Interaction between intrinsic positive end-expiratory pressure and externally applied positive end-expiratory pressure during controlled mechanical ventilation

OBJECTIVE: To assess the interaction between intrinsic and externally applied positive end-expiratory pressure (intrinsic PEEP and administered PEEP) in mechanically ventilated patients. DESIGN: Prospective study. SETTING: Intensive care unit of a university hospital. PATIENTS: Twelve consecutive critically ill patients. INTERVENTIONS: Application of an external PEEP of the same value as the intrinsic PEEP. MEASUREMENTS AND MAIN RESULTS: We found that when the administered PEEP was increased from 0 to the baseline value of intrinsic PEEP, mean intrinsic PEEP decreased from 6.5 +/- 4.2 (SD) to 1.3 +/- 0.7 cm H2O (p = .001). The mean end-inspiratory pressure was increased from 20.3 +/- 4.6 to 23.1 +/- 6.1 cm H2O (p < .05). The difference between the modification of intrinsic PEEP and the change in the end-inspiratory pressure was not significantly > 0 cm H2O. Thus, the increase in end-inspiratory pressure may be directly attributable to the increase in total PEEP (administered PEEP plus intrinsic PEEP). None of the other measurements of pulmonary mechanics changed (peak pressure, inspiratory resistance, compliance, and trapped-gas volume). CONCLUSIONS: The administration of positive end-expiratory pressure equal to the intrinsic positive end-expiratory pressure causes the almost total disappearance of the intrinsic positive end-expiratory pressure. When the administered positive end-expiratory pressure does not exceed the intrinsic positive end-expiratory pressure, the former is applied almost in its entirety to the patient's external circuit. The administration of positive end-expiratory pressure without prior quantification of the intrinsic positive end-expiratory pressure results in an overestimation of the beneficial effects of the administered positive end-expiratory pressure on the quasi-static compliance.