>REVIEW ARTICLE |
https://doi.org/10.5005/ijrc-1-1-101 |
Protective lung ventilation in Acute Respiratory Distress Syndrome: What is new?
Email: sgnair@aims.amrita.edu
Professor and Head, Anaesthesia and Critical Care, Amrita Institute of Medical Sciences Cochin, Kerala
Senior Consultant, Anaesthesiologist Lissie Hospital, Cochin, Kerala
Abstract
Acute respiratory distress syndrome (ARDS) is associated with high mortality and morbidity. Traditional ventilation with large tidal volumes aimed at normalisation of blood gases is usually associated with significant injury to the lungs. Protective lung ventilation (PLV) with smaller tidal volumes, higher levels of positive end-expiratory pressure (PEEP) and control of plateau pressures is the accepted mode of ventilation in patients with ARDS. Recruitment manoeuvres have been part of the protocol in PLV. However, considerable controversy surrounds the optimal PEEP that may be applied to these patients. Similarly, there is no consensus on the usefulness of recruitment manoeuvres in these sick patients. This review tries to find some answers to the best way to use PLV, PEEP and recruitment manoeuvres in patients with ARDS.
Keywords: Acute respiratory distress syndrome, protective lung ventilation, positive end-expiratory pressure, recruitment manoeuvre.
How to cite this article: Nair SG, Iyengar V Protective lung ventilation in acute respiratory distress syndrome: what is new? Ind J Resp Care 2012; 1(2):101-11.
Introduction
Mortality in patients with acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) approaches 40-60 percent.1 Mechanical ventilation is critical in patients with ARDS to maintain adequate gas exchange and reduce the work of breathing. Traditional ventilation uses 10-15 mL/kg of tidal volume (VT) which is much more than in normal patients at rest (7-8 mL/kg). However, this is frequently necessary to maintain normal values of pH and partial pressure of carbon dioxide in blood (PaCO2). Ventilation with large tidal volumes can cause disruption of pulmonary epithelium and endothelium, produce intense hypoxaemia, atelectasis and release of inflammatory mediators. This review will specifically look at protective lung ventilation (PLV) and try to answer some of the controversies surrounding this strategy. However, rescue methods such as airway pressure release ventilation (APRV), prone ventilation or high frequency oscillatory ventilation will not be discussed.
Ventilator induced lung injury
The parenchymal lung injury associated with ARDS is heterogeneous in nature. Using computed tomography of the chest, Gattinoni has described 3 general regions of the lung: a region of normal lung tissue, primarily in the nondependent areas; a region of densely consolidated, fluid filled or atelectatic tissue, primarily in the dependent regions; and a region that is collapsed during expiration but recruitable during inspiration.2Lung injury associated with ARDS can be compounded by mechanical ventilation. Ventilation with high airway pressures can result in barotrauma. Incidence of barotrauma is high with plateau pressures (Pplat) > 35 cm H2O than when the Pplat are maintained < 30 cm H2O.1 The healthy region of the lung which receives most of the tidal ventilation tends to get over-distended. This does not manifest as air leak but rather as a diffuse alveolar damage called volutrauma. Volutrauma usually occurs in the nondependent areas of the lung with normal alveolar units and normal airway resistance. Injury can also occur when lung units are allowed to repeatedly open and close with tidal ventilation (atelectrauma). Both atelectrauma and volutrauma can trigger release of inflammatory mediators (biotrauma) and induce bacterial translocation which may incite multiorgan dysfunction syndrome (MODS) and increase morbidity and mortality.3,4
These patients need high inspired concentrations of oxygen (FiO2) which can also be toxic in humans. The toxic threshold for inducing damage is debatable. It is generally desirable to reduce the FiO2 to < 60 percent. Inspired oxygen concentrations < 40 percent can be tolerated for prolonged periods without lung injury.
Concept of protective lung ventilation (PLV)
The concept of lung protective ventilation strategy is aimed at opening up as many alveolar units as possible and maintaining the patency of the airways during the course of the respiratory cycle while ensuring adequate oxygenation, carbon dioxide removal and acceptable blood pH. Figure 1 shows the pressure volume relationship of a patient with ARDS.
