Siham Koriche

Hiba Azzeddine

Saadia Boudekhani-Abbas

Anne-Laure Helbert

Francois Brisset

Thierry Baudin

Djamel Bradai

On some microstructural parameter variation of a Ni-W alloy processed by Groove Pressing

Keywords: Ni-W alloy, Groove pressing, Stored energy, Microstructure

The microstructure and texture of a Ni-14W (wt.%) alloy processed by groove pressing (GP) at 450°C to 4 cycles were characterized using Electron BackScatter Diffraction. In addition, the stored energy was estimated using the kernel average misorientation (KAM) approach. The microstructure undergoes a slight grain refinement with increasing GP cycles. The stored energy increases with the GP cycle number due to the increase of the dislocation density that saturates up 4 GP cycles. The texture was characterized by the domination of the Cube {001}<100> component and the remnant of some deformation texture components.

Introduction

Recently, Nickel and its alloys have increasingly attracted scientific investigations due to their potential applications as coated conductor substrates for photovoltaic purposes [START_REF] Bhattacharjee | Effect of processing variables on cube texture formation in powder metallurgically prepared Ni and Ni-W alloy tapes for use as substrates for coated conductor applications[END_REF]. In particular and for technical applications, Ni-W alloy is the most material used as highly Cube textured {001}<100> coated superconductor substrates as shown in the Rolling Assisted Biaxially Textured Substrates (RABITS TM ) technique [START_REF] Borisov | Composite Textured Substrate Tape for Second-Generation High-Temperature Superconductors with Low Magnetic Susceptibility[END_REF]. Indeed, the easy forming of Cube {001}<100> texture after heavy deformation and subsequent annealing is a great advantage for coating [START_REF] Zhao | Development of cube textured Ni-5at.% W alloy substrates for coated conductor application using a melting process[END_REF]. The domination of Cube {001}<100> texture in the substrate materials ensures that the grain boundary network consists of low angle grain boundaries (LAGB) which is absolutely necessary to achieve high critical current density [START_REF] Zhao | Development of cube textured Ni-5at.% W alloy substrates for coated conductor application using a melting process[END_REF]. The amount of Cubeoriented grains depends strongly on the prior thermo-mechanical processing. Therefore, conventional processing techniques such as rolling but also Severe Plastic Deformation (SPD) processing like Accumulative Roll Bonding (ARB) were experimented in order to investigate their effect on the texture development [43]. For example, the texture after ARB processing of Ni-14W (wt.%) alloy was characterized by the typical rolling components (Copper, S and Brass), which showed a tendency toward stabilization after 4 ARB cycles [START_REF] Boudekhani-Abbas | Microstructure, Texture and Mechanical Properties of Ni-W alloy after Accumulative Roll Bonding[END_REF]. Today, Groove Pressing (GP) technique is known as the suitable SPD technique for fabricating sheet materials having very interesting combination of mechanical and physical properties [START_REF] Gupta | Constrained groove pressing for sheet metal processing[END_REF]. In the GP process, the sheet is subjected to repetitive shear deformation under plane strain deformation conditions by alternately pressing between asymmetrically grooved and flat dies [START_REF] Gupta | Constrained groove pressing for sheet metal processing[END_REF]. Previously, the deformation microstructure, texture and mechanical properties of a Ni-14W (wt.%) alloy were investigated after GP at 450°C to 4 cycles using Electron BackScatter Diffraction (EBSD) and microhardness measurements [START_REF] Koriche | On the groove pressing of Ni-W Alloy: microstructure, texture and mechanical properties evolution[END_REF]. The results showed that the effect of GP processing on the improvement of plastic anisotropy and mechanical properties depended strongly on the initial state of the alloy before the processing [START_REF] Koriche | On the groove pressing of Ni-W Alloy: microstructure, texture and mechanical properties evolution[END_REF]. However, some important properties such as the stored energy and the evolution of Grain Boundary Character Distribution (GBCD) of Ni-14W alloy after GP processing have not been investigated, yet. Therfore, the evolutions of the microstructural parameter such as dislocation density, stored energy and type of grain boundaries of the GP processed Ni-14W alloy are investigated in the present study.

Experimental procedure

A commercial Ni-14W (wt.%) alloy was kindly provided by APERAM Alloys Imphy society, France in the form of recrystallized strips of 1 mm in thickness. Rectangular strips with dimension of 35 mm × 20 mm were pressed between asymmetrically grooved dies, with a groove angle of 45° and width of 4 mm [START_REF] Koriche | On the groove pressing of Ni-W Alloy: microstructure, texture and mechanical properties evolution[END_REF] and then subsequently between flat dies. Before each press stage, the strips were preheated at 450°C for 5 minutes. A total of 4 GP cycles was successfully obtained. More details on the sample preparation and the GP processing history were given earlier [START_REF] Koriche | On the groove pressing of Ni-W Alloy: microstructure, texture and mechanical properties evolution[END_REF]. EBSD measurements were performed in the Longitudinal Direction (LD) -Normal Direction (ND) plane of the samples using a Scanning Electron Microscope FEG-SEM SUPRA 55 VP operating at 20 kV. The EBSD data acquisition and analysis were undertaken using TSL Orientation Imaging Microscopy, OIM TM software. The scanned areas were 100×100 μm 2 with a step size of 0.1 μm. The grain size was determined using a grain tolerance angle of 5° and the minimum grain size was chosen as 2 pixels. The quantitative texture analysis was carried out by calculating the Orientation Distribution Function (ODF) using MTex software [START_REF] Bachmann | Texture Analysis with MTEX -Free and Open Source Software Toolbox[END_REF]. The dislocation density was estimated using KAM approach. Usually, KAM is used to quantify the average misorientation between a given point and its nearest neighbors where KAM reflects the dislocation density, ρ on individual measurement points [START_REF] Ateba Betanda | Measurement of stored energy in Fe-48%Ni alloys strongly cold-rolled using three approaches: Neutron diffraction, Dillamore and KAM approaches[END_REF]. Then, the stored energy was calculated from the dislocation density using the following equation [START_REF] Ateba Betanda | Measurement of stored energy in Fe-48%Ni alloys strongly cold-rolled using three approaches: Neutron diffraction, Dillamore and KAM approaches[END_REF]:

E KAM = ½μb 2 = αμb KAM /2 nd (1)
where θ KAM is the mean misorientation angle between the point and its 3 rd neighbors here (n = 3) excluding misorientation greater than 5°, α is a parameter which depends on the grain boundary type with value of 3. b = 0.24 nm is the Burgers vector, d is the EBSD step size and μ = 89.5 GPa is the shear modulus for Ni.

Results and discussion

Fig. 1 presents the evolution of the Image Quality (IQ) maps of Ni-14W alloy after 1, 2, 3 and 4 GP cycles, respectively. The grain boundaries are devised into two families: Low-Angle Grain Boundaries (LAGBs) when misorientation between the neighboring grains is 2° ≤ θ ≤ 15°, and High-Angle Grain Boundaries (HAGBs) when the misorientation angle is θ >15°. The grain boundary types are superposed with IQ maps in Fig. 1 (red, green and blue lines represent LAGBs (2°-5°), LAGBs (5°-15°) and HAGBs (> 15°), respectively). Moreover, the fraction of the different grain boundaries is summarized in Table 1. Fig. 1a and Table 1 show that after 1 cycle the sample contains about 83% of HAGBs and relatively high fraction of twins (43 %). Increasing the number of GP cycles leads to an increase of LAGBs especially those with misorientation in the range 2-5°. This increase of LAGBs and especially of very LAGBs is attributed to the accumulation of dislocations and the formation of sub-grains. Furthermore, the twin fraction decreases with increasing GP cycles to reach a fraction of 20 % after 4 GP cycles. It is well known that the twinning could occur only under high strain rate and low-temperature deformation conditions [START_REF] Liao | Deformation twinning in nanocrystalline copper at room temperature and low strain rate[END_REF]. Moreover, small grain size prevents the occurrence of twinning [START_REF] Liao | Deformation twinning in nanocrystalline copper at room temperature and low strain rate[END_REF]. It was reported earlier that the grain size of Ni-W alloy slightly decreases from 6.5 μm (initial state) to 4.5 μm after 4 GP cycles [START_REF] Koriche | On the groove pressing of Ni-W Alloy: microstructure, texture and mechanical properties evolution[END_REF]. The small decrease of the grain size is expected since the deformation processing was performed at high temperature (450 °C). Fig. 2 shows the KAM maps of the Ni-14W alloy after GP processing up to 4 cycles. The estimation of the dislocation density and the corresponding stored energy from the KAM approach are summarized in Table 2.

