Open lung ventilation: Waiting for outcome studies?*

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One of the most important advances in mechanical ventilation in recent years has been the recognition that a protective ventilatory strategy with low tidal volume decreases morbidity and mortality when applied properly in patients with acute respiratory distress syndrome (ARDS) (1). The application of low tidal volume ventilation prevents injury from alveolar overdistension but does not avoid potential injury caused by the repetitive alveolar opening and closing. Positive end-expiratory pressure (PEEP) prevents end-expiratory alveolar collapse and improves ventilation/perfusion matching as well as gas exchange in ARDS patients (2).
Recruitment maneuvers (RMs), known also as “open lung ventilation,” were first encouraged by a randomized controlled trial performed by Amato et al (3). The open lung strategy allows a sustained increase in airway pressure to open collapsed alveoli followed by sufficient PEEP application to maintain the lungs open. The percentage of potentially recruitable lung varies among patients and directly correlates with the maintained percentage of open lung after application of PEEP (4).
Most studies of RMs have used physiologic end points, including the study by Dr. Toth and colleagues (5) appearing in this issue of Critical Care Medicine. The effects on oxygenation during open lung ventilation have been variable between studies and may be related to the heterogeneity of patients studied or the method used for recruitment (6). Sustained improvement in oxygenation with open lung ventilation was better achieved in paralyzed patients (7) and if larger tidal volume (8) or relatively lower but adequate PEEP levels (9) were applied during RMs. In animal models of acute lung injury, RMs did not worsen right or left ventricular function (10). The loss of hypoxic pulmonary vasoconstriction due to alveolar recruitment may counteract the negative hemodynamic effect of high airway pressure use. Adverse hemodynamic effects of RMs appear to be more common in patients with reduced chest wall compliance or limited oxygenation response from recruitment (11).
Improved gas exchange seen during RMs is likely related to opening of the atelectatic alveoli from lung recruitment. The impact of RMs on extravascular lung water (EVLW) is less clear, and the relationship between application of PEEP, EVLW, and oxygenation remains complex and controversial. Experimental studies on this subject have yielded variable results. Application of PEEP has caused decreased (12), increased (13), or unchanged (13) EVLW measurements. The presence of intravascular occlusions and hence perfusion defects in patients with ARDS may account for the variability when measuring EVLW.
Dr. Toth and colleagues (5) investigated the relationship between the application of PEEP during recruitment maneuvers and oxygenation, EVLW, and hemodynamic changes. Their open lung ventilation algorithm incorporated application of continuous positive airway pressure of 40 cm H2O for 40 secs followed by a decremental PEEP protocol to determine the PEEP level that maintains oxygenation after lung recruitment in 18 sedated paralyzed ARDS patients ventilated with a pressure control mode.
The findings of this study once again illustrate the beneficial effect of RMs on gas exchange and oxygenation as well as the relative safety of this strategy (14). Oxygenation improved significantly after lung inflation compared with baseline and remained significantly elevated at 30 mins but not at 1 hr postintervention. Conversely, sustained increase in oxygenation (4–6 hrs postinflation) was described by similar work applying decremental PEEP protocols after lung opening (15, 16). A lower PEEP achieved during titration, patient heterogeneity, and different methods of recruitment probably accounted for variability of results between these studies. Overall RMs were well tolerated in the study by Dr. Toth and colleagues (5). Significant respiratory acidosis developed, but acid-base balance returned to baseline 30 mins after lung recruitment, whereas only two patients required increased inotropic support to maintain cardiac output.
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