To assess the interobserver and intraobserver variability in the clinical evaluation of the quasi-static pressure-volume (P-V) curve, we analyzed 24 sets of inflation and deflation P-V curves obtained from patients with ARDS. We used a recently described sigmoidal equation to curve-fit the P-V data sets and objectively define the point of maximum compliance increase of the inflation limb (P mci,i ) and the true inflection point of the deflation limb (P inf,d ). These points were compared with graphic determinations of lower Pflex by seven clinicians. The graphic and curve-fitting methods were also compared for their ability to reproduce the same parameter value in data sets with reduced number of data points. The sigmoidal equation fit the P-V data with great accuracy (R 2 ϭ 0.9992). The inspiratory limb of the quasi-static pressure-volume (P-V) curve of the total respiratory system has been used to guide mechanical ventilation in the acute respiratory distress syndrome (ARDS) (1, 2). The form of the curve is sigmoidal, with upward concavity at low inflation pressures and downward concavity at higher inflation pressures (3). The pressure at which a rapid increase in compliance occurs is thought to reflect recruitment of atelectatic alveolar units (1). Beyond this point, the "linear" portion of the curve is thought to reflect alveolar compliance. It has thus been postulated that setting PEEP above a point on the inspiratory limb of the P-V curve termed "Pflex" or "lower inflection point" (LIP) might optimize alveolar recruitment (4). In fact, by setting PEEP ϭ Pflex ϩ 2 cm H 2 O in a pressure-limited, "open lung" ventilator strategy in ARDS, Amato and colleagues (5) demonstrated reduced barotrauma, a higher weaning rate, and improved survival at 28 d when compared with a "conventional" ventilator strategy without guidance from P-V curves. Despite these encouraging results, there are few theoretical or experimental reasons to justify the clinical use of Pflex to optimize alveolar recruitment. First, PEEP is conceptually used to prevent derecruitment after a sustained inflation maneuver. Because derecruitment is a deflation phenomenon, it seems reasonable that it would be best identified on the deflation, and not the inflation, limb of the P-V curve (6). Second, there is most likely not a single pressure where recruitment or derecruitment occurs, but rather a distribution of pressures around which the phenomenon progressively takes place. These points have been recognized by other investigators (7). Holzapfel and colleagues (8) compared the reduction in shunt fraction with features of the inflation and deflation P-V curves as PEEP was progressively increased in patients with ARDS. They found that the maximum reduction in shunt correlated best with the true inflection point (the point where concavity changes direction) of the deflation limb of the P-V curve. It has also been shown by nitrogen washout studies in anesthetized patients that the deflation limb of the P-V curve can be used to estimate the pressure required to raise FRC above its closing volume (9). Others have shown with mathematical models that alveolar collapse may occur before the true inflection point has been reached (10). These studies provide support for
The data point with the largest second derivative REF .
An Objective Analysis of the Pressure-Volume Curve in the Acute Respiratory Distress Syndrome
In American Journal of Respiratory and Critical Care Medicine
Medicine