It is expected that the KAM value continuously increases with increasing strain but in the present processed alloy the average KAM value initially increases after 2 GP cycles and then tends to saturate until 4 GP cycles. Moreover, the KAM maps show clearly that the grains were not uniformly deformed taking into account the color coded. Indeed, it was demonstrated that the activation and accumulation of dislocations depend strongly on the orientation of the grains [START_REF] Azzeddine | On the stored energy evolution after accumulative roll-bonding of Invar alloy[END_REF]. The analysis shows that the S {231}<346> and Copper {112}<111> components develop significantly higher stored energies than the Brass {110}<112> and Cube {001}<100> components in Fe-36Ni (wt.%) alloy processed by ARB up to 6 cycles [START_REF] Azzeddine | On the stored energy evolution after accumulative roll-bonding of Invar alloy[END_REF]. Consequently, the dislocation density increases from 7.55 10 14 m -2 after 1 GP cycle to 1.51 10 14 m -2 after 2 GP cycles and then saturates up 4 GP cycles. Similar trends can be obviously noticed for the evolution of the stored energy (as shown in Table 2). 3. Furthermore, the evolution of the fraction of these texture components as function of GP cycle number is shown in Fig. 4. The initial texture, not presented here, was characterized by the domination of the Cube component and the presence of the S {231}<346> and the Copper {112}<111> components with a very weak intensity [START_REF] Koriche | On the groove pressing of Ni-W Alloy: microstructure, texture and mechanical properties evolution[END_REF]. The texture after GP processing is very similar to the initial one (Fig. 3). Nevertheless, the Cube component acuity diminishes when the GP cycle number increases. Indeed, the fraction of the Cube components decreases from about 9% for 1 GP cycle to about 5.5% for 4 GP cycles (Fig. 4). Moreover, the S component fraction decreases rapidly after 2 GP cycles and the Brass and Copper component ones remain quite stable. It is clearly seen that the fraction of Brass component is higher than the Copper one which should be an indication of the formation of a Brass-type texture rather than a Copper-type texture [START_REF] Koriche | On the groove pressing of Ni-W Alloy: microstructure, texture and mechanical properties evolution[END_REF] due to the relatively low stacking fault energy of the studied alloy. Actually, it was reported that the variation of the deformation texture during GP processing of pure Ni sheets depended strongly on the design of groove dies [START_REF] Wang | Influences of die structure on constrained groove pressing of commercially pure Ni sheets[END_REF]. A new texture component is developed in Ni sheets when using groove dies with a width of 3 mm and an angle of 45°. Contrastingly, the Ni sheet exhibits the same texture when using groove dies with a width of 2 mm and an angle of 37 or 45° [START_REF] Kumar | Mechanical behaviour and microstructural evolution of constrained groove pressed nickel sheets[END_REF]. Based on these observations, it can be speculated that the no change in the texture in the present Ni-W alloy could be attributed to the use of groove dies with a width of 4 mm and an angle of 45° [START_REF] Koriche | On the groove pressing of Ni-W Alloy: microstructure, texture and mechanical properties evolution[END_REF].

Conclusion

The microstructural parameters and the texture of the Ni-14W alloy processed by GP up 4 cycles were investigated using EBSD measurements. The following conclusions can be drawn:  The microstructure undergoes a slight grain refinement with increasing GP cycles.  The stored energy increases until 2 GP cycles due to the accumulation of the dislocation density and then a steady state appears up 4 GP cycles.  The texture is characterized by the domination of the Cube {001}<100> component and the presence of remnant typical deformation components (Brass, Copper and S).

Fig. 1 .

 1 Fig. 1. IQ maps of the Ni-14W alloy after GP to: (a) 1 GP cycle, (b) 2 GP cycles, (c) 3 GP cycles and (d) 4 GP cycles. LAGBs (2°-5°), LAGBs (5°-15°) and HAGBs (> 15°) are colored in red, green and blue, respectively)

Fig. 2 .

 2 Fig. 2. KAM maps of the Ni-14W alloy after GP to 1, 2, 3 and 4 GP cycles, respectively.

Fig. 3 .

 3 Fig. 3. presents the evolution of deformation texture in terms of recalculated {111} pole figure of the Ni-14W alloy after GP up to 4 cycles. The main ideal texture component positions of FCC alloys are also presented and their description is given in Table3. Furthermore, the evolution of the fraction of these texture components as function of GP cycle number is shown in Fig.4.

Fig. 3 .

 3 Fig. 3. Recalculated {111} pole figure of the Ni-14W alloy after GP to: (a) 1 GP cycle, (b) 2 GP cycles, (c) 3 GP cycles and (d) 4 GP cycles.

Fig. 4 .

 4 Fig. 4. Evolution of the texture component fraction as function of the GP cycle number for Ni-14W alloy.

Table 1 .

 1 Fraction of LAGBs and HAGBs in the microstructures of the processed Ni-14W alloy.

	Number of	2-5°	5-15°	15-65°
	GP cycles
	1	14.0	2.9	83.1
	2	34.3	5.2	60.5
	3	30.8	3.7	65.5
	4	41.8	5.1	53.2

Table 2 .

 2 Average KAM value, dislocation density, and stored energy in the Ni-14W after GP up 4 cycles.

	Number of	KAM(°)	 (× 10 14 m -2 )	EKAM (J/mol)
	GP cycles
	1	1.04	07.5	13.6
	2	1.51	10.9	19.8
	3	1.53	11.1	20.0

Table 3 .

 3 Main ideal texture components of FCC alloys.

	Component	{hkl}<uvw>		Euler Angles
			φ1		φ2
	Brass	{110}<112>	35°	45°	0°
	Cube	{001}<100>	0°	0°	0°
	Copper	{112}<111>	90°	35°	45°
	S	{231}<346>	59°	37°	63°

1.54 11.2 20.2

Acknowledgments. The authors are deeply grateful to Yanick ATEBA-BETANDA and Thierry WAECKERLE from APERAM Alloys Imphy Society, France, for kindly providing the Ni-14W alloy.


MD PhD Alexandre Demoule

MD Laurent Brochard

MD Martin Dres

MD Leo Heunks

MD Amal Jubran

MD Franco Laghi

Armand Mekontso-Dessap

MD Stefano Nava

MD Lamia Ouanes-Besbes

Oscar Peñuelas

MD Armand Mekontso- Dessap

MD Lise Piquillioud

MD, PhD Theodoros Vassilakopoulos

MD Jordi Mancebo

HOW TO VENTILATE OBSTRUCTIVE AND ASTHMATIC PATIENTS

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

Introduction

Respiratory failure from acute exacerbations of chronic obstructive pulmonary disease (COPD) and severe asthma are characterized by acute worsening of respiratory symptoms associated with a development of severe airflow limitation, gas-trapping, dynamic hyperinflation and intrinsic positive end-expiratory pressure (PEEPi). In the most severe cases, these exacerbations may cause acute respiratory failure, which may require mechanical ventilation This review focuses on strategies for ventilation and provides the physiological background that underlies these strategies. Even though the pathogenesis and clinical course of asthma and COPD differ, the ventilator support management of both conditions is similar in various respects.