Figure 1: Shows a pressure volume loop from a patient with ARDS. Point A shows the lower inflection point (Pflex.i). Point B represents the upper inflection point on the inspiratory limb. The bracketed area represents the ‘safe zone’ of ventilation in ARDS. As can be observed from the figure, allowing the PEEP to fall below point A would result in collapse of alveoli while ventilating above point B would result in over-distention of alveoli.
Initial application of pressure to the patient’s airway does not result in an increase in the lung volume as most alveolar units remain collapsed. Once a critical pressure is achieved in the airway, a sudden increase in lung volume is noticed (critical opening pressure or lower inflection point (PFlex.i)). With increasing pressures, the slope of pressure-volume (P-V) curve rises sharply, signifying improved compliance and easier inflation. When the lung is near total inflation, the curve once again becomes flatter indicating that further increase in airway pressure is unlikely to recruit more alveolar units (upper inflection point). It is also important to note that for any given pressure, a larger lung volume exists during the deflation part of the curve than during inflation. This volume difference (hysteresis) is to a large extent dependent on the surfactant function, the degree of lung impairment and recruitability of lung units. Less hysteresis is seen with normal function or less recruitable alveolar units. The primary principles behind protective lung ventilation are, therefore, to:
a) Avoid repetitive opening and closure of the alveolar units by application of adequate positive end expiratory pressure (PEEP) to maintain patency of lung units throughout the respiratory cycle.
b) Ventilate patients using airway pressures that limit the fluctuations in pressure between the set PEEP and the upper inflection point to prevent overdistension of alveoli. Ideally ventilation should take place in the "safe zone" of the P-V loop (Figure 1). The upper limit of airway pressure in PLV is generally accepted as < 30 cm H2O.
Oxygenation target of PLV
Oxygenation targets include a PaO2 55 - 80 mm Hg or systemic arterial oxygen saturation (SaO2) of 88 – 92 percent.
Ventilatory strategies during PLV
The ARDS Network study, which to date is the only ventilator related study showing an outcome benefit, is the benchmark for management of patients with ARDS.5 There are three important criteria to be followed in this strategy of ventilation.
a) VT should be maintained at < 6 mL/kg of predicted body weight irrespective of the mode of ventilation.
b) Maintain the Pplat < 30 cm H2O irrespective of the mode of ventilation or PEEP levels.
c) Use of 'adequate PEEP' to ensure that alveoli do not collapse at end-expiration.
The combined end points of 6 mL/kg VT and a Pplat < 30 cm H2O can result in unacceptably low blood pH due to elevated PaCO2. To overcome this, the respiratory rate may be increased to a maximum of 35 breaths/minute. Increasing respiratory rates beyond this point can result in generation of auto-PEEP and may be counterproductive. A low pH may be due to high PaCO2 as well as metabolic acidosis which is not uncommon in patients with multi-organ dysfunction. Buffering agents may have to be used if the blood pH falls below 7.15.
If the Pplat rises > 30 cm H2O with a VT of 6 mL/kg, further reduction in the VT (to 4 mL/kg) may be required. Figure 2 shows a flow chart that may be used to manage patients with the above mentioned primary targets.
Failure of PLV strategy: Breaching the set targets
The Pplat threshold of 30 cm H2O can be exceeded under the following circumstances:6
a) If the patient has severe hypoxaemia (PaO2 < 50 mm Hg or SaO2 < 88%) despite FiO2 1.0 and has a Pplat around 30 cm H2O even with a VT of 4 mL/kg to allow for further increases in PEEP to improve oxygenation.
b) If the patient has pH < 7.15 despite buffering, to allow for a larger VT to reduce CO2 load.
Management of refractory hypoxaemia
Despite the valiant efforts made to adjust the ventilation of patients with ARDS, a significant number of patients can still remain hypoxaemic. PLV is considered as a failure under the following conditions:
a) Despite FiO2 1.0, PEEP > 20 cm H2O with a PLV for > 24 h; the PaO2 is < 55 mm Hg.
b) PLV > 24 - 72 h with FiO2 > 0.7 and PEEP > 15 cm H2O to maintain adequate oxygenation.
