MECHANICAL EFFECTS OF PEEP IN PATIENTS WITH ADULT RESPIRATORY-DISTRESS SYNDROME

In 10 patients with adult respiratory distress syndrome, we studied the effects on respiratory system mechanics of two levels of positive end-expiratory pressure (PEEP), best PEEP (BP) and half of this value (HBP), using a respiratory inductive plethysmograph (RIP) combined with a super syringe. We found the following. 1) Inflation compliance of pressure-volume (PV) curves did not change significantly. 2) End-expiratory volume increased with HBP and further with BP (279 ± 186 and 464 ± 313 ml, respectively, P < 0.01). This increase was positively correlated with inflation compliance for HBP and BP (r = 0.794, P < 0.01 and r = 0.876, P < 0.02, respectively). 3) No dynamic hyperinflation was detected on mechanical ventilation at zero end-expiratory pressure (ZEEP), and the time constant of the respiratory system was in the normal range (0.79 ± 0.21 s). 4) Hysteresis of PVrip curves, which were corrected for gas exchange, decreased significantly with PEEP (P < 0.05). We conclude that PEEP does not change inflation PV curve but induces an increase in intrathoracic volume whose magnitude is related to compliance and PEEP level. The reduction of hysteresis with PEEP suggests less gas trapping and thus a functional improvement.