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From the Authors:

The method we have used does not require any underlying assumption about hysteresis. To calculate recruitment as the difference between two curves, we measured the trapped volume above the relaxation volume of the respiratory system by passive spirometry and traced a new pressure–volume curve from this lung volume. The only assumption is that the relaxation volume does not change throughout the experiment. This method does not measure the progressive derecruitment that could occur over a longer period of time at low or zero end-expiratory pressure. We believe, however, that this would require other methods to estimate lung volume because the so-called relaxation volume of the respiratory system would change. The same technique can be applied with expiratory curves.

Crotti and coworkers used the lung computer tomography scan in five patients and had a different study design and analysis (2). Their data show (Figure 6 [2]) that a positive end-expiratory pressure (PEEP) of 10 cm H₂O is necessary to keep derecruitment just below 50% of the previously recruited lung, whereas a PEEP of 5 cm H₂O would be associated with 60% of derecruitment. Figure 7 (2) allows the evaluation of the incremental fractional derecruitment observed at each PEEP level compared with the preceding pressure. In this figure, the fractional derecruitment observed at a PEEP of 20 cm H₂O (compared with end-inspiratory plateau pressure) is surprisingly higher or similar than between all inferior levels, including a PEEP of 0 cm H₂O. This graph could lead to the conclusion that maximal derecruitment occurred at a PEEP of 20 cm H₂O. However, we definitely would not use this argument to recommend such PEEP levels.

Both studies are thus consistent in showing, as stated by Dr. Jardin, that prevention of cyclic collapse and reopening, if desirable, could only be obtained by a PEEP close to the plateau pressure. Neither of these two studies can tell us what level of PEEP might be used in patients, because they explore only one physiological effect of this therapy, ignoring for instance the hemodynamic effects of ventilation, dear to Dr. Jardin's heart. Both studies say, however, that if it is important to minimize derecruitment, a PEEP of at least 10 cm H₂O needs to be used to prevent 50% of derecruitment.

Laurent Brochard^a and Salvatore Maggiore^b

^a Hôpital Henri Mondor, Université Paris 12 Creteil, France
^b Universitá Cattolica del Sacro Cuore Rome, Italy

REFERENCES

1. Maggiore SM, Jonson B, Richard J-C, Jaber S, Lemaire F, Brochard L. Alveolar derecruitment at decremental positive end-expiratory pressure levels in acute lung injury: comparison with the lower inflection point, oxygenation, and compliance. Am J Respir Crit Care Med 2001;164:795–801.[Abstract/Free Full Text]
2. Crotti S, Mascheroni D, Caironi P, Pelosi P, Ronzoni G, Mondino M, Marini J, Gattinoni L. Recruitment and derecruitment during acute respiratory failure: a clinical study. Am J Respir Crit Care Med 2001; 164:131–140.[Abstract/Free Full Text]

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