Rescue therapy should be initiated in these patients rather than take a 'wait and watch' approach. In addition to increasing the FiO2, alternative techniques of ventilation such as airway pressure release ventilation (APRV), inverse ratio ventilation (IRV), prone ventilation and high frequency oscillatory ventilation should be tried in these patients. Each of these ventilation strategies has its own benefits and drawbacks. Inhaled nitric oxide has also been used to counter the hypoxaemia associated with ARDS with variable results.
In addition to increasing FiO2 in response to refractory hypoxaemia, increasing the mean airway pressure (MAP) may also help. This can be achieved by increasing the duration of inspiration (increasing pause time during VCV or preferably using PCV), increasing PEEP and increasing ventilation pressure. Establishing an optimal PEEP is the first step as this is the most practical approach to management of hypoxaemia.
Pressure control versus volume control ventilation
PCV is time cycled and pressure targeted. During inspiration, flow from the ventilator pressurises the circuit and the lungs. As the lungs get inflated, flow from the ventilator progressively reduces. If inspiration is long enough to achieve zero flow from the ventilator, the lungs are maximally inflated for that pressure and the peak airway pressures (Ppeak) on PCV will be the same as Pplat. PCV is often used as a backup in patients with worsening oxygenation. PCV is initiated when
Figure 2: Flow chart of PLV
a) Pplat on VCV is above the acceptable Pplat
b) When patients are uncomfortable with VCV or when oxygenation is low
c) When prolonged inspiratory time (Ti) is required to increase the mean airway pressure (MAP).
It is important to remember that when Pplat is excessive during VCV and patient is switched to PCV, a lower Pplat (pressure control level) is only possible at the expense of a lower VT and probably permissive hypercapnoea. A potential disadvantage of PCV is that as the lung mechanics become worse, the VT delivered will automatically reduce and in the same token, when lung mechanics improve, the delivered VT will increase resulting in volutrauma. The variable flow available with PCV may be more acceptable for a patient exhibiting significant respiratory efforts and who are asynchronous with the fixed flows of VCV. This can reduce the work of breathing and the requirement for sedatives and muscle relaxants.
PCV is best suited for improvement in oxygenation. Longer Ti in PCV may result in recruitment of lung units with longer time constants. As the Ti increases, the MAP also progressively increases contributing to improved oxygenation. However, care must be taken to prevent development of autoPEEP. The expiratory flow wave form may be useful in assessment of autoPEEP. The Ti should be prolonged not more than to the point of creating an autoPEEP regardless of the I:E ratio, unless the intent is to create an autoPEEP. A concern with autoPEEP is that it is not uniformly distributed especially in diseased lung such as ARDS. As auto-PEEP is created, the driving pressure and thus, the delivered VT reduces.. This will protect the lung against increased inspiratory pressures unlike VCV.
Setting an optimal PEEP
Confusion still prevails regarding the ideal PEEP to be used. The initial PEEP is set at 8-12 cm H2O and then either the low PEEP table or the high PEEP table may be followed. The ARDS Net study used a maximum PEEP of 24 cm H2O to attain the target oxygenation.5 The FiO2 and PEEP adjustments were done based on the PaO2 attained. In this study, although there was a provision to increase PEEP up to 24 cm H2O in case target oxygenation was not attained, this level of PEEP was not required in any of the patients. The ALVEOLI study used higher and lower PEEP tables.7 However, after recruitment of 171 patients the higher PEEP table was re-adjusted to obtain a minimum PEEP of 12 cm H2O. If the oxygenation target cannot be achieved with the highest level of PEEP suggested (24 cm H2O) with FiO2 1.0, then the PEEP could further be increased in steps of 2-5 cm H2O to a maximum of 34 cm H2O.6 If there was no significant improvement in oxygenation within four hours, then the PEEP should be reduced to 24 cm H2O. This additional management strategy may be required only in a small subgroup of patients with severe forms of H1N1 infections.