Acute respiratory failure in COPD and asthma: Magnitude of the problem COPD exacerbations are common and have important clinical consequences, including acute decline in quality of life, temporary or permanent reduction in lung function and exercise capacity, hospitalization, and increased mortality. They have also a major economic impact. According to cohorts studies that enrolled unselected critically ill patients receiving mechanical ventilation (either invasive or non-invasive) for more than twelve hours [START_REF] Peñuelas | Inter-country variability over time in the mortality of mechanically ventilated patients[END_REF], the proportion of patients managed for COPD exacerbation decreased from 10% in 1998 to 7% in 2016 (Figure 1). This trend paralleled an increased rate of non-invasive ventilation (NIV) use as first ventilatory support in the intensive care unit (ICU) admission (from 16% in 1998 to 51 % in 2017). Simultaneously, overall mortality decreased (Figure 1).

Severe asthma exacerbation causing respiratory failure imposes significant morbidity and mortality related mechanical ventilation-associated complications (eg, barotrauma, cardiovascular collapse, atelectasis, and pneumonia). Severe asthma exacerbation represents approximately 1% of mechanically ventilated patients admitted in the ICU [START_REF] Peñuelas | Inter-country variability over time in the mortality of mechanically ventilated patients[END_REF]. NIV use increased in these patients from 3% in 1998 to 34% in 2016 [START_REF] Pendergraft | Rates and characteristics of intensive care unit admissions and intubations among asthma-related hospitalizations[END_REF].

Respiratory system mechanics and gas exchange

In term of respiratory system mechanics, asthma and COPD are characterized by the development of dynamic hyperinflation, defined as the increase in the relaxation volume of the respiratory system at the end of a tidal expiration. In healthy subjects the end-expiratory, alveolar and airway pressures are zero relative to the atmosphere, and pleural pressure is negative. In the presence of dynamic hyperinflation, the alveolar pressure remains positive throughout expiration, leading to development of auto-PEEP, also termed or intrinsic PEEP or PEEPi [START_REF] Vassilakopoulos | What's new about pulmonary hyperinflation in mechanically ventilated critical patients[END_REF] (Figure 2). The etiology of PEEPi is multifactorial including increased expiratory flow limitation due to reduced lung recoil pressure (emphysema) leading to small airway collapse that increases airway resistance, shortened expiratory time due to increased respiratory rate, and increased tidal volume due an augmentation or respiratory drive (and therefore a higher volume to exhale) [START_REF] Vassilakopoulos | Understanding wasted/ineffective efforts in mechanically ventilated COPD patients using the Campbell diagram[END_REF].

The consequences of dynamic hyperinflation depend on whether patients are passively ventilated or triggering their ventilator. In passively ventilated patients dynamic hyperinflation increases delivered mechanical power [START_REF] Marini | Dynamic predictors of VILI risk: beyond the driving pressure[END_REF] with its associated risk for barotrauma, and hemodynamic compromise [START_REF] Marini | Dynamic hyperinflation and auto-positive end-expiratory pressure: lessons learned over 30 years[END_REF]. In patients triggering the ventilator, initiation of inspiratory flow requires inspiratory force to overcome PEEPi [START_REF] Smith | Impact of PEEP on lung mechanics and work of breathing in severe airflow obstruction[END_REF], which increases the inspiratory effort during the triggering phase. Ultimately, this increased effort may fail to trigger de ventilator, leading to ineffective triggering, one of the most frequent dysynchrony [START_REF] Vassilakopoulos | Understanding wasted/ineffective efforts in mechanically ventilated COPD patients using the Campbell diagram[END_REF].

Patients with COPD have complex patterns of V/Q distributions: low V/Q regions that remain perfused, high V/Q regions and mixed patterns. COPD patients often exhibit small amounts of shunt (typically less than 10% of cardiac output) [START_REF] Rodriguez-Roisin | Effect of mechanical ventilation on gas exchange[END_REF].

Severe asthma exacerbation is characterized by markedly increased airway resistance due to bronchospasm and airway inflammation. Expiratory flow is dramatically reduced with resultant dynamic hyperinflation [START_REF] Oddo | Management of mechanical ventilation in acute severe asthma: practical aspects[END_REF]. This leads to increased alveolar dead space and hypercapnia but also increased risk for barotrauma and hemodynamic compromise. Hypoxemia in asthma is characterized by the presence of low V/Q units; hypoxemia is usually attenuated by compensatory redistribution of blood flow mediated by hypoxic vasoconstriction and changes cardiac output [START_REF] Rodriguez-Roisin | Effect of mechanical ventilation on gas exchange[END_REF][START_REF] Young | Gas exchange in disease: asthma, chronic obstructive pulmonary disease, cystic fibrosis, and interstitial lung disease[END_REF]. It has been described that in asthma patients, hypercapnia is mainly due to increased dead space ventilation caused by alveolar overdistension [START_REF] Young | Gas exchange in disease: asthma, chronic obstructive pulmonary disease, cystic fibrosis, and interstitial lung disease[END_REF][START_REF] Leatherman | Mechanical Ventilation for Severe Asthma[END_REF]. Nevertheless this mechanism has not been proved [START_REF] Rodriguez-Roisin | Effect of mechanical ventilation on gas exchange[END_REF].

Heart lung interactions in the mechanically ventilated COPD patient

The pathophysiological changes in the pulmonary system may have adverse effects on cardiac function. COPD is associated with pulmonary hypertension, increased pulmonary vascular resistance, right ventricle dilatation and right ventricle hypertrophy.

Both left ventricle systolic and diastolic functions are often impaired in COPD patients.

Among 148 patients admitted to the ICU for severe COPD exacerbation, 31% had an exacerbation that was definitely associated with left-heart dysfunction [START_REF] Abroug | Association of Left-Heart Dysfunction with Severe Exacerbation of Chronic Obstructive Pulmonary Disease[END_REF]. These cardiac alterations are caused by dynamic hyperinflation and the large swings in negative intrathoracic pressure developed by the respiratory muscles to overcome the inspiratory elastic threshold caused by PEEPi and the increased airway resistance.

Dynamic hyperinflation is more detrimental to left ventricle hemodynamics than large swings in negative intrathoracic pressure [START_REF] Cheyne | Heart-lung interaction in a model of COPD: importance of lung volume and direct ventricular interaction[END_REF]. Direct ventricular interaction and significant septal flattening appear to be responsible for the reduced left ventricle enddiastolic volume and stroke volume [START_REF] Cheyne | Hemodynamic effects of incremental lung hyperinflation[END_REF]. Dynamic hyperinflation worsens the increase in right ventricular impedance (afterload effect), while large negative intrathoracic pressure swings increase the venous return to the right ventricle (preload effect). Both favor direct ventricular interaction with leftward shift of the septum.

Application of an external PEEP up to values approaching PEEPi does not result in hemodynamic impairment in COPD [START_REF] Vassilakopoulos | Understanding wasted/ineffective efforts in mechanically ventilated COPD patients using the Campbell diagram[END_REF]. Higher PEEP levels reduce cardiac index [START_REF] Dambrosio | Effects of positive endexpiratory pressure on right ventricular function in COPD patients during acute ventilatory failure[END_REF]. However, the effects of external PEEP on lung mechanics and hemodynamics depend on many factors, such as airways characteristics, lung volumes, intravascular volume status, vasomotor tone, etc., making the individual patient's response difficult to predict [START_REF] Georgopoulos | Effects of extrinsic positive endexpiratory pressure on mechanically ventilated patients with chronic obstructive pulmonary disease and dynamic hyperinflation[END_REF].

