaires que le volume maximal de l'OG, même après ajustement aux facteurs de risque traditionnels, la masse VG et la partie N-terminale du peptide natriurétique de type B (NT-proBNP) [START_REF] Graça | Left atrial dysfunction in type 2 diabetes mellitus: insights from cardiac MRI[END_REF].

Déformation myocardique multi-chambres

Plus récemment, ces mêmes données IRM, acquises de manière systématique à chaque examen sans nécessité de nouvelles séquences, ni injection de produits de contraste, ont été utilisées pour estimer la déformation myocardique (« strain ») et sa vitesse de déformation (« strain-rate ») dans les différentes cavités cardiaques, grâce à des méthodes de suivi de texture (« feature tracking ») au cours du cycle cardiaque (figure 1). Ces mesures sont reproductibles [START_REF] Lamy | Scan-rescan reproducibility of ventricular and atrial MRI feature tracking strain[END_REF], capables de détecter les modifications cardiaques infracliniques liées à l'âge [START_REF] Evin | Left atrial aging: a cardiac magnetic resonance feature-tracking study[END_REF], et ont montré une association forte avec le taux de fibrose et de graisse mesuré en histologie [START_REF] Huber | Cardiac MR strain: a noninvasive biomarker of fibrofatty remodeling of the left atrial myocardium[END_REF] sur des prélèvements chirurgicaux au décours d'une atriotomie pour insuffisance valvulaire mitrale. Cette déformation peut être mesurée en longitudinal, circonférentiel et radial, et ce pour toutes les cavités cardiaques, avec une évaluation tri-phasique de la fonction atriale. Les études récentes centrées sur cette mesure de la déformation myocardique chez le patient atteint d'un DT2 en IRM montrent leur capacité à détecter l'atteinte myocardique à son stade précoce, à la fois sur le VG [START_REF] Xie | Assessment of left ventricular deformation in patients with type 2 diabetes mellitus by cardiac magnetic resonance tissue tracking[END_REF], le ventricule droit [START_REF] Shao | Early detection of left atrial and bi-ventricular myocardial strain abnormalities by MRI feature tracking in normotensive or hypertensive T2DM patients with preserved LV function[END_REF], et l'OG [START_REF] Shao | Early detection of left atrial and bi-ventricular myocardial strain abnormalities by MRI feature tracking in normotensive or hypertensive T2DM patients with preserved LV function[END_REF].

Caractérisation tissulaire et métabolisme 3.1.

Fibrose myocardique de remplacement L'IRM cardiaque se distingue des autres techniques par ses nombreuses séquences dédiées à la caractérisation du tissu et de la fibrose myocardiques. La technique la plus largement utilisée est celle de l'étude du rehaussement tardif (LGE) myocardique lié à une rétention de produit de contraste. Cette technique consiste à acquérir des images en coupe dans différents plans cardiaques couvrant le VG de son apex à sa base, 10 à 15 minutes après l'injection d'un produit de contraste gadoliné. Le gadolinium étant un agent extracellulaire, sa quantité dans le myocarde sain à 10 minutes de l'injection est minime, alors qu'elle est considérable dans l'espace extracellulaire de remplacement en cas de destruction cardiomyocytaire (infarctus du myocarde [IDM], myocardite, fibrose cicatricielle). Cette zone apparaît alors en hypersignal (blanc) sur les images IRM tardives (figure 2), tandis que le myocarde sain apparait en hyposignal (noir) [START_REF] Kim | The use of contrast-enhanced magnetic resonance imaging to identify reversible myocardial dysfunction[END_REF]. La présence de rehaussement tardif chez le patient diabétique sans preuves cliniques d'événement cardiaque est révélatrice d'un IDM silencieux et méconnu qui est associé à un mauvais pronostic [START_REF] Kwong | Incidence and prognostic implication of unrecognized myocardial scar characterized by cardiac magnetic resonance in diabetic patients without clinical evidence of myocardial infarction[END_REF][START_REF] Yang | Prognosis of unrecognised myocardial infarction determined by electrocardiography or cardiac magnetic resonance imaging: systematic review and meta-analysis[END_REF], et ce indépendamment des données d'électrocardiographie et de fonction ventriculaire gauche, mesurée ou non en IRM.

3.2.

