By Dean R. Hess, PhD, RRT
In the care of mechanically ventilated patients with the acute respiratory distress syndrome (ARDS), an issue that has generated much discussion in recent years is that of positive end-expiratory pressure (PEEP), recruitment maneuvers, and the so-called "open lung" approach. What is the role of PEEP and recruitment maneuvers in patients with ARDS?1 How important is it to aggressively apply strategies to improve arterial oxygenation? This question brings us to the issue of physiologic benefits, such as blood gas results, vs patient-important outcomes, such as survival. As clinicians, we have traditionally made adjustments on the ventilator to improve the patient’s blood gases. However, what is important for the patient is whether they survive their acute respiratory failure. Most patients are not interested in their PaO2. I suspect that few patients (or their families) ask about their blood gases results. However, they are very interested in whether they are likely to survive their mechanical ventilation course.
There is accumulating literature that suggests that PaO2 is not a good predictor of survival. Inhaled nitric oxide improves the PaO2 in most patients with ARDS but does not affect survival.2 Mask CPAP in patients with acute hypoxemic respiratory failure improves the PaO2, but it does not affect the intubation rate or survival.3 We know from the ARDS Network study that higher tidal volumes in patients with ARDS result in a higher PaO2, but survival is better with lower tidal volumes.4 Prone positioning improves PaO2 in patients with ARDS, but it does not affect mortality.5
Rationale for PEEP
I suggest that the reason to use PEEP in patients with acute lung injury is as part of a lung-protective strategy, rather than simply a way that we can increase the PaO2 and lower the FIO2. Collapsed alveoli adjacent to open alveoli can promote the development of shear forces within the lung during tidal ventilation that could potentially be injurious.6 Perhaps we can eliminate these high shear forces if we use an adequate amount of PEEP to maintain alveolar recruitment. There are data published more than 25 years ago showing that if the lungs in a small animal model are ventilated with an inspiratory pressure of 45 cm H2O and no PEEP, much injury quickly occurs in the lungs. However, with the same inflating pressure and a PEEP of 10 cm H2O, the lungs are protected against much of the injury associated with this high inflation pressure,7 suggesting that PEEP may be part of a lung protection strategy.
What is an appropriate level of PEEP in patients with acute lung injury or ARDS? Although few would argue that an appropriate level of PEEP is important, evidence is lacking upon which to base firm guidelines for setting PEEP. Advocates of the open-lung approach argue that high levels of PEEP should be used to maximize alveolar recruitment. Those who advocate strict adherence with the ARDS Network approach use the combination of PEEP and FIO2 that maintains the PaO2 between 55 and 80 mm Hg (or an SpO2 between 88% and 95%). Higher levels of PEEP often—but not always—improve the PaO2. Although there is considerable emotion associated with the appropriate setting of PEEP, there is a lack of evidence that higher PEEP levels improve patient-important outcomes like survival. In fact, a recently completed (still unpublished) ARDS Network study compared a modest level of PEEP to a higher level of PEEP in patients with acute lung injury and ARDS and failed to demonstrate a survival benefit for the use higher PEEP levels.
In recent years, there has been enthusiasm toward the use of recruitment maneuvers in an attempt to open collapsed lung tissue. The rationale for this approach is that an open lung is part of a lung protection strategy. A recruitment maneuver is a sustained increase in airway pressure, typically performed by increasing the PEEP setting on the ventilator to 30-40 cm H2O for 30-40 seconds, after which a sufficient amount of PEEP is applied to keep the lungs open. This generally results in levels of PEEP higher than what many clinicians are accustomed to using. There have been a number of strategies reported as recruitment maneuvers. These include increasing the PEEP setting on the ventilator, using sustained inflations, the use of frequent sighs on the ventilator, use of ventilator modes that encourage spontaneous breathing, high frequency oscillation, and prone positioning.
There are some physiologic data reported to suggest that recruitment maneuvers and the open-lung approach might be helpful. As part of a lung protective ventilation strategy producing improved survival, Amato et al used recruitment maneuvers.8 There are CT images showing much dependent lung collapse that improved after an aggressive recruitment maneuver9 and a case series reporting an improvement in arterial oxygenation following the recruitment maneuver.10
However, other recent studies have questioned the benefit of recruitment maneuvers. Villagra et al11 reported that recruitment maneuvers have no short-term benefit on oxygenation. Grasso et al12 reported that recruitment maneuvers were only useful to improve oxygenation in patients with early ARDS and those without an impairment in chest wall mechanics. In patients with brain injury, Bein et al13 reported that recruitment maneuvers marginally improved arterial oxygenation and adversely affected cerebral hemodynamics.
The effect of recruitment maneuvers on arterial oxygenation may depend on whether the pathophysiology of ARDS is primarily atelectasis, consolidation, or edema. If the underlying pathology is atelectasis, then one might expect a large benefit from recruitment maneuvers. In many patients with ARDS, however, the underlying pathophysiology is more likely to be consolidation or edema. In the case of consolidation, a recruitment maneuver may be more harmful than beneficial because it may cause over-distention injury of the unaffected lung. A similar argument might be made in the case of edema. Interestingly, Crotti et al14 reported a potential for recruitment of only about 6% of lung parenchyma. In a recent commentary, Hubmayr15 made a convincing argument that the dependent lung in ARDS is primarily fluid-filled rather than collapsed. If this is the case, then the benefit of open-lung strategies (ie, recruitment maneuvers and high PEEP) for lung protection are called into question.
Although there has been a lot of enthusiasm about using lung recruitment maneuvers in patients with ARDS, I submit that there is still a lot we need to learn about this. We do not know which are the best patients in whom to apply recruitment maneuvers; we do not know which technique (if any) is the best one to use; we do not know how to set the PEEP after we do recruitment maneuvers; and we do not know how to monitor the effect of recruitment. We do not know the safety of these maneuvers. Although I have seen impressive increases in PaO2 for some patients after a recruitment maneuver, I have also seen significant hemodynamic compromise and subcutaneous emphysema.
Perhaps most important, we do not know the effect of recruitment maneuvers on patient-important outcomes like survival. The question that remains in relation to using recruitment maneuvers and high levels of PEEP is whether open lungs are "happy" lungs or simply "pretty" lungs. As stated by Hubmayr,15 ". . . maximizing oxygen tension through the use of aggressive recruitment may be gratifying in the short term, but at this point, who can say that it prevents lung injury and promotes alveolar repair?"
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13. Bein T, et al. Lung recruitment maneuver in patients with cerebral injury: effect of intracranial pressure and cerebral metabolism. Intensive Care Med. 2002;28: 554-558.
14. Crotti S, et al. Recruitment and derecruitment during acute respiratory failure. A clinical study. Am J Respir Crit Care Med. 2001;164:131-140.
15. Hubmayr RD. Perspective on lung injury and recruitment. A skeptical look at the opening and collapse story. Am J Respir Crit Care Med. 2002;165:1647-1653.