Finally, patients with COPD are at increased risk for difficult weaning, and are susceptible to develop weaning induced pulmonary edema in particular [START_REF] Liu | Cardiac dysfunction induced by weaning from mechanical ventilation: incidence, risk factors, and effects of fluid removal[END_REF]. Diuretics and nitroglycerin are efficient in treating weaning-induced pulmonary edema in selected COPD patients [START_REF] Lemaire | Acute left ventricular dysfunction during unsuccessful weaning from mechanical ventilation[END_REF][START_REF] Routsi | Nitroglycerin can facilitate weaning of difficult-to-wean chronic obstructive pulmonary disease patients: a prospective interventional non-randomized study[END_REF]. Similar heart lung interactions are observed in patients with severe asthma exacerbation because of the severity of hyperinflation and they may develop severe hypotension [START_REF] Corbridge | The assessment and management of adults with status asthmaticus[END_REF].

Noninvasive ventilation in COPD 20 years after

Delivering mechanical ventilation without intubation in patients with CO2 retention had been attempted during the 1960s [START_REF] Sadoul | Traitement par ventilation instrumentale de 100 cas d'insuffisance respiratoire aiguë sévère (PaCO2 égale ou supérieure à 70 mmHg) chez des pulmonaires chroniques[END_REF] but without a great acceptance, intubation with invasive mechanical ventilation was the rule for patients admitted for respiratory failure.

In the late 1980s several groups treated patients with chronic or acute on chronic hypercapnic respiratory failure with a face mask [START_REF] Meduri | Noninvasive face mask ventilation in patients with acute respiratory failure[END_REF][START_REF] Bach | Intermittent positive pressure ventilation via the mouth as an alternative to tracheostomy for 257 ventilator users[END_REF][START_REF] Leger | Acute respiratory failure in COPD patient treated with non invasive intermittent mechanical ventilation (control mode) with nasal mask[END_REF][START_REF] Brochard | Reversal of acute exacerbations of chronic obstructive lung disease by inspiratory assistance with a face mask[END_REF]. The success largely resulted from combining physiological assessment of the mechanisms of respiratory failure (including respiratory muscle function [START_REF] Brochard | Reversal of acute exacerbations of chronic obstructive lung disease by inspiratory assistance with a face mask[END_REF]), with new technologies (pressure support ventilation [START_REF] Brochard | Inspiratory pressure support prevents diaphragmatic fatigue during weaning from mechanical ventilation[END_REF]). In the early 1990s several studies demonstrated the efficacy of positive pressure ventilation usually delivered with pressure support ventilation and PEEP [START_REF] Brochard | Noninvasive ventilation for acute exacerbations of chronic obstructive pulmonary disease[END_REF][START_REF] Bott | Randomised controlled trial of nasal ventilation in acute ventilatory failure due to chronic obstructive airways disease[END_REF]. It is remarkable that some of the best results were obtained without any PEEP [START_REF] Brochard | Reversal of acute exacerbations of chronic obstructive lung disease by inspiratory assistance with a face mask[END_REF],

highlighting the importance of pressure delivered during inspiration. Randomized clinical trials showed that the intubation rate was dramatically reduced, resulting in improved outcomes, with less complications related to invasive mechanical ventilation and improved hospital survival [START_REF] Demoule | Benefits and risks of success or failure of noninvasive ventilation[END_REF][START_REF] Girou | Secular trends in nosocomial infections and mortality associated with noninvasive ventilation in patients with exacerbation of COPD and pulmonary edema[END_REF][START_REF] Girou | Association of noninvasive ventilation with nosocomial infections and survival in critically ill patients[END_REF] (Figure 3).

Implementing NIV into practice took more than a decade [START_REF] Chandra | Outcomes of noninvasive ventilation for acute exacerbations of chronic obstructive pulmonary disease in the United -25 -States, 1998-2008[END_REF] but NIV became the reference for treating acute respiratory failure due to severe COPD exacerbation, saving thousands of lives [START_REF] Chandra | Outcomes of noninvasive ventilation for acute exacerbations of chronic obstructive pulmonary disease in the United -25 -States, 1998-2008[END_REF]. Technological improvements continued, stimulated by the need for efficient techniques in the hospital and the extensive use of home NIV, which required more comfortable and user-friendly equipment [START_REF] Murphy | Effect of Home Noninvasive Ventilation With Oxygen Therapy vs Oxygen Therapy Alone on Hospital Readmission or Death After an Acute COPD Exacerbation: A Randomized Clinical Trial[END_REF]. Automated management of leaks became progressively the rule and ICU ventilators finally became as efficient as dedicated ventilators to compensate for leaks and reduce patient-ventilator dyssynchronies [START_REF] Carteaux | Patient-ventilator asynchrony during noninvasive ventilation: a bench and clinical study[END_REF][START_REF] Doorduin | Automated patient-ventilator interaction analysis during neurally adjusted non-invasive ventilation and pressure support ventilation in chronic obstructive pulmonary disease[END_REF].

NIV is sometimes proposed as a ceiling of therapy for ventilator assistance [START_REF] Nava | Non-invasive ventilation in elderly patients with acute hypercapnic respiratory failure: a randomised controlled trial[END_REF][START_REF] Demoule | Changing use of noninvasive ventilation in critically ill patients: trends over 15 years in francophone countries[END_REF][START_REF] Azoulay | Noninvasive mechanical ventilation in patients having declined tracheal intubation[END_REF]. In patients with "do not intubate-orders" who were managed with NIV, a multicenter French study showed patients had good quality of life six-month after discharge; caregivers of patients treated with NIV had similar level of stress and anxiety compared with caregivers of patients with no limitation in therapy [START_REF] Azoulay | Noninvasive mechanical ventilation in patients having declined tracheal intubation[END_REF]. Because NIV relieves dyspnea, the technique has been also used to relieve dyspnea in dying patients receiving palliative care [START_REF] Nava | Palliative use of non-invasive ventilation in end-of-life patients with solid tumours: a randomised feasibility trial[END_REF], although this approach has not gained a widespread use.

In severe asthma exacerbation, NIV is not recommended [START_REF] Conte | Management of severe asthma exacerbation: guidelines from the Société Française de Médecine d'Urgence, the Société de Réanimation de Langue Française and the French Group for Pediatric Intensive Care and Emergencies[END_REF]. However, a recent study has shown that NIV in severe asthma exacerbation was associated with improved outcome, although it has to be used cautiously [START_REF] Althoff | Noninvasive Ventilation Use in Critically Ill Patients with Acute Asthma Exacerbations[END_REF].

Management of invasive ventilation

Invasive ventilation is indicated in patients who, for instance, have a respiratory arrest or failed NIV for any reason including persistent clinical signs of increased work of breathing. As previously detailed (see above "Respiratory mechanics and gas exchange") acute exacerbations of COPD are characterized by dynamic hyperinflation leading to development of PEEPi. Dynamic hyperinflation and PEEPi should be considered if expiratory flow does not cease at end-expiration (Figure 4). With controlled mechanical ventilation, total PEEP is measured during end expiratory occlusion. The reference standard technique to quantify dynamic hyperinflation is measurement of volume at end-inspiration [START_REF] Tuxen | The effects of ventilatory pattern on hyperinflation, airway pressures, and circulation in mechanical ventilation of patients with severe airflow obstruction[END_REF]. As this is cumbersome in clinical practice, end-inspiratory plateau pressure (Pplat) during controlled mechanical ventilation is a reasonable, yet less sensitive surrogate to monitor hyperinflation [START_REF] Tuxen | The effects of ventilatory pattern on hyperinflation, airway pressures, and circulation in mechanical ventilation of patients with severe airflow obstruction[END_REF]. Pplat, is measured with end-inspiratory occlusion for ± 3 sec. Peak pressure is not a reliable measure for hyperinflation.