Fibrose interstitielle et microcirculation myocardique Au-delà de cette imagerie de la fibrose dense de remplacement, l'IRM cardiaque dispose aujourd'hui d'une nouvelle séquence dite « T1 mapping » ou de cartographie du T1 qui permet de caractériser la fibrose interstitielle (figure 3). La cartographie du T1 fournit une imagerie quantitative des temps de relaxation longitudinal dits T1 et peut être réalisée sans et avec injection de gadolinium. La combinaison des cartographies T1 pré-et post-injection avec le taux d'hématocrite du patient permet d'estimer le volume extracellulaire. Les paramètres quantitatifs du T1 permettent de différencier les patients ayant une cardiomyopathie des sujets sains, qu'elle que soit la fraction d'éjection. Ces données de quantification de la fibrose interstitielle en IRM sont aussi corrélées aux capacités d'exercice, à la dysfonction cardiaque infraclinique, à l'analyse histologique du myocarde, ainsi qu'au pronostic cardiovasculaire du patient [START_REF] Kammerlander | T1 mapping by CMR imaging: from histological validation to clinical implication[END_REF][START_REF] Puntmann | International T1 Multicentre CMR Outcome Study. T1-mapping and outcome in nonischemic cardiomyopathy: all-cause mortality and heart failure[END_REF]. Par ailleurs, il a été montré chez le patient diabétique sans rehaussement tardif que la présence de fibrose interstitielle caractérisée par la cartographie T1 était associée à une dysfonction diastolique et systolique longitudinale du VG [START_REF] Ng | Association between diffuse myocardial fibrosis by cardiac magnetic resonance contrast-enhanced T1 mapping and subclinical myocardial dysfunction in diabetic patients: a pilot study[END_REF]. Levelt et al. [START_REF] Levelt | Adenosine stress CMR T1-mapping detects early microvascular dysfunction in patients with type 2 diabetes mellitus without obstructive coronary artery disease[END_REF] ont réalisé une cartographie du T1 sans injection, au repos et sous stress pharmacologique, révélant un taux d'augmentation du T1 natif sous stress chez les patients atteints d'un DT2 non coronariens et non hypertendus inférieur à l'augmentation constatée chez des sujets sains appariés, démasquant ainsi une probable dysfonction microvasculaire chez les sujets diabétiques. Celle-ci, longtemps discutée dans la littérature comme étiologie possible de la dysfonction myocardique dans la population diabétique, souffrait d'un manque de preuves de son existence formelle. L'IRM myocardique revisite ainsi ce concept important, en permettant de démasquer les patients potentiellement à risque d'évoluer vers l'insuffisance cardiaque sans atteinte macrovasculaire des coronaires. Une autre approche pouvant caractériser cette atteinte microvasculaire au niveau du myocarde du patient diabétique en IRM est la séquence de perfusion de premier passage acquise pendant l'injection intraveineuse du produit de contraste gadoliné, avant les acquisitions de rehaussement tardif et de cartographie T1 post-injection. Cette séquence permet de déterminer la perfusion du myocarde au cours du premier passage de l'agent de contraste. Des modifications de la cinétique de rehaussement du myocarde au premier passage du produit de contraste peuvent être observées en condition de repos et de stress et donner lieu à une quantification ou semiquantification grâce à des techniques de traitement d'image. La quantification des paramètres de rehaussement (temps du pic de rehaussement, intensité maximale du signal) met en évidence l'atteinte macro-et microvasculaire induite par le diabète au niveau myocardique. En effet, Liu et al. [START_REF] Liu | Left ventricular subclinical myocardial dysfunction in uncomplicated type 2 diabetes mellitus is associated with impaired myocardial perfusion: a contrast-enhanced cardiovascular magnetic resonance study[END_REF] ont analysé les images de perfusion en IRM chez des volontaires sains et des patients atteints de DT2 diagnostiqués depuis plus, ou moins de 5 ans, révélant : un pic de rehaussement plus tardif et une intensité maximale moins élevée chez les diabétiques dans le stade précoce de leur maladie. Cette imagerie réalisée au repos et sous stress pharmacologique permet d'estimer la réserve de perfusion myocardique calculée comme le rapport entre le flux sanguin myocardique sous stress et au repos. Cette dernière était significativement réduite chez les patients diabétiques malgré l'absence de sténoses coronariennes > 30 % [START_REF] Larghat | The microvascular effects of insulin resistance and diabetes on cardiac structure, function, and perfusion: a cardiovascular magnetic resonance study[END_REF], suggérant une part significative d'atteinte microvasculaire.