Individualised approach: Open lung concept
An individualised approach to setting the PEEP was described by Lachmann in the open lung concept (OLC).9 The prerequisites in this concept included the use of pressure controlled ventilation, adequate fluid loading and inotropes to ensure that hypotension does not occur during the manoeuvre, use of repeated arterial blood gases as an index of alveolar recruitment and an inspiratory to expiratory ratio (I:E) that ensures end expiratory flows of zero. The OLC attempts to place the patient on the expiratory limb of the P-V curve in steps:
a) An initial arbitrary PEEP set between 15 cm H2O and Ppeak at 35 cm H2O.
b) Then increase PEEP in steps of 2-3 cm H2O every two min till PEEP reaches 20 cm H2O (Ppeak will reach 40 cm H2O).
c) Increase pressure control levels in steps of 5 cm H2O every two min (Ppeak will now be 50 cm H2O).
d) Gradually reduce pressure control level to identify the pressure level required to deliver a VT of 6 mL/kg.
e) Gradually reduce the end-expiratory pressure through a decremental PEEP trial (2 cm H2O every two min) until the critical closing pressure is reached indicating lung derecruitment (identified by a large decrease in tidal volume with a small decrease in pressure level or a large decrease in dynamic compliance).
f) Perform another recruitment manoeuvre to open the lung to total lung capacity (use PCV of 50 cm H2O).
g) Keep the lung open by setting the PEEP 2 cm H2O above the PEEP associated with the critical closing pressure.
h) Reduce pressure control level to deliver a VT of 6 ml/kg.
Figure 3: The figure represents the points at which optimal PEEP may be applied. Point A represents Pflexi. Pmcii represents the point of maximum compliance increase. Point B represents the upper inflection point. Pflexd represents the point of deflection on the expiratory limb of the curve. Pmcdd represents the point of maximum decrease in compliance on the deflation limb of the curve.
Evidence for PEEP
Amato and colleagues compared traditional ventilation using large tidal volume, minimum PEEP and normal PaCO2 versus PLV using low tidal volume, high PEEP and permissive hypercapnoea in 51 patients with ARDS.10 PLV resulted in significant reduction in mortality, early weaning from mechanical ventilation and reduction in barotrauma despite use of higher PEEP. Three randomised controlled trials (RCT’s) have looked at high PEEP versus low PEEP in ARDS patients. The Assessment of Low tidal Volume and increased End-expiratory volume to Obviate Lung Injury (ALVEOLI) study by Bower and colleagues utilised a high PEEP versus a low PEEP in 549 patients.7 The study was truncated because of projected futility of outcome. Although the authors found significant improvement in oxygenation and lung compliance in the high PEEP group, there was no mortality benefit or reduction in ventilator or ICU free days. The Expiratory Pressure (EXPRESS) study which included 768 patients, compared a minimal distension group with a high PEEP group in which the PEEP was adjusted so that the Pplat was maintained between 28 - 30 cm H2O (low VT in both groups).11 This study was also terminated at the 18th interim analysis due to futility of outcome. There was no difference in the 28-day or 60-day mortality. However, patients who received high PEEP had more ventilator free days and organ failure free days. The Lung Open Ventilation study (LOVS) differed from the previous study in that the experimental group was allowed higher Pplat up to 40 cm H2O and lung recruitment was used in the high PEEP group.12 This study involved 983 patients and showed that although there was no outcome benefit with higher PEEP, patients who received higher PEEP had lower incidence of refractory hypoxaemia, death during refractory hypoxaemia and required lesser rescue therapies.
A number of meta-analyses have looked at the benefits of high versus low PEEP. The data from the three RCTs mentioned above were pooled together by Briel and colleagues (2299 patients).13 Although there was no benefit in terms of hospital mortality, the higher PEEP group showed a statistically significant reduction in ICU deaths. Patients in the high PEEP group had less refractory hypoxaemia and fewer patients died during rescue therapy.
The study also showed that patients who were already in ARDS (PaO2/FiO2 < 200) at the time of recruitment into the study had lower mortality and more ventilator free days when they were allocated to the higher PEEP group. Patients who were in ALI (PaO2/FiO2 >200) at the time of entry into the study fared worse when they were allocated to the high PEEP group. The study highlighted that high PEEP was more useful in patients with worse disease with recruitable alveoli.
