In this issue of Anesthesiology, Lim et al.  provide experimental evidence that the positive pressure ventilation–induced re-aeration of a surfactant-depleted collapsed lung can be optimized by using a recruitment maneuver. 1They also convincingly demonstrate that this potential beneficial effect is accompanied by the overinflation of other lung regions.

For the first time in an experimental study, computed tomographic sections of the whole lung were obtained at end-expiration and end-inspiration at different intrathoracic pressures. Scanning the whole lung is essential for an accurate determination of lung volumes of gas and tissue, lung aeration, and alveolar recruitment. 2In a majority of patients with acute respiratory distress syndrome (ARDS) when lying supine, lung re-aration resulting from positive end-expiratory pressure (PEEP) decreases from the apex to the diaphragm. 3In contrast, lung overinflation predominates in caudal and nondependent lung regions. 4As a consequence, assessing changes in lung aeration resulting from PEEP on a single computed tomographic section presents the risk of grossly mis-estimating alveolar recruitment and lung overinflation. In their study, Lim et al.  used a multi-detector Light Speed Scanner, which allowed scanning of the whole lung with an accurate spatial resolution (voxel size ranging between 1.2 and 1.74 mm3) during a breath-holding lasting less than 10 s.

An ovine model of surfactant depletion–induced lung lavage was selected because it promotes alveolar collapse and fits the traditional concept viewing the ARDS lung as “collapsing” under its own weight. In addition, it has been shown to be the most responsive to recruitment maneuvers. 5Throughout the experiment, pure oxygen was administered, a condition known to potentiate alveolar collapse. 6The sheep were also lying supine, a posture that frequently generates atelectasis of dorsal lung regions and is well known to enhance the response to recruitment maneuvers in animals with oleic acid-induced lung injury 7or in patients with ARDS. 8In other words, the most favorable experimental settings were selected for demonstrating the beneficial effects of recruitment maneuvers. For these reasons, the results of the present study cannot be automatically extrapolated to adults with ARDS in whom alveolar flooding rather than lung collapse is the main cause of the loss of aeration. 9,10Additional studies performed in oleic acid-induced injured lungs 11are required to assess whether the benefits of recruitment maneuver hold true when alveolar spaces are filled with fluid, hemorrhagic edema, inflammation, and tissue debris.

The recruitment maneuver performed in the present study has several particularities that deserve to be outlined. In contrast to classic recruitment maneuvers consisting of a 40–60 cmH2O inspiratory pressure sustained during 15–60 s, 5,7,12–17a 40-cmH2O peak inspiratory pressure was applied during a 120-s period of pressure-controlled ventilation, during which PEEP was gradually increased above and then decreased to the lower inflection point + 2 cmH2O. Theoretically, such a long-lasting maneuver should prevent the expiratory derecruitment of lung regions that are progressively reopened during the successive inspiratory phases performed at the pressure of 40 cmH2O. 18As correctly pointed out by the authors in the discussion, lung recruitment is basically an inspiratory phenomenon occurring during tidal ventilation, whereas PEEP prevents expiratory derecruitment. However, before adopting such a recruitment maneuver in clinical practice, it seems essential to verify experimentally that it is more efficient than a sustained inflation for recruiting a surfactant-deficient collapsed lung.

An interesting result of the study, although not entirely new, is that a recruitment maneuver seems superior to a simple PEEP titration for decreasing pulmonary shunt, improving arterial oxygenation, and reopening a surfactant-deficient lung. This beneficial effect is accompanied by an increase in respiratory compliance and a decrease in plateau inspiratory pressure, reflecting the greater lung re-aeration. This finding is in accordance with previous studies demonstrating that at a given PEEP, the end-expiratory aeration is markedly influenced by the preceding end-inspiratory pressure: the higher the end-inspiratory pressure (or volume), the higher the end-expiratory aeration. 13,19–21In fact, as confirmed by Lim et al. , the administration of a recruitment maneuver boosts the ventilatory cycle onto the deflation limb of the pressure-volume curve. 22If the duration of the recruitment maneuver is long enough, then the beneficial effects may be long-lasting, as previously demonstrated in patients with ARDS. 18However, it must be outlined that the superiority of a recruitment maneuver over a simple PEEP titration for recruiting collapse prone lungs was not retrieved in patients with ARDS. 23In a series of 17 patients whose ARDS was predominantly caused by a primary insult to the lung, Villagra et al.  23compared the effects of a simple PEEP titration with the effects of a recruitment maneuver very similar to the one used by Lim et al.  The superimposed recruitment maneuver provided no additional increase in arterial oxygenation in most of the patients whose lungs were probably not collapsed but rather were filled with edema, blood, and inflammation. In such patients, lung recruitment could be optimized by setting the PEEP 3 to 4 cmH2O above the lower inflection point of the pressure-volume curve.

In fact, the most striking contribution of Lim et al.  is to bring convincing evidence that providing lung recruitment may be associated with a risk of overinflating significant parts of the lungs. At a PEEP of 3 cmH2O, 40% of the lungs were normally aerated, with a wide range of CT attentuations, indicating a nonhomogeneous regional distribution of the lung collapse. As observed in humans, in whom “focal” loss of aeration predominating in lower lobes is the most frequent morphologic pattern characterizing ARDS, 24lung collapse was found predominantly in caudal and dependent parts of the lungs. According to the “sponge” theory, the “superimposed” pressure increases in dependent lung regions, which require high intrathoracic pressures to be reopened. These high opening pressures were generated during the recruitment maneuver, explaining a recruitment of caudal lung regions that was not obtained with a simple PEEP titration. Unfortunately, a high opening pressure for a collapsed lung becomes a high “distending” pressure for a normally aerated lung, and it is not surprising that both methods of recruitment induced significant lung overinflation. At end-expiration, 12% of the overall lung volume was overinflated, reintroducing a risk of ventilator-induced lung injury. These experimental findings are in accordance with several human studies that clearly reported the simultaneous onset of alveolar recruitment and lung overinflation in patients with ARDS receiving PEEP levels ranging between 10 and 20 cmH2O. 3,25–28Experimentally, lung overinflation was also reported in Escherichia coli  bronchopneumonia 29and in oleic acid-induced lung injury, 11two models in which alveolar flooding rather than lung collapse plays a determinant role in the loss of aeration. Interestingly, lung overinflation slightly increased at end-inspiration, reaching 14% of the overall lung volume and suggesting that tidal ventilation essentially resulted in lung recruitment. Similarly, lung overinflation was not significantly greater following the recruitment maneuver, suggesting a predominant effect on lung re-aeration.

With regard to the clinical application of recruitment maneuvers and, more generally, to the different techniques of alveolar recruitment, it seems essential to consider the risk of overdistension and to avoid focusing exclusively on the potential for recruitment. 30Optimizing alveolar recruitment can be defined as providing the greatest lung re-aeration without inducing significant lung overinflation. Because a recruitment maneuver is not likely to increase lung overinflation, as demonstrated by Lim et al. , it could be an attractive adjunct to PEEP for optimizing the re-aration of a collapse-prone lung.

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