Spectroscopie IRM et métabolisme myocardique in vivo

La spectroscopie par résonance magnétique, qui recueille le signal de résonance du proton ( 1 H) dans un volume d'intérêt du myocarde a également été utilisée pour mesurer la teneur en triglycérides du myocarde dans le DT2, reflétant ainsi la stéatose myocardique qui est présente chez le patient DT2, y compris avec fraction d'éjection préservée [START_REF] Mcgavock | Cardiac steatosis in diabetes mellitus: a 1 H-magnetic resonance spectroscopy study[END_REF]. Cette stéatose a été associée à la dysfonction systolique (diminution de la déformation) et au remodelage concentrique du VG chez des patients DT2 non hypertendus [START_REF] Levelt | Relationship between left ventricular structural and metabolic remodeling in type 2 diabetes[END_REF], ainsi qu'à l'altération de la fonction diastolique VG [START_REF] Rijzewijk | Myocardial steatosis is an independent predictor of diastolic dysfunction in type 2 diabetes mellitus[END_REF][START_REF] Korosoglou | Left ventricular diastolic function in type 2 diabetes mellitus is associated with myocardial triglyceride content but not with impaired myocardial perfusion reserve[END_REF]. Une autre étude récente, réalisée sur des patients atteints d'un DT2 et des volontaires sains, a montré que cette mesure de triglycérides était positivement associée à la baisse de la déformation et de la perfusion myocardiques [START_REF] Gao | Metabolic syndrome and myocardium steatosis in subclinical type 2 diabetes mellitus: a 1 H-magnetic resonance spectroscopy study[END_REF]. Enfin, la stéatose myocardique présente dans le DT2 a été également associée à la diminution de la performance énergétique de la chaîne respiratoire du myocarde mesurée par spectroscopie IRM du phosphore 31 ( 31 P) via le rapport de la phosphocréatine à l'adénosine triphosphate (PCr/ATP) [START_REF] Levelt | Relationship between left ventricular structural and metabolic remodeling in type 2 diabetes[END_REF] qui est elle-même associée à un remodelage concentrique du VG. La même équipe a également montré que ce déficit énergétique chez le patient DT2 est exacerbé sous stress et corrélé à un déficit de perfusion myocardique [START_REF] Levelt | Cardiac energetics, oxygenation, and perfusion during increased workload in patients with type 2 diabetes mellitus[END_REF]. Parallèlement à la caractérisation du myocarde, l'IRM a également été utilisée pour identifier et quantifier la graisse épicardique (figure 4). La graisse épicardique, dont l'augmentation a été associée au DT2 et à la résistance à l'insuline, indépendamment de l'indice de masse corporelle (IMC) du patient, est liée à une diminution de la déformation systolique [START_REF] Levelt | Ectopic and visceral fat deposition in lean and obese patients with type 2 diabetes[END_REF]. Une autre étude a montré que le volume de graisse épicardique était significativement plus élevé chez les insuffisants cardiaques et qu'il était associé à la présence d'une fibrillation atriale et d'un DT2 chez des insuffisants cardiaques avec une fraction d'éjection VG > 40 % [START_REF] Van Woerden | Epicardial fat in heart failure patients with mid-range and preserved ejection fraction[END_REF].

Rigidité aortique et couplage aorte-ventricule gauche

L'IRM permet également de mesurer de nouveaux biomarqueurs de la rigidité locale et régionale de l'aorte thoracique (figure 5). La distensibilité aortique s'est avérée être un marqueur précoce du vieillissement vasculaire dans la population générale [START_REF] Redheuil | Reduced ascending aortic strain and distensibility: earliest manifestations of vascular aging in humans[END_REF]. L'augmentation de la rigidité aortique proximale mesurée par IRM a été associée à une diminution de la fonction systolique et diastolique et à un remodelage concentrique du VG chez des sujets sains [START_REF] Redheuil | Age-related changes in aortic arch geometry: relationship with proximal aortic function and left ventricular mass and remodeling[END_REF][START_REF] Redheuil | Left ventricular and proximal aorta coupling in magnetic resonance imaging: aging together?[END_REF][START_REF] Bollache | Abnormalities in aortic properties: a potential link between left ventricular diastolic function and ventricular-aortic coupling in sickle cell disease[END_REF][START_REF] Bollache | How to estimate aortic characteristic impedance from magnetic resonance and applanation tonometry data?[END_REF][START_REF] Bargiotas | MR and applanation tonometry derived aortic impedance: association with aging and left ventricular remodeling[END_REF]. Enfin, la distensibilité aortique mesurée en IRM était prédictive de la mortalité et des événements cardiovasculaires, chez les individus à risque faible à intermédiaire sans maladie cardiovasculaire manifeste, au-delà des facteurs de risque traditionnels et des marqueurs de l'athérosclérose infracliniques, tels que le calcium coronarien et l'épaisseur de l'intima-média carotidienne [START_REF] Redheuil | Proximal aortic distensibility is an independent predictor of all-cause mortality and incident CV events: the MESA study[END_REF]. De surcroît, la distensibilité aortique était altérée chez les patients atteints d'un diabète de type 1 asymptomatiques en lien avec un mauvais contrôle glycémique, en particulier chez les plus jeunes, une atteinte rénale (macroalbuminurie) et l'hypertension artérielle [START_REF] Turkbey | Diabetes Control and Complications Trial/Epidemiology of Diabetes Interventions and Complications Research Group. Aortic distensibility in type 1 diabetes[END_REF]. Le lien entre la rigidité aortique et le remodelage ventriculaire mesurés en IRM a également été étudié chez des patients avec un DT2 et sans maladies cardiovasculaires connues, indiquant que la distensibilité aortique était indépendamment associée au remodelage concentrique du VG [START_REF] Gulsin | Relation of aortic stiffness to left ventricular remodeling in younger adults with type 2 diabetes[END_REF]. La distensibilité aortique mesurée en IRM chez des patients DT2 avec un risque vasculaire faible est également indépendamment associée à un mauvais contrôle de la glycémie, ainsi qu'aux événements cardiovasculaires [START_REF] Swoboda | Cardiovascular magnetic resonance measures of aortic stiffness in asymptomatic patients with type 2 diabetes: association with glycaemic control and clinical outcomes[END_REF]. En outre, elle peut être réversible lors d'un traitement par inhibition du système rénine-angiotensinealdostérone.