Phoenix et al included 2482 patients from six studies in their meta-analysis.14 They found an early mortality benefit in the high PEEP group. This was attributed to the disproportionate effect of the three smaller studies. A repeat meta-analysis with the 3 larger studies which accounted for 85 percent of the patients showed a trend towards improved mortality with high PEEP although this was not statistically significant. The authors concluded that PEEP may have an independent clinical benefit in very sick patients with ARDS.
Putensen looked at nine studies which included varying combinations of high versus low VT and high PEEP versus low PEEP.15 In addition to their conclusion that low VT was associated with lower hospital mortality, they also showed that high PEEP had no advantage over low PEEP. However, the number of rescue therapies for life-threatening hypoxaemia and death during rescue therapy was significantly lower in the group receiving high PEEP.
A recent meta-analysis looked at four studies which included the three RCTs mentioned earlier and a study where transpulmonary pressure (PTP) was used for optimisation of PEEP.16 This study included 2360 patients and showed a nonsignificant trend towards reduced mortality in the high PEEP group.
To determine whether the higher or lower PEEP table should be used in individual patients, Ramnath and colleagues suggest that every patient should be assessed for the presence of recruitable lung units by increasing PEEP from 5-15 cm H2O and observed for any improvement in compliance and dead space fraction.17 If the patient responds favourably, one should use the high PEEP table and if there is minimal or no response, the lower PEEP table should be used.
Conclusions regarding optimal PEEP use
From the above discussion, it can be assumed that for patients with ALI, the low PEEP table from the ARDS net study may be most appropriate. In patients with severe ARDS and refractory hypoxaemia, either the high PEEP table, or individualising the PEEP to respiratory mechanics may be beneficial.
Lung recruitment manoeuvres (RM)
Various methods have been used for lung recruitment. The most commonly used method for lung recruitment is with PCV with a constant drive pressure where the PEEP is transiently increased to 35-45 cm H2O for a period of 30-40 s. Arnal and colleagues showed that most of the recruitment occurs during the first 10 s of the manoeuvre and haemodynamic changes occurred after 30 s of sustained inflation.18 Marini suggested that stepwise increase in PEEP accompanied by multiple tidal ventilation at each level of PEEP would help stabilise the functional residual capacity at each level of PEEP increment.19 This technique has been described as the ‘staircase recruitment manoeuvre’ by Hodgson and colleagues.20 Another technique is to maintain a constant, moderate level of PEEP while the pressure control level is progressively increased to a maximum of 45-60 cm H2O. When the recruitment manoeuvre is completed, the pressure should be sufficient enough to open as many lung units as possible. Some intensivists use an arbitrary upper limit target pressure of 40-45 cm H2O although it may be as high as 55-60 cm H2O.
Evidence for RM
Three RCTs have looked at the implications of RM. In 30 patients with ARDS and PLV, the application of RM was associated with significant improvement in PaO2/FiO2 ratio and intrapulmonary shunt fraction (Qp/Qt). However, oxygenation indices returned to baseline values within 30 min of the RM.21 Gattinoni et al showed that the amount of recruitable lung varied greatly. Approximately 24 percent of the lung could not be recruited in ARDS patients. In his study, patients with more recruitable lung had lower oxygenation indices, poorer respiratory system compliance, higher dead space ventilation and higher mortality.22 Brower et al evaluated 72 patients after sustained RM at 35-40 cm H2O for 30 s. Response to RM was variable. The greatest improvement in oxygenation occurred at 10 min after RM and was associated with reduction in systolic blood pressure. There were no significant changes in FiO2/PEEP required after a RM.23
A recent meta-analysis looked at 40 studies that evaluated RMs: 4 were RCT’s, 32 were prospective studies and 4 were retrospective cohort studies.24 The sustained inflation technique (CPAP at 3040 cm H2O for 30-40 s) was the most commonly used technique (45 percent), followed by high pressure control (23 percent), incremental PEEP (20 percent) and high VT (10 percent). Oxygenation was significantly increased after a RM. However, many studies reported a rapid decline in saturation within the next 15-30 min. Adverse events included hypotension and desaturation which were transient and did not interfere with the RM. Significant adverse events including arrhythmias and barotrauma were rare. Sustained inflation with CPAP resulted in greater improvement in oxygenation index as compared to other techniques of RM. The authors concluded that given the uncertain benefits of transient oxygenation improvement, routine use of RM can neither be recommended nor discouraged. RM should be considered on an individual basis in patients with severe lung injury with life-threatening hypoxaemia.