It is important to stress that in the early phase of mechanical ventilation, the goal in these patients is not to normalize blood gases but prevent complications due to hyperinflation while keeping a pH around 7.25-7.30.

Many ventilator modes are used in intubated patients with COPD, however, it is unknown whether one is superior to another. A common mode is volume assist-control ventilation. With volume assist-control ventilation, inspiratory flow waveform can be set in the square pattern to facilitate monitoring of mechanics. To limit hyperinflation, minute ventilation is minimized, and sufficient time is allowed for expiration. As a reasonable starting point, moderate tidal volume, around 6 to 8 ml/Kg and respiratory rate at 12 /minute, with constant inspiratory flow delivered at 60-90 L/min has been proposed [START_REF] Mancebo | Assist-Control Ventilation[END_REF].

The inspiration to expiration ratio is kept low, e.g. 1:4. If with these ventilator settings Plat is not too high (e.g. <28cmH2O), respiratory rate can be increased to improve gas exchange. In case Pplat is high (e.g. >28cmH2O), minute ventilation should be reduced by limiting tidal volume and/or respiratory rate in patients with PEEPi. Increasing expiration time at similar minute ventilation (e.g. by increasing inspiratory airflow thus decreasing inspiratory time with constant respiratory rate and tidal volume) has much smaller effect on hyperinflation [START_REF] Tuxen | The effects of ventilatory pattern on hyperinflation, airway pressures, and circulation in mechanical ventilation of patients with severe airflow obstruction[END_REF].

Selecting appropriate PEEP in acute COPD exacerbation may be complex and dedends on whether the patient triggers or not his ventilator. In general, at the early phase of intubation, patients do not trigger their ventilator In theory, zero PEEP would be optimal in these COPD patients with "pure" high airway resistance, as PEEP reduces expiratory driving pressure and therefore is expected to increase hyperinflation.

However, physiology appears more complex. Indeed, in a study of eight intubated patients (four with asthma and four with COPD), Caramez et al reported and three responses to PEEP on hyperinflation [START_REF] Caramez | Paradoxical responses to positive end-expiratory pressure in patients with airway obstruction during controlled ventilation[END_REF]: 1) no change in hyperinflation (assessed by Pplat and by changes in end expiratory lung volume measured with inductive plethysmography) during progressive increase in PEEP until a threshold; 2) any increase in PEEP increases Pplat and end expiratory lung volume and 3) a "paradoxical response" (observed in three patients with asthma and in two patients with COPD) where increases in PEEP decreases Plat and end expiratory lung volume. A paradoxical response may be expected in patients with expiratory flow limitation with highly heterogeneous lungs [START_REF] Marini | Dynamic hyperinflation and auto-positive end-expiratory pressure: lessons learned over 30 years[END_REF][START_REF] Leatherman | Effect of prolongation of expiratory time on dynamic hyperinflation in mechanically ventilated patients with severe asthma[END_REF]. Indeed, expiratory flow limitation generating PEEPi is a complex phenomenon, whose mechanisms, diagnosis, physiologic and clinical consequences and therapeutic approaches have been extensively described elsewhere [START_REF] Junhasavasdikul | Expiratory Flow Limitation During Mechanical Ventilation[END_REF]. At bedside, the effect of PEEP on hyperinflation is unpredictable [START_REF] Caramez | Paradoxical responses to positive end-expiratory pressure in patients with airway obstruction during controlled ventilation[END_REF] and it is therefore advised to measure Pplat while titrating PEEP. If a trial of PEEP may be cautiously attempted, while measuring frequently Pplat. It should be immediately stopped if Pplat increases [START_REF] Leatherman | Mechanical Ventilation for Severe Asthma[END_REF].

As soon as patient is able to trigger the ventilator, a moderate external PEEP is added to counterbalance PEEPi and hence to reduce effort to trigger the ventilator and improve patient-ventilator interaction [START_REF] Laghi | Mechanical ventilation in chronic obstructive pulmonary disease[END_REF]. It is of notice that patients with COPD are susceptible to ventilator-induced hyperinflation and dysynchronies such as ineffective triggering (also called ineffective efforts or wasted efforts, Figure 5) [START_REF] Laghi | Mechanical ventilation in chronic obstructive pulmonary disease[END_REF]. Because PEEPi increases the effort required to trigger the ventilator, weak respiratory effort may fail triggering the ventilator [START_REF] Parthasarathy | Cycling of inspiratory and expiratory muscle groups with the ventilator in airflow limitation[END_REF]. Ineffective triggering is associated with a less sensitive inspiratory trigger, higher level of pressure support higher tidal volume, and higher pH [START_REF] Laghi | Mechanical ventilation in chronic obstructive pulmonary disease[END_REF]. In patients with a high prevalence of ineffective triggering, markedly reducing pressure support or inspiratory duration to reach a tidal volume of about 6 ml/kg predicted body weight eliminated ineffective triggering in two-thirds of patients [START_REF] Thille | Reduction of patient-ventilator asynchrony by reducing tidal volume during pressure-support ventilation[END_REF]. When pressure support is used, the pressure support level should not be set too high in order to limit tidal volume (around 6 to 8 ml/Kg) and subsequent dynamic hyperinflation [START_REF] Thille | Reduction of patient-ventilator asynchrony by reducing tidal volume during pressure-support ventilation[END_REF].

Shortening insufflation time by decreasing the level of the expiratory trigger (also called cycling-off) may also help reducing dynamic hyperinflation [START_REF] Thille | Reduction of patient-ventilator asynchrony by reducing tidal volume during pressure-support ventilation[END_REF].

In severe asthma exacerbation mechanical ventilation is associated with increased risk of complications and significant mortality [START_REF] Brenner | Intubation and mechanical ventilation of the asthmatic patient in respiratory failure[END_REF]. Therefore, indication for intubation should be limited to patients in life-threatening conditions (respiratory arrest, bradypnea, altered consciousness, patients totally exhausted and/or with severe and worsening hypercapnia or major respiratory distress despite adequate medical treatment). In a mechanically ventilated patient with severe asthma exacerbation, the aim is to limit -14 -hyperinflation, not normalize PaCO2, which might increase dramatically due to the major reduction of minute ventilation [START_REF] Darioli | Mechanical controlled hypoventilation in status asthmaticus[END_REF]. Because of the major increase in expiratory resistance due to airway obstruction related to edema and bronchospasm) pulmonary hyperinflation might be extremely in asthmatics. Key ventilatory strategy is to minimize hyperinflation, which is best achieved by reducing minute ventilation (low tidal volume, low respiratory rate [START_REF] Tuxen | The effects of ventilatory pattern on hyperinflation, airway pressures, and circulation in mechanical ventilation of patients with severe airflow obstruction[END_REF] and high inspiratory airflow rate with the objective of targeting an inspiratory-to-expiratory time of 1:4 to 1:6) lengthens expiratory time and will reduce hyperinflation. Low level of external PEEP (≤ 5 cmH2O) is recommended by some authors [START_REF] Leatherman | Mechanical ventilation for severe asthma[END_REF]. The degree of hyperinflation must be closely monitored (using endinspiratory and end-expiratory holds) with the aim of having plateau pressure < 28-30 cmH2O and the lowest possible total PEEP [START_REF] Brenner | Intubation and mechanical ventilation of the asthmatic patient in respiratory failure[END_REF]. Using a volumetric mode with square wave airflow delivery allows easier monitoring.