Conclusion

L'IRM myocardique est en train de révolutionner notre analyse du myocarde, non seulement sur le plan anatomique, mais également sur le plan fonctionnel. En effet, cette technique d'imagerie permet d'accéder à une plus grande précision d'analyse de la structure et composition du myocarde (fibrose, lipides, zones ischémiques ou infarcies passées cliniquement inaperçues) et de sa fonctionnalité (contractilité globale et segmentaire), bien au-delà de la seule mesure de la fraction d'éjection ventriculaire. De plus, la combinaison de cette analyse myocardique fine et de la rigidité aortique centrale permet une mise en évidence très précoce de l'atteinte cardiovasculaire, bien avant la survenue d'évènements cliniques. La découverte de micro-infarctus passés inaperçus, d'une fibrose débutante ou d'une dysfonction contractile, peuvent permettre de reclasser des patients considérés a priori comme en prévention primaire vers un statut de prévention secondaire, ce qui change significativement la perception du risque cardiovasculaire et les stratégies correspondantes de prise en charge individuelle. Ainsi, au-delà de la prouesse technique de l'analyse multiparamétrique d'un organe contractile in vivo, les informations mises à disposition par l'IRM trouvent aujourd'hui, dans l'exploration des conséquences cardiovasculaires du diabète, un champ d'application majeur.

Les points essentiels

• L'imagerie par résonance magnétique (IRM), technique d'imagerie non invasive, permet une imagerie précise et quantitative de deux organes cibles du diabète : le coeur et les artères. -

----------------------------------------------------------------------------------------------------------------

Déclaration de liens d'intérêts

Les auteurs déclarent ne pas avoir de liens d'intérêts.

----------------------------------------------------------------------------------------------------------------

Figure 1 .

 1 Figure 1. Analyse de texture pour l'estimation automatisée de la déformation de l'oreillette gauche au cours du cycle cardiaque en imagerie par résonance magnétique (IRM). L'image 4 cavités de gauche correspond à une phase du cycle cardiaque de la séquence ciné analysée. À droite, sont illustrées les courbes de déformation globale (rouge) et segmentaire (couleurs associées aux segments délimités sur l'image de gauche). Les trois phases de la déformation atriale sont illustrées sur les courbes (Analyse réalisée par le logiciel académique Cardio-Track ; Laboratoire d'Imagerie Biomédicale, Sorbonne Université).

Figure 2 .Figure 3 .

 23 Figure 2. Image IRM de rehaussement tardif d'un patient ayant un infarctus du myocarde

Figure 4 . 1 Figure 5 .

 415 Figure 4. Délimitation de la graisse épicardique (à gauche) et de la graisse cardiaque totale

  • L'IRM permet une mesure quantitative des modifications géométriques, fonctionnelles, tissulaires (perfusion, fibrose et graisse) et métaboliques du myocarde. • L'IRM permet une exploration des conséquences des altérations macrovasculaires et microvasculaires liées au diabète sur le coeur. • L'IRM met en évidence des atteintes myocardiques ischémiques ou non ischémiques, avec ou sans atteinte macrovasculaire coronaire, et souvent asymptomatiques. • La rigidité de l'aorte, mesurée in vivo en IRM est un biomarqueur prédictif de mortalité et d'événements cardiovasculaires chez des individus à risque cardiovasculaire a priori faible à intermédiaire. • La rigidité aortique mesurée en IRM est fortement liée au remodelage concentrique du ventricule gauche.


Thibaut Fortané
email: tfortane@gmail.com

Michael Bouyer

Malo Le Hanneur

Baptiste Belvisi

Guillaume Courtiol

Kevin Chevalier

Caroline Dainotto

Marie Loret

Agathe Kling

Antonin Bentejac

Thibault Lafosse

Thiery Pr

PHD Begué

Dr Marion Burnier
email: marion.burnier@wanadoo.fr

Epidemiology of hand traumas during the COVID-19 confinement period

Keywords: Epidemiology, hand injury, confinement, emergency, hand surgery, Covid19

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Materials and methods: we conducted a retrospective epidemiological study to analyze two periods between the same dates in 2019 and 2020, for all the duration of the confinement period. We compared the numbers of consultations in the emergency department, elective surgeries, hand and upper limb emergency cases in our center and urgent limb surgeries in the nearby hospital. Then we compared the mechanisms and severity of injuries and the type of surgery.

