Lower inflection point and recruitment with PEEP in ventilated patients with acute respiratory failure.

The lower inflection point (LIP) on the total respiratory system pressure-volume (P-V) curve is widely used to set positive end-expiratory pressure (PEEP) in patients with acute respiratory failure (ARF) on the assumption that LIP represents alveolar recruitment. The aims of this work were to study the relationship between LIP and recruited volume (RV) and to propose a simple method to quantify the RV. In 23 patients with ARF, respiratory system P-V curves were obtained by means of both constant-flow and rapid occlusion technique at four different levels of PEEP and were superimposed on the same P-V plot. The RV was measured as the volume difference at a pressure of 20 cm H(2)O. A third measurement of the RV was done by comparing the exhaled volumes after the same distending pressure of 20 cm H(2)O was applied (equal pressure method). RV increased with PEEP (P < 0.0001); the equal pressure method compares favorably with the other methods (P = 0.0001 by correlation), although individual data cannot be superimposed. No significant difference was found when RV was compared with PEEP in the group of patients with a LIP < or =5 cm H(2)O and the group with a LIP >5 cm H(2)O (76.9 +/- 94.3 vs. 61.2 +/- 51.3, 267.7 +/- 109.9 vs. 209.6 +/- 73.9, and 428.2 +/- 216.3 vs. 375.8 +/- 145.3 ml with PEEP of 5, 10, and 15 cm H(2)O, respectively). A RV was found even when a LIP was not present. We conclude that the recruitment phenomenon is not closely related to the presence of a LIP and that a simple method can be used to measure RV.