© 2002 American Thoracic Society
Alveolar derecruitment at decremental positive end-expiratory pressure levels in acute lung injuryTo the Editor:In a mechanical study that appeared in a recent issue of AJRCCM, Maggiore and coworkers measured derecruitment in a group of 14 patients with acute respiratory distress syndrome (ARDS), by superimposing inspiratory pressure–volume (P/V) curves obtained at decremental positive end-expiratory pressures (PEEPs) (1). For this purpose, they used a symmetric technique that they have previously used to measure recruitment with incremental PEEPs (2). Thus, this methodology implied that recruitment and derecruitment are symmetrical and that no hysteresis is present between inspiratory and expiratory curves in ARDS patients. Hysteresis is a characteristic of surface tension of a normal lung (3), which may well have disappeared in a severely injured lung. The inspiratory low-flow P/V curve is easily obtained with modern respirators, and many reports have been devoted to this curve, with the aim of determining its clinical value, if any. Conversely, because the low-flow expiratory curve is more difficult to obtain, it is usually neglected, and the question of the hysteresis evaded. For example, Amato and coworkers have proposed to use the inspiratory P/V curve to titrate end-expiratory pressure in a given patient (4). A remarkable study by Crotti and coworkers has recently demonstrated that hysteresis is actually present under the clinical conditions of acute lung injury (5). Through measurements in aerated lung from computed tomography scans at different inspiratory and expiratory airway pressures, these authors showed that "threshold opening pressure," a threshold of airway pressure where a maximal increase in aeration occurred, and "threshold closing pressure," a threshold of airway pressure where a maximal decrease in aeration occurred, are actually very different. In this paper, whereas "threshold opening pressure" was close to 20 cm H2O, "threshold closing pressure" was only 5 cm H2O. In other words, setting PEEP at 6 cm H2O, i.e., just above "threshold closing pressure," would avoid cyclic collapse and reopening in Crotti's patients, information of great import for clinicians. The results obtained by Maggiore and coworkers are very different and derecruitment appeared as a continuous phenomenon without any threshold (1). However, the methodology used, neglecting hysteresis, might be inaccurate. If this is not the case, prevention of cyclic collapse and reopening in Maggiore's patients could only be obtained by a PEEP close to the plateau pressure.
University Hospital Ambroise Paré Boulogne Cedex, France REFERENCES
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