Results: between 2019 and 2020 there was a decrease of consultations in the emergency department in our institution of 52%, a decrease of total elective surgeries of 75%, a decrease in surgeries for urgent peripheral limb injuries of 50%, whereas the hand and upper limb emergency remained stable or even increased by 4% regard to occupational and domestic accidents.

There was a significant difference in the mechanism of injury with an increase of domestic accident and a decrease of occupational, road traffic and sport accidents. Severity of the injuries increased, with augmentation of the number of tissues involved and longer expected time of recovery.

Conclusion:

during the confinement period of the Covid-19 pandemic, despite an important reduction of medical activities, the amount and severity of hand emergency cases increased. A

Introduction

Hand injuries are common emergency situations, requiring surgical management in specialized centers on a daily basis [START_REF] Emmett | A review and analysis of 11,000 fractures seen in a private practice of orthopaedic surgery, 1937-1956[END_REF][START_REF] Chung | The frequency and epidemiology of hand and forearm fractures in the United States[END_REF]. More than a quarter of all unintentional injuries are hand injuries [START_REF] Larsen | The epidemiology of hand injuries in The Netherlands and Denmark[END_REF][START_REF] Smith | Causes and consequences of hand injury[END_REF]. They are mainly caused by accidents in daily life situations while other traumatic injuries of the upper limb are often observed in the context of sports injuries or road traffic accident [START_REF] Larsen | The epidemiology of hand injuries in The Netherlands and Denmark[END_REF][START_REF] Angermann | Injuries to the hand and wrist. A study of 50,272 injuries[END_REF]. There are two main types of hand injuries requiring surgical treatment.

Wounds with potential complex lesions of major structures in the deep tissues (e.g., tendons, nerves, arteries, ligaments and joint, bones), which require emergency management in order to save the hand function (e.g., devascularization, open fractures, flexor tendons lesions…) [START_REF] Altman | Initial management of hand injuries in the emergency patient[END_REF][START_REF] Ghosh | A study of hand injury and emergency management in a developing country[END_REF]. The second category includes all closed injuries involving bones and joints but without skin lesions, such as closed fractures and sprains, which require an urgent care but can be deferred for 7 to 10 days.

The orthopedic surgical activity finds a majority of wounds on hand trauma, while on the rest of the upper limb we reported a greater number of closed fractures [START_REF] Crowe | Global trends of hand and wrist trauma: a systematic analysis of fracture and digit amputation using the Global Burden of Disease 2017 Study[END_REF].

In 2020, during the confinement period secondary to the Covid-19 pandemic, there was a significant decrease in professional activities, physical activities and transportation. We then hypothesized that despite the confinement of the population and the decrease in most of the medical and elective surgical activities, traumas of the hand operated in emergency would continue [START_REF] Ducournau | COVID-19: Initial experience of an international group of hand surgeons[END_REF]. In addition, we expected a significant reduction in closed trauma of the peripheral limb treated with osteosynthesis, and an increase of number and severity of complex cases.

Materials and methods

We conducted a retrospective epidemiological study in our center to analyze the medical and surgical activities, along with hand and upper limb injuries within the period of lockdown of the population related to the Covid-19 pandemic inducing a cutback in all professional work activities, physical activities and transportation. Our center is a provincial town with a population of about 210,000 inhabitants. It drains a large population base from the northern French Alpine massif, south of Geneva, north of Grenoble and east of Lyon.

Two periods of eight weeks were compared. The first from Wednesday the 18 th of March in 2020 to Tuesday the 10 th of May in 2020. The time of inclusion was the same as the confinement period as it corresponds to a period during which information emanating from the local health authorities (i.e., hospital direction comity, regional medical council and medical insurance companies) were unanimous (i.e. surgical activity restricted to critical emergencies, trauma, infections, tumors). After the 10 th of May 2020, information regarding the authorized activity in hospitals and health care centers authorized a partial resumption of medical activity.

The second was a control period of two months from the 18 th of March, 2019 to the 10 th of May, 2019.For these two periods, we retrospectively collected the number of patients coming to the emergency department of our hospital for other motives than Covid-19-related symptoms, the number of elective cases managed in our hospital and the number of patients operated for upper limb and hand injuries; regarding those cases, we looked more precisely at the energy of the accidents.

In addition to our private clinic in Annecy, Rhônes-Alpes, France, we collected the data regarding orthopedics traumas, which were operated at the trauma center of the local public hospital, Annecy, Rhônes-Alpes, France, as the regional health organization (Agence Régionale de Santé) of our area distributes the traumas management between our private center (i.e., hand traumas and microsurgical reconstruction) and the public hospital (i.e., polytrauma and peripheral traumatology).