A recent paper has looked into some of the controversies surrounding RM.25 Patients who responded transiently to RM were recruited adequately but were subsequently ventilated with inadequate PEEP while patients who had sustained improvement in oxygenation were ventilated with adequate PEEP after their RM. RM’s may not help when it is applied late in ARDS or when inadequate pressures are applied during a RM.
Conclusions regarding RM
Current evidence suggests that RM should not be used in all patients with ARDS unless they have severe refractory hypoxaemia. In patients with severe refractory hypoxaemia, RMs should be used early as a rescue manoeuvre to open the lung when setting PEEP or following evidence of acute lung derecruitment such as ventilator circuit disconnection.
Objective assessment of lung recruitment and optimal PEEP
Various techniques have been used to objectively assess lung recruitment. Borges et al showed that the combined value of PaO2 + PaCO2 of more than 400 indicated complete recruitment.26 Hodgson and colleagues showed that the PaO2/FiO2 ratio correlated with systemic saturation and lung compliance during lung recruitment manoeuvres.20 In another study, changes in dynamic compliance correlated well with changes in FRC and dead space.27 In a piglet model, Hanson and colleagues showed that maximal tidal elimination of CO2 (VTCO2) correlated with complete lung recruitment while a drop in dynamic compliance during decremental PEEP trial correlated with lung derecruitment.28
Caramez compared various techniques including maximum PaO2, minimum PaCO2, combined PaO2 + PaCO2, Pflex.i, upper deflection point, Pmci.i and Pmcd.d to identify optimal PEEP (Figure 3).29 They found no statistically significant difference between any of these variables. The PEEP obtained with the point of maximum deflation on expiratory limb and points of maximal compliance decrease (expiratory limb) were significantly higher while the Pflex.i and minimum PaCO2 were significantly lower than other variables.
A theoretical advantage of using an objective based assessment of lung recruitment is that an occasional patient may require less recruitment pressure than the arbitrary value, or more likely, the lungs may not be fully recruited with the target values that have been used.
Clinical application of RM and setting of PEEP
A practical way of applying RM in the clinical setting is to set the patient on PCV after deep sedation or paralysis of patient. The PEEP is set at 20 cm H2O and the PC level at 40 cm H2O. The RM is carried out for 2-3 min based on patient tolerance. After the RM, the patient is changed to VCV at 6 mL/kg or less and a decremental PEEP trial is applied. The VCV is used at this point because most modern ventilators instantaneously give the lung compliance on this mode. The patient is allowed to stabilise at this level of PEEP. Compliance stabilises in 3-5 min. The PaO2 or PaO2 + PaCO2 may also be used to identify the optimal PEEP. The PEEP is then reduced in steps of 2 cm H2O till a drop in compliance or any of the other parameters is noticed. Once the critical closing pressure is identified, a repeat RM is done and the PEEP is set 2 cm H2O above the critical closing pressure. The RM can be tried at a higher PEEP and pressure control levels in case adequate recruitment is not appreciated.
New concepts in PLV The stress index
The stress index utilises the patient’s pressure time wave form during constant flow generated, volume controlled ventilation, to reflect tidal recruitment and overdistention. Figure 4 shows the three basic shapes: the upward sloping convex shape suggesting alveolar overdistention (stress index < 1), the upward sloping concave shape suggesting suboptimal ventilation (stress index > 1) and the straight line implying no further recruitment or overdistention. Grasso and colleagues using stress index revealed significant overdistension in patients managed with ARDS Net protocol and stress index based management resulted in better lung compliance and minimal release of inflammatory mediators.30
Targeting transpulmonary pressure
The airway pressure is affected by chest wall compliance, respiratory muscle activity and intraabdominal pressure. The transpulmonary pressure [PTP (airway pressure – pleural pressure)] is however, unaffected by the above factors as well as any intra-alveolar pathology. PTP reflects the actual lung distending pressure.31 In clinical practice, this is estimated by a balloon catheter placed in the oesophagus. Talmor and colleagues compared PTP guided ventilation versus traditional ARDS network based ventilation in 61 patients with ARDS.32 PTP guided ventilation resulted in higher PaO2/FiO2 and better lung compliance than traditional practice. Grasso and colleagues in a similar study showed that PTP rather than unadjusted airway pressure guided the optimum PEEP required while keeping the end inspiratory PTP within acceptable limits.33 Among 14 patients with severe H1N1 infection referred for extra corporeal membrane oxygenation (ECMO), the authors found seven patients whose Pplat was below acceptable levels. In these patients optimisation of PEEP levels resulted in improved oxygenation and avoidance of ECMO.