Weaning from mechanical ventilation

Readiness to wean needs to be screened on a daily basis (Figure 2) [START_REF] Boles | Weaning from mechanical ventilation[END_REF]. Such screening involves among aothers the respiratory frequency to tidal volume ratio (f/VT) [START_REF] Yang | A prospective study of indexes predicting the outcome of trials of weaning from mechanical ventilation[END_REF][START_REF] Tobin | Variable performance of weaning-predictor tests: role of Bayes' theorem and spectrum and test-referral bias[END_REF]. In ready-to-wean patients, a spontaneous breathing trial is performed, ether with T-tube or pressure support ventilation [START_REF] Esteban | A comparison of four methods of weaning patients from mechanical ventilation. Spanish Lung Failure Collaborative Group[END_REF]. In patients who tolerate the spontaneous breathing trial, then one proceeds to extubation.

In COPD patients, prophylactic post-extubation NIV and high flow nasal canula both decrease acute respiratory failure and subsequent reintubation (Figure 2) [START_REF] Ferrer | Non-invasive ventilation after extubation in hypercapnic patients with chronic respiratory disorders: randomised controlled trial[END_REF][START_REF] Hernández | Effect of Postextubation High-Flow Nasal Cannula vs Noninvasive Ventilation on Reintubation and Postextubation Respiratory Failure in High-Risk Patients: A Randomized Clinical Trial[END_REF].

The addition of NIV session to high flow nasal cannula seems to be more efficient to prevent reintubation than high flow nasal cannula alone [START_REF] Thille | Effect of Postextubation High-Flow Nasal Oxygen With Noninvasive Ventilation vs High-Flow Nasal Oxygen Alone on Reintubation Among Patients at High Risk of Extubation Failure: A Randomized Clinical Trial[END_REF]. Finally, NIV can also hasten weaning in COPD patients who repeatedly fail spontaneous breathing trials [START_REF] Burns | Noninvasive positivepressure ventilation as a weaning strategy for intubated adults with respiratory failure[END_REF].

-15 -In a series of 208 patients intubated for hypercapnic respiratory failure 4.4% required ventilation via tracheostomy at discharge [START_REF] Girault | Noninvasive ventilation and weaning in patients with chronic hypercapnic respiratory failure: a randomized multicenter trial[END_REF]. The decision for tracheostomy includes considerations about risks of the procedure versus anticipated but unproven benefits [START_REF] Esperanza | What's new in intensive care: tracheostomy-what is known and what remains to be determined[END_REF]. In the difficult-to-wean patient, tracheostomy should be the subject of a multidisciplinary discussion [START_REF] Trouillet | Tracheotomy in the intensive care -29 -unit: guidelines from a French expert panel[END_REF]. The patient and his or her family must be informed that tracheotomy does not alter the prognosis of the causal disease. Although tracheotomy can improve comfort [START_REF] Blot | Early tracheotomy versus prolonged endotracheal intubation in unselected severely ill ICU patients[END_REF], it may unduly prolong suffering associated with the underlying illness. In a context of chronic respiratory failure, these ethical considerations must be carefully thought through and discussed with the patient and his or her family before performing a tracheotomy.

Long-term outcome

In a small series of patients with COPD requiring prolonged mechanical ventilation (> 21 days), two-year survival was 40% (68% for patients who weaned from the ventilator and 22% for those who did not wean) [START_REF] Nava | Survival and prediction of successful ventilator weaning in COPD patients requiring mechanical ventilation for more than 21 days[END_REF]. In a cohort of patients (59% with COPD) requiring prolonged ventilation at a weaning center, one-year survival was 49% [START_REF] Schönhofer | Survival of mechanically ventilated patients admitted to a specialised weaning centre[END_REF].

A prospective longitudinal study investigated the effect of prolonged mechanical ventilation on survival and quality of life in 315 patients with various causes of respiratory failure (95 had COPD as primary or secondary cause of respiratory failure) [START_REF] Jubran | Effect of pressure support vs unassisted breathing through a tracheostomy collar on weaning duration in patients requiring prolonged mechanical ventilation: a randomized trial[END_REF][START_REF] Jubran | Long-Term Outcome after Prolonged Mechanical Ventilation. A Long-Term Acute-Care Hospital Study[END_REF]. Among them, 54% were detached from the ventilator and 30% were still attached to the ventilator at discharge from weaning facility, 53% patients died at six months and 57% died at twelve months. The one-year survival was 63% for ventilatordetached patients and 22% for the ventilator-attached patients. Survival was not influenced by the underlying cause of respiratory failure, including COPD [START_REF] Jubran | Long-Term Outcome after Prolonged Mechanical Ventilation. A Long-Term Acute-Care Hospital Study[END_REF]. At six months, functional status improved; improvement in functional status was related to improvement in handgrip strength. By twelve months, physical-summary score and mental-summary score of SF36 returned to pre-illness values and 85% of patients indicated willingness to undergo ventilation again [START_REF] Jubran | Long-Term Outcome after Prolonged Mechanical Ventilation. A Long-Term Acute-Care Hospital Study[END_REF][START_REF] Nava | Patient-Clinician Alliance during Prolonged Mechanical Ventilation. "Never Give Up on a Dream[END_REF].

Are there particular features in middle-income countries?

The burden of COPD and asthma is disproportionally high in low-resource countries due to high indoor/outdoor air pollution (smoking, exposure to coal indoor and to dust in the workplace) [START_REF] Brakema | The socioeconomic burden of chronic lung disease in low-resource settings across the globe -an observational FRESH AIR study[END_REF][START_REF] Menezes | Worldwide burden of -30 -COPD in high-and low-income countries. Part II. Burden of chronic obstructive lung disease in Latin America: the PLATINO study[END_REF]. The death toll of chronic respiratory diseases imposes a real challenge to the public health systems in developing countries, since the highest risk of dying from non-communicable disease is observed in low-middle income countries [START_REF]NCD Countdown 2030: worldwide trends in non-communicable disease mortality and progress towards Sustainable Development Goal target 3.4[END_REF].

In addition, ICUs are sparse, and resources are constrained in most low-to-middle income countries. In particular, the availability of invasive mechanical ventilation is limited, and its use is associated with a high risk of mortality especially from ventilator associated pneumonia [START_REF] Mandelzweig | Non-invasive ventilation in children and adults in low-and low-middle income countries: A systematic review and meta-analysis[END_REF][START_REF] Rosenthal | International Nosocomial Infection Control Consortium (INICC) report, data summary of 36 countries, for 2004-2009[END_REF][START_REF] Markwart | Epidemiology and burden of sepsis acquired in hospitals and intensive care units: a systematic review and meta-analysis[END_REF]. Such as in high-income countries, NIV should be preferred to invasive ventilation, in particular in COPD exacerbation. A recent metaanalysis summarizing the experience with NIV in these countries reported a moderate risk of mortality in adults (16%), and rates of NIV failure of 28% [START_REF] Mandelzweig | Non-invasive ventilation in children and adults in low-and low-middle income countries: A systematic review and meta-analysis[END_REF]. For COPD exacerbation, the use of NIV as the primary ventilatory mode increased from 29% in 2000 to 97% in 2012 % [76] [76]. This change was associated with a gradual fall in the rate of NIV failure (learning curve), of ventilator associated pneumonia, and in the concurrent use of antibiotics [START_REF] Ouanes | Trends in use and impact on outcome of empiric antibiotic therapy and non-invasive ventilation in COPD patients with acute exacerbation[END_REF]. These data suggest that guidelines regarding the preferential use of NIV therapy are not specific to high income countries and should be apply to low and middle-income countries.