In France, during the two month of lockdown, social policies were defined with few authorized activities. Only authorized movements were those: in the professional context only if essential and that cannot be carried out by telework; to make purchases of basic necessities; for medical reasons, administrative or judicial summons; recreational and individual activity within the limit of 1 kilometer and 1 hour around the home.

-Analysis criteria

The primary analysis criterion was the comparison of the number of patients between these two periods.

The secondary analysis criteria were the difference between the type, mechanism and severity of the hand and upper limb injuries operated at our institution. We also analyzed the influence of the age in the group of patients operated for peripheral limb trauma in the public hospital.

-Definitions A hand injury was defined as any closed or open injury to the wrist and/or the hand, substantially involving skin, muscle, tendon, bone and joint, nerve and/or vessels [START_REF] Czarnecki | CRITERIA FOR INCLUSION IN A EUROPEAN EMERGENCY[END_REF].

An upper limb injury was defined as an injury to the forearm, elbow, arm and shoulder.

A high-energy injury was defined by road, ski and mountain accidents, industrials machine and "do it yourself" tools accidents.

Peripheral limb trauma was defined as the traumas to the musculoskeletal system, excluding spine and skull and could include the upper limb, excluding hands.

Combined injury criteria was analyzed by looking at the number of structures damaged. The skin was considered only in case of a skin loss requiring a flap. Nerves and arteries from a collateral pedicle were considered as one tissue (i.e., neurovascular bundle) on fingers. We described a severity score as the mean of 3 criteria: high-energy injury, combined injury and expected delay before resuming manual daily activity longer or equal to 3 months, marking 1 point for each criteria.

-Inclusion and exclusion criteria

For the primary analysis criteria, we included all patients coming to the emergency department of our institution for a motive of consultation other than Covid-19-related symptoms, and all patients who underwent surgery at our institution regardless of the specialties. For the analysis of the epidemiological data about upper limb and hand emergencies, inclusion criteria were all upper limb injuries that happened during the period of confinement and required surgical management within the 10 days following the initial trauma.

The same criteria were applied to the group of patients managed in the public hospital for peripheral limb injuries.

The epidemiological data collected were gender, age, job, type of surgery, mechanism of injury and time between the date of injury, the first consultation and surgery. The type of surgery was separated into three categories: osteosynthesis and ligaments suture; wound with soft tissues lesions (skin, muscle, tendon, bone and joint, ligaments, nerves, vessels); infections and hematoma. The mechanism of injury was separated into four categories: occupational accidents; home accidents; motor vehicle accident (MVA) and sports traumatisms.

-Statistic analysis R for Mac OS X (R Foundation for Statistical Computing, CRAN) and Stata (StataCorp LP, College Station, TX, USA) were used for the statistical analysis. Data were presented as mean and standard deviation (SD). We performed paired Student t-tests if the sample was normally distributed and the homogeneity of variance was given. Otherwise, non-parametric Mann-Whitney tests were used. Normality was verified using Shapiro-Wilk tests and Gaussian aspects of variable distribution on histograms. Homogeneity of variances was verified using Bartlett tests. Chi-squared analyses with individual comparisons were made with Fisher exact tests and χ 2 tests, as appropriate. A P-value less than .05 was considered statistically significant.

The study was approved by the local review board (number 2217606 v 0), and conducted according to good clinical practice and applicable laws, and the 1964 Declaration of Helsinki.

Written consent was given by each patient.

Results

Between the two periods, we observed in our private institution a decrease of only 10% of hand or upper limb injuries operated in emergency. More specifically, within the hand and upper limb injuries, when high energy injuries were excluded (i.e. transport and sport injuries) we observed an increase by 4%. Meanwhile, a decrease of 52% of the consultations in the emergency department, 75% of elective cases operated in our institution was observed.

A decrease of 50% of peripheral limb injuries operated in the nearby public hospital, was observed.

Table 1 and Figure 1 describes the repartition of epidemiologic and surgical data between the two periods and the two medical centers.

-Epidemiological data

The main epidemiological data are summarized in Table 2.

The mechanism of injury is presented in Figure 2. There was a significant difference in repartition of trauma etiology between 2019 and 2020 with a decrease by 50% of occupational accident, a decrease of sport accident by 73% and an increase of domestic injury by 40%, (fisher test, p < 0.01).

-Severity score

We observed significant augmentation of severity of hand and upper limb trauma in 2020 in comparison to 2019. The severity score increased from 0.8±0.9 to 1.1±0;9 between 2019 and 2020 (p<0,01). Severity of trauma is presented in Table 3 and Figure 3. There was a significant augmentation of combined injury and expected delay before resuming manual daily activity from 2019 to 2020.