Summary
Traditional ventilation with large tidal volumes can result in barotrauma, atelectrauma and volutrauma. PLV using low VT and optimal PEEP has shown the best outcome in patients with ARDS. In a clinical setting, the ideal PEEP can be identified through the P-V loop or drop in compliance. High PEEP should be used only in patients with severe hypoxaemia. It should not be tried in patients with ALI. Recruitment manoeuvres may be used only as rescue therapy. Objective assessment should be used for evaluating lung recruitment. PLV cannot always salvage patients and alternative modes of ventilation or therapy should be tried in case of failure. Recent concepts such as the PTP and stress index may become more practical in the coming years.
Figure 4: Stress index. Evaluation of pressure time waveform on constant flow generated volume control ventilation. Stress index < 1 indicates alveolar overdistension, Stress index > 1 indicated alveolar under Alling and stress index of 1 indicates adequate alveolar Alling.
References
1. Doyle L, Szaflarski N, Modin GW, Wiener-Kronish JP, Matthay MA. Identification of patients with acute lung injury. Predictors of mortality. Am J Respir Crit Care Med 1995; 152:1818-24.
2. Gattinoni L, Caironi P, Pelosi P, Goodman LR. What has computed tomography taught us about acute respiratory distress syndrome? Am J Respir Crit Care Med 2001; 164:1701-11.
3. Murphy DB, Cregg N, Tremblay L et al. Adverse ventilatory strategy causes pulmonary-to-systemic translocation of endotoxins. Am J Respir Crit Care Med 2000; 162:27-33.
4. Slutsky AS, Tremblay LN. Multiple system organ dysfunction: Is mechanical ventilation a contributing factor? Am J Respir Crit Care Med 1998; 157:1721-5.
5. Brower RG, Matthay MA, Morris A et al Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. N Eng J Med 2000; 342:1301-8.
6. Haas CF. Mechanical ventilation with lung protective strategies: What works? Crit Care Clin 2011; 27:46986.
7. Brower RG, Lanken PN, Macintyre N et al. Higher versus lower positive end-expiratory pressures in patients with acute respiratory distress syndrome. N Eng J Med 2004; 351:327-36.
8. Albaiceta GM, Luyando LH, Parra D et al. Inspiratory versus expiratory pressure-volume curves to set end-expiratory pressure in acute lung injury. Intensive Care Med 2005; 31:1370-8.
9. Lachmann B. Open the lung and keep it open. Intensive Care Med 1992; 18:319-21.
10. Amato MB, Barbas CS, Medeiros DM et al. Effect of protective lung ventilation strategy on mortality in the acute respiratory distress syndrome. N Eng J Med 1998; 338:347-54.
11. Mercat A, Richard JC, Vielle B et al. Positive end-expiratory pressure setting in adults with acute lung injury and acute respiratory distress syndrome. A randomized controlled trail. JAMA 2008; 299:646-55.
12. Meade MO, Cook DJ, Guyatt GH et al. Ventilation strategy using low tidal volumes, recruitment maneuvers and high positive end-expiratory pressures for acute lung injury and acute respiratory distress syndrome. A randomized controlled trial. JAMA 2008; 299:637-45.
13. Briel M, Meade M, Mercat A et al. Higher vs. lower positive end-expiratory pressure in patients with acute respiratory distress syndrome and acute lung injury. JAMA 2010; 303:865-73.