New avenues of research

NIV has well known drawbacks. Indeed, skill of the caregivers is important to the success of this technique. In addition, patient tolerance may be poor due to patient discomfort, dyspnea, skin damage, and claustrophobia [START_REF] Dangers | Dyspnoea in patients receiving noninvasive ventilation for acute respiratory failure: prevalence, risk factors and prognostic impact: A prospective observational study[END_REF]. High-flow nasal cannula may be an alternative method [START_REF] Seo | High-flow nasal cannula oxygen therapy for acute hypoxemic respiratory failure in patients with chronic lung disease in terms of -31 -hospital outcomes[END_REF]. In a recent study conducted in 12 hypercapnic COPD patients with mild to moderate exacerbation who had initially required NIV, applying high-flow nasal cannula at 30 L/min for a short duration reduces inspiratory effort, and resulted in similar effect than NIV delivered at modest levels of pressure support.

Given the fact that approximately 15% of COPD patients fail NIV, attempts to improve the efficacy of NIV have been tried. Among those attempts is the inhalation of gas mixtures made up of helium and oxygen, which requires a complex setting and a specific ventilator. Because of its low density compared with air, helium/oxygen markedly enhanced the ability of NIV to reduce patients effort and to improve gas exchange [START_REF] Jaber | Noninvasive ventilation with heliumoxygen in acute exacerbations of chronic obstructive pulmonary disease[END_REF]. However, despite some improvement of several physiologic variables, RCTs did not show a clinical benefit (i.e. reduction in intubation rate or mortality) [START_REF] Jolliet | A Multicenter Randomized Trial Assessing the Efficacy of Helium/Oxygen in Severe Exacerbations of Chronic Obstructive Pulmonary Disease[END_REF][START_REF] Maggiore | A multicenter, randomized trial of noninvasive ventilation with helium-oxygen mixture in exacerbations of chronic obstructive lung disease[END_REF] with the use of helium/oxygen mixture. The relatively low rate of intubation already achieved with NIV alone may account for the lack of benefit with helium/oxygen mixture.

More recently, the use of extracorporeal CO2 removal has been considered as a possible adjunct to NIV to avoid intubation in patients not responding to NIV [START_REF] Sorbo | Extracorporeal Co2 removal in hypercapnic patients at risk of noninvasive ventilation failure: a matched cohort study with historical control[END_REF].

Combining NIV with direct removal of CO2 is postulated to improve alveolar ventilation and relieve the workload of respiratory muscle. Extracorporeal CO2 removal can also be used to accelerate weaning from endotracheal intubation [START_REF] Pisani | Effects of Extracorporeal CO2 Removal on Inspiratory Effort and Respiratory Pattern in Patients Who Fail Weaning from Mechanical Ventilation[END_REF]; it may prevent ineffective shallow-breathing pattern and reduce inspiratory work by maintaining stable PaCO2 levels during unsupported breathing. Although devices eliminate carbon dioxide efficiently, experimental evidence of their effectiveness in patients with COPD is limited.

-18 -Demonstrating benefits in COPD will be challenging because of complications associated with extracorporeal CO2 removal [START_REF] Sklar | Extracorporeal carbon dioxide removal in patients with chronic obstructive pulmonary disease: a systematic review[END_REF].

Another strategy to improve the prognosis of COPD patients is to optimize patient ventilator interaction. Unlike the traditional pneumatic modes, neurally adjusted ventilator assist (NAVA) is a mode that triggers, cycles, and regulates inspiratory airflow based on the diaphragmatic electromyography signal. The level of ventilatory assistance is proportional to the patient's breathing effort. The level of ventilatory assistance is proportional to the patient's breathing effort, there is no influence of PEEPi during the ventilator assistance since it starts with the patient's breathing effort, and there should not be any influence from leaks during NIV due to better patient-ventilator interaction [START_REF] Sinderby | Neural control of mechanical ventilation in respiratory failure[END_REF][START_REF] Sinderby | Neurally adjusted ventilatory assist: first indications of clinical outcomes[END_REF]. Several studies have compared pressure support ventilation to NAVA, with the majority of studies showing that NAVA improves patient-ventilator interaction, diaphragm efficiency and patient comfort [START_REF] Demoule | Neurally adjusted ventilatory assist as an alternative to pressure support ventilation in adults: a French multicentre randomized trial[END_REF]. However, no clear clinical advantage of NAVA has been demonstrated between these modes although NAVA might be beneficial in difficult weaning [START_REF] Demoule | Neurally adjusted ventilatory assist as an alternative to pressure support ventilation in adults: a French multicentre randomized trial[END_REF][START_REF] Liu | Neurally Adjusted Ventilatory Assist versus Pressure Support Ventilation in Difficult Weaning: A Randomized Trial[END_REF].

A striking feature of patients treated with NIV or invasive ventilation is the high rate of ICU or hospital readmissions [START_REF] Adler | Comorbidities and Readmissions in Survivors of Acute Hypercapnic Respiratory Failure[END_REF]. At least 50% of patients surviving an ICU stay will be readmitted within a year and this percentage can reach 80% in some studies. Two factors may explain this high rate. First patients may continue to need ventilation at home [START_REF] Macrea | Long-Term Noninvasive Ventilation in Chronic Stable Hypercapnic Chronic Obstructive Pulmonary Disease. An Official American Thoracic Society Clinical Practice Guideline[END_REF], a practice which has not been developed widely. Recent trials of home NIV for patients surviving an ICU admission suggest important potential benefits, even if not all patients will accept this modality [START_REF] Murphy | Effect of Home Noninvasive Ventilation With Oxygen Therapy vs Oxygen Therapy Alone on Hospital Readmission or Death After an Acute COPD Exacerbation: A Randomized Clinical Trial[END_REF]. Second, many of these patients have untreated or undiagnosed comorbidities, especially sleep-related breathing disorders and cardiac dysfunction [START_REF] Adler | Obstructive sleep apnea in patients surviving acute hypercapnic respiratory failure is best predicted by static hyperinflation[END_REF][START_REF] Adler | Comorbidities and Subgroups of Patients Surviving Severe Acute Hypercapnic Respiratory Failure in the Intensive Care Unit[END_REF]. The organization of the healthcare system is such that many of these -19 -patients continue to be undertreated and will not have a proper work-up before their subsequent admissions. New approaches are needed to reduce this high readmission rate.