-Age analysis

In the group of patients operated from peripheral limb injuries at the local public hospital, the mean age was not different in 2020 (62±28 years) and 2019 (57±28 years), p > 0.05. When this population was separated into three groups (<65, 65-80 and >80 years old) the decrease in surgical activity were respectively by 60%, 55% and 31%.

In our private clinic, the mean age was not different between 2020 (44±21 years) and 2019 (41±21 years). We didn't observe any difference when the population was separated into the three same groups.

Discussion

In this study, we highlighted an augmentation of surgeries for hand emergencies, in opposition with a global medical and surgical decrease in activity. Despite a massive reduction of all surgical and medical activity in our institution and the local public hospital, with the number of patients coming to the emergency department for another reason of consultation than Covid19, the activity of the elective surgery, and the surgical trauma activity all decreased by more than half (respectively 52%, 75% and 50%). On the contrary the amount of upper limb injuries and hand injuries requiring an urgent surgical management was almost the same (decrease of only 10%). Furthermore, the amount of hand and upper limb injuries, not related to sports and transportation, has increased. We could therefore answer to our hypothesis, stating that hand emergency activity remains equivalent independently from an overall cutback into any other activity.

As a center to which patients are referred from nearby hospitals, (being the only hand and microsurgery unit in the area), and knowing that general hospitals focused on managing Covid19, a greater number of upper limb injuries could have been expected during this period.

In our area, sports practiced take place in mountains, and accidents may be serious with upper limb injuries (proximal humerus, clavicle and elbow fractures). Indeed, many of these injuries are secondary to sports trauma or motor vehicle accidents. During this period of the year, people are usually outdoor, playing sports, which increases our upper limb trauma surgery activity. During the confinement period, people stayed at home. As a result, we have seen an increase in domestic injuries such as simple hand soft tissues injuries with knives or glass. On the contrary, there is a decrease in upper limb injuries linked to the prohibition of mountain sports (especially skiing) and the decrease in road traffic.

The discrepancy established between the great decrease of activity in the emergency department and the elective cases and the less significant decrease in the amount of upper limb injuries managed in our institution can also be explained by the fact that while many healthcare professionals had to manage Covid19 infection related cases (explaining the general decline in medical activities), our institution focused on the management of hand and upper limb injuries which became the main reason for consultation in our department, and in the emergency department of our institution [START_REF]Considerations for Obstetric Care during the COVID-19 Pandemic[END_REF][START_REF] Mak | Oculoplastic surgery practice during the COVID-19 novel coronavirus pandemic: experience sharing from Hong Kong[END_REF].

During the confinement, the trend of the amount of major structure lesions, and the severity of the damaged caused by domestic accidents seemed to highlight an increase compared to the previous year. Consequently, we designed a severity score showing an increase of combined injury and an augmentation of the expected delay before resuming manual daily activity.

We believe, since people were confined at home, there were more accidents as they performed handy or "do it yourself" work, manipulating tools they were not used to, such as chain or circular saws.

The most severe lesions affecting many tissues (tendon, artery, nerve, and skin-loss lesion mainly) were indeed mainly due to gardening activities (wounds by chainsaw or hedge trimmer) and do-it-yourself activities (circular saw).

The distribution of the mechanism of accident in 2019 is similar to those reported in other studies particularly with regard to occupational activities (between 25 and 30%) [START_REF] Smith | Causes and consequences of hand injury[END_REF][START_REF] Angermann | Injuries to the hand and wrist. A study of 50,272 injuries[END_REF][START_REF] Nieminen | Hand injuries in Finland[END_REF][START_REF] Marty | Hand injuries and occupational accidents. Statistics and prevention[END_REF].

Sports injuries are less represented compared to the literature, because in this study we only evaluated the number of injuries requiring surgery, whereas many are managed in the emergency department or in our clinic. Domestic accidents are comparable to the literature and as shown by Campbell, falling and punching were the commonest mechanisms of fracture whereas glass, knives and "do-it-yourself" materials were most frequently implicated in wounds [START_REF] Campbell | Hand injuries at leisure[END_REF]. Domestic accidents were responsible for the majority of the traumas observed during this period. We have observed a decrease of work accident during this time. This is consistent with the cutback in professional activity linked to confinement. Sorock showed that hand injuries during occupational activities are mainly due to industries [START_REF] Sorock | Epidemiology of occupational acute traumatic hand injuries: a literature review[END_REF]. As they almost all closed, we observed a decrease in occupational hand injuries.