14. Phoenix SI, Paravastu S, Columb M, Vincent JL, Nirmalan M. Does a higher positive end-expiratory pressure decrease mortality in acute respiratory distress syndrome? A systematic review and meta-analysis. Anesthesiology 2009; 110:1098105.
15. Putensen C, Theuerkauf N, Zinserling J, Wrigge H, Pelosi P. . Meta-analysis: Ventilation strategies and outcomes of the acute respiratory distress syndrome and acute lung injury. Ann Intern Med 2009; 151:566-76.
16. Dasenbrook EC, Needham DM, Brower RG, Fan E. Higher PEEP in patients with acute lung injury: A systematic review and meta-analysis. Respir Care 2011; 56:568-75.
17. Ramnath VR, Hess DR, Thompson BT. Conventional mechanical ventilation in acute lung injury and acute respiratory distress syndrome. Clin Chest Med 2006; 27: 601-613.
18. Arnal JM, Paquet J, Wysocki M et al. Optimal duration of a sustained inflation recruitment maneuver in ARDS patients. Intensive Care Med 2011; 37:1588-94.
19. Marini JJ. Recruitment by sustained inflation: time for a change. Intensive Care Med 2011; 37: 1572-4.
20. Hodgson CL, Tuxen DV, Bailey MJ et al. A positive response to a recruitment maneuver with PEEP titration in patients with ARDS, regardless of transient oxygen desaturation during the maneuver. J Intensive Care Med 2011; 26:41-9.
21. Oczenski W, Hormann C, Keller C et al. Recruitment maneuvers after appositive end-expiratory pressure trial did not induce sustained effects in early adult respiratory distress syndrome. Anesthesiology 2004; 101 :620-5.
22. Gattinoni L, Casroni P, Cressoni M et al. Lung recruitment in patients with acute respiratory distress syndrome. N Eng J Med 2006; 354:177586.
23. Brower RG, Morris A, Macintyre N et al. Effects of recruitment maneuvers in patients with acute lung injury and acute respiratory distress syndrome ventilated with high positive end-expiratory pressure. Crit Care Med 2003; 31 :2592-7.
24. Fan E, Wilcox ME, Brower RG et al. Recruitment maneuvers for acute lung injury. A systematic review. Am J Respir Crit Care Med 2008; 178:1156-63.
25. Kacmarek RM, Villar J. Lung recruitment maneuvers during acute respiratory distress syndrome: is it useful? Minerva Anestesiol 2011; 77:85-9.
26. Borges JB, Okamoto VN, Matos GF et al. Reversibility of lung collapse and hypoxemia in early acute respiratory distress syndrome. Am J Respir Crit Care Med 2006; 174:268-78.
27. Maisch S, Reissmann H, Fuellekrug B et al. Compliance and dead space fraction indicate an optimal level of positive end-expiratory pressure after recruitment in anesthetized patients. Anesth Analg 2008; 106:175-81.
28. Hanson A, Göthberg S, Nilsson K, Larsson LE, Hedenstierna G. VTCO2 and dynamic compliance-guided lung recruitment in surfactant depleted piglets: a computed tomography study. Pediatr Crit Care Med 2009; 10:687-92.
29. Caramez MP, Kacmarek RM, Helmy M et al. A comparison of methods to identify open lung PEEP. Intensive Care Med 2009; 35:740-7.
30. Grasso S, Stripoli T, De Michele M et al. ARDSnet ventilator protocol and alveolar hyperventilation. Role of positive end-expiratory pressure. Am J Respir Crit Care Med 2007; 176:761-7.
31. Richard JC, Marini JJ. Transpulmonary pressure as a surrogate of plateau pressure for lung protective strategy: not perfect but more physiologic. Intensive Care Med 2012; 38:339-41.
32. Talmor D, Sarge T, Malhotra A et al. Mechanical ventilation guided by esophageal pressure in acute lung injury. N Eng J Med 2008; 359:2095-104.
33. Grasso S, Terragni P, Birocco A et al. ECMO criteria for influenza A (H1N1) - associated ARDS: role of transpulmonary pressure. Intensive Care Med 2012; 38:395-403.