Ultrasound is a feasible technique at the bedside and is increasingly used in the ICU. Ultrasound can evaluate the respiratory muscle function and help manage mechanically ventilated patients [START_REF] Tuinman | Respiratory muscle ultrasonography: methodology, basic and advanced principles and clinical applications in ICU and ED patients-a narrative review[END_REF]. It can give a gross estimation of the diaphragm function by providing the thickening fraction or diaphragm excursion [START_REF] Dubé | Ultrasound evaluation of diaphragm function in mechanically ventilated patients: comparison to phrenic stimulation and prognostic[END_REF]. In patients presenting with acute exacerbations of COPD in the emergency room, a thickening fraction <20%as a surrogate of diaphragm dysfunctionwas associated with NIV failure [START_REF] Marchioni | Ultrasound-assessed diaphragmatic impairment is a predictor of outcomes in patients with acute exacerbation of chronic obstructive pulmonary disease undergoing noninvasive ventilation[END_REF]; these results, have not been prospectively validated. Patient-ventilator dyssynchronies are frequent during acute exacerbations of COPD but their detection can be challenging. Diaphragm ultrasound may be useful by displaying the signal of diaphragm excursion in combination with airway pressure tracings [START_REF] Soilemezi | Understanding Patient-Ventilator Asynchrony Using Diaphragmatic Ultrasonography[END_REF]. Ultrasound can also examine extradiaphragmatic inspiratory muscle function such as the intercostal parasternal muscle [START_REF] Dres | Usefulness of Parasternal Intercostal Muscle Ultrasound during Weaning from Mechanical Ventilation[END_REF]. Technically, parasternal intercostal ultrasound can be performed with a 10-15 MHz linear probe positioned at the second intercostal space; the muscle is then visualized between the two ribs (Figure 6). A study describing the use of parasternal intercostal muscle ultrasound in stable COPD patients, demonstrated feasibility and reproducibility of muscle thickness measurements [START_REF] Wallbridge | Parasternal intercostal muscle ultrasound in chronic obstructive pulmonary disease correlates with spirometric severity[END_REF]. In mechanically ventilated patients, a dose response relationship between respiratory load and the parasternal intercostal thickening fraction has been recently reported [START_REF] Dres | Usefulness of Parasternal Intercostal Muscle Ultrasound during Weaning from Mechanical Ventilation[END_REF]; a significant association was observed between parasternal intercostal thickening fraction and presence of diaphragm dysfunction [START_REF] Dres | Usefulness of Parasternal Intercostal Muscle Ultrasound during Weaning from Mechanical Ventilation[END_REF]. Ultrasound can also be applied to image the lungs in COPD. and may be useful in differentiating causes of acute dyspnea in these [START_REF] Mayo | Thoracic ultrasonography: a narrative review[END_REF]. It can also help to identify pneumothorax, pleural effusion, consolidation or cardiogenic edema. In summary, ultrasound of the respiratory muscles and the lungs in combination with In normal subjects, the end expiratory lung volume (EELV,Normal) is the relaxation volume of the respiratory system or Functional Residual Capacity, FRC), where no inward or outward recoil pressure exists (the pressure of the respiratory system is 0 cmH2O relative to the atmosphere). To trigger the ventilator, the inspiratory muscles of the patient have to develop an inspiratory effort ≥ the trigger sensitivity set on the ventilator (2 cmH2O in the example). A tidal breath of 500 ml delivered by the ventilator will increase the volume of the respiratory system to its end inspiratory lung volume (EILV,Normal). The normal elastic work of breathing (Wel,n) represented by the triangular area is not excessive.

In hyperinflated COPD or asthma patients, the end expiratory lung volume (EELV,Hyperinfl) is above the relaxation volume of the respiratory system, increasing Functional Residual Capacity, ΔFRC where Δ denotes the increase in volume from the normal FRC), where an inward recoil pressure exists (the pressure of the respiratory system is 7 cmH2O relative to the atmosphere). This pressure is called the intrinsic positive end expiratory pressure (PEEPi). (This pressure is usually measured by the end-expiratory occlusion method with the patient relaxed). To trigger the ventilator, the inspiratory muscles of the patient have first to develop an inspiratory effort to overcome the positive inward recoil of the respiratory system present at the end of expiration (7 cmH2O, PEEPi) and then the trigger sensitivity set on the ventilator (2 cmH2O in the example). The required pressure to effectively trigger the ventilator (Peffect,trigger = 7+2=9 cmH2O in the example). In case they fail to generate this amount (9 cmH2O), an effective triggering effort ensues, which does not trigger the ventilator. A similar tidal breath of 500 ml delivered by the ventilator will increase the volume of the respiratory system to its new end inspiratory lung volume (EILV,hyperinfl), where there is risk of overdistension (stress and strain of the lung) with its potentially injurious sequalae (This is Pplateau if measured by the end-inspiratory occlusion method with the patient relaxed). The elastic work of breathing is mainly attributed to PEEPi (square shaded area, WPEEPi) and is greatly increased leading to increased delivered mechanical power.

Figure 1 .

 1 Figure 1. Proportion of severe acute chronic obstructive pulmonary disease (COPD)

Figure 2 .

 2 Figure 2. Pressure volume (P-V) relationship of the respiratory system when

Figure 3 .Figure 4 .

 34 Figure 3. Therapeutic options at the different stages of patient management

Figure 5 .

 5 Figure 5. Tracings from top to bottom of airway pressure (Paw), airflow (Flow),

Figure 6 .

 6 Figure 6. Schematic representation (A) and ultrasound images of the parasternal

Figure 6

 6 Figure 1

support from Philips, personal fees from Baxter, personal fees from Hamilton, personal -3 -fees and non-financial support from Fisher & Paykel, grants from French Ministry of Health, personal fees from Getinge, grants and personal fees from Respinor, grants and non-financial support from Lungpacer, outside the submitted work. L Brochard conducts an investigator-initiated trial on PAV+ (NCT02447692) funded by the Canadian Institute for Health Research and a partnership with Medtronic Covidien; his laboratory also receives grants and non-financial support from Fisher & Paykel, non-financial support from Air Liquide Medical System, non-financial support from Philips, non-financial support from Sentec, other from General Electric (patent). M Dres reports personal fees from Lungpacer Med Inc, grants from French Ministry of Heath outside the submitted work. L Heunks reports a research grant paid to institution from Liberate Medical (USA) and speakers fee from Getinge Critical Care. A Jubran reports grant from the National Institute of Health (RO1-NR016055). F Laghi reports research grants from the National Institutes of Health, VA Research Service, Liberate Medical LLC, and the National Science Foundation, all outside the submitted work. A Mekontso-Dessap reports reports research grants from Fischer Paykel, Baxter, Philips, Ferring and GSK; participation to advisory board for Air Liquide, Baxter, and Amomed, lectures for Getingue and Addmedica. S Nava report advisory board for Philips and Breas and speaking fee from Resmed Italy outside the submitted work. O from Medtronic, personal fees from Janssen, grants from Covidien (Medtronic) and CIHR, and reimboursement of travel and hotel expenses to attend a meeting from IMT Medical, all outside the submitted work.

Social network: A state of the art review on mechanical ventilation in COPD and asthma patients, from ICU admission to ICU discharge. Take home message: This review summarizes practical ventilator strategies to manage patients with asthma and chronic obstructive pulmonary disease (COPD). The causes, impact and management of dynamic hyperinflation are discussed, as well as heart lung interaction. We expose the importance of non-invasive ventilation to prevent intubation.

We provide key messages regarding ventilator settings in intubated patients. Future adjunctive strategies are discussed physical examination may can be an effective diagnostic tool and a helpful technique to adjust and guide ventilator management. Whether such ultrasound imaging of the respiratory muscles improves patient outcomes remains to be determined. In addition, training and skills are required to ensure a safe and worthwhile implementation.

Summary

Mechanical ventilation is the cornerstone of the management of COPD and asthma patients confronting life-threatening respiratory failure. Although NIV prevents invasive ventilation in the majority of patients with COPD exacerbation, future efforts should focus on the improvement of the efficacy on NIV. Invasive mechanical ventilation is applied to patients who fail NIV and are subsequently intubated. The major goal during invasive mechanical ventilation is to limit hyperinflation, which is achieved by the use of low minute ventilation and low mean expiratory flow (i.e. low tidal volumes and prolonged expiratory time). Normalization of blood gas is not a therapeutic goal. Low level of external PEEP may be applied to patients triggering their ventilator. Mechanical ventilation of asthma patients follows the same rules except that the use of NIV is not presently recommended despite promising recent data. Weaning should be performed as expeditiously as possible with daily screening test followed by a trial of spontaneous breathing. In selected patients, prophylactic post-extubation NIV prevents postextubation acute respiratory failure and subsequent reintubation. High flow nasal cannula seems as efficient as NIV to prevent reintubation and the combination of NIV and high flow nasal cannula maybe even more efficient. Finally, tracheostomy should be the subject of a multidisciplinary discussion.