Rettig showed that most sport injuries at the hand appear to be associated with competition as opposed to the practice arena [START_REF] Rettig | Epidemiology of hand and wrist injuries in sports[END_REF]. This explains the great decrease in sport injuries during confinement. Despite the fact that sports injuries are mostly closed injuries (sprain or fractures) [START_REF] Bergfeld | Soft playing splint for protection of significant hand and wrist injuries in sports[END_REF][START_REF] Dehaven | Athletic injuries: comparison by age, sport, and gender[END_REF], and that the number of sport injury decreased, we did not observe any decrease on closed injury of the hand. This is explained by the fact that closed hand injuries are mainly caused by domestic accidents (falls, punching) and are only few sports injuries. [START_REF] Campbell | Hand injuries at leisure[END_REF]. When we separated the population of the public hospital into three groups (<65, 65-80 and >80 years old) there seems to be a tense to a greater decrease in general orthopedic surgery activity in the youths than in the elderly. Even though we could not statistically demonstrate this constatation, it is easily explained since the injuries of the elderly are mainly home accidents. Conversely, many sports injuries and motor vehicle accident (MVA) of young people have decreased. In our institution we did not highlight such a population difference because in hand injuries the population remains the same.

A strong point of our study is to take into account the entire duration of confinement. Even if this duration is only two months and on only two center in Annecy, it was long enough to highlight a specific trend regarding the augmentation of hand traumas compared to the decrease of the rest of the surgical and medical activity. Still we acknowledged that in order to increase the power of our conclusions, a further retrospective, multicentric study, with a larger population, more representative of the country should be performed [START_REF]Surgery in the time of Ebola: how events impacted on a single surgical institution in Sierra Leone[END_REF][START_REF] Ribacke | The Impact of the West Africa Ebola Outbreak on Obstetric Health Care in Sierra Leone[END_REF].

We agree with Wilson that traumatic emergencies are linked to weather conditions [START_REF] Wilson | The Effect of Season and Weather on Orthopaedic Trauma: Consult Volume Is Significantly Correlated with Daily Weather[END_REF].

Consequently, we chose the same period than the previous year in order to obtain similar weather conditions over weather and season. In order to avoid any selection bias, we chose a duration of inclusion long enough to have a comparable weather conditions between the two periods.

Selection bias could also appear if a different population had been referred to us over the period of inclusion. However, as our institution remains the only hand emergency center in the area, this parameter could not influence the hand injury population. Indeed, we usually receive thoroughly all hand emergencies.

Regarding the emergencies of peripheral limbs which could have been referred to our institution, as the nearby hospital kept part of its surgical activity, our population was not changed.

Conclusion

During the confinement period caused by the Covid19 pandemia, most elective medical and surgical activities sustained a severe drawdown. However, the amount of upper limb injury related surgeries declined less than the rest of the other hospital activities, and the amount of hand traumas which required surgical care increased. Despite the decrease of all medical activities more than half, the amount of upper limb and hand injuries remained stable or even increased regarding to occupational and domestic accidents.

Since hand injuries must be managed in specialized centers and do not decrease even when half the population of the world is confined in their homes, and despite a cutback in the activity of the emergency room and in elective or general trauma cases, medical care continuity should always be maintained for hand trauma, whatever the sanitary situation becomes. When healthcare providers focus on managing the Covid19 pandemia, sometimes despite their area of expertise, hand and orthopedic surgeons should keep providing an onduty organization to be able to operate on hand injuries and trauma.

Figure 1 -Figure 2

 12 Figure 1-Modifications of medical and surgical activities between 2019 and 2020

Table 1 -

 1 Modifications of medical and surgical activities between

	2019 and 2020
		2019	2020	Increase /
				decrease
	Hand or upper limb	136	123	-10%
	emergencies 1
	Hand emergencies without	112	117	+4%
	road and sport injuries
	Elective surgeries 1	2964	742	-75%
	Consultations in emergency	2742	1309	-52%
	unit 2
	Peripheral limb injury 3	245	122	-50%
	1 Orthopaedic department in private clinic
	2 Emergency department in private clinic
	3 Orthopaedic department in public hospital
	Data are presented as number of patients in absolutes values

Data are presented as mean ± standard deviation, unless otherwise stated No: number of cases/patients in absolutes values

The severity score is calculated by scoring 1 point for each of the three previous criteria in the table (patient with expected delay before resuming manual daily activity ≥3 months = 1 point)

National Institutes of Health (NIH); Welcome Trust; Howard Hughes Medical Institute (HHMI); or other(s).

Ethical approval

Since this study was a retrospective chart review, no formal approval from the IRB of our institution was required. All investigations were conducted in accordance with the 1964 Declaration of Helsinki ethical standards and the MR-003 reference methodology*; the study was registered in the National Committee for the Computer Sciences and Liberties (Commission Nationale de l'Informatique et des Libertés -CNIL) database register (number 2217606 v 0)

Conflict of interest

The authors declare that they have no conflict of interests. None of them has a financial interest in any of the products, devices, or drugs mentioned in this manuscript. They have not received or will

Conflicts of interest

No conflicts of interest and no benefits from any commercial entity related to the subject of this article.