Interpretation of the pulmonary artery occlusion pressure in mechanically ventilated patients with large respiratory excursions in intrathoracic pressure

Objective: To assess the reliability of the pulmonary artery occlusion pressure (Ppao) when respiratory excursions in intrathoracic pressure are prominent. Design: We studied 24 critically ill patients who had 15 mm Hg or more of respiratory excursion in their Ppao tracing. Large respiratory excursions resulted from respiratory muscle activity that persisted despite sedation and mechanical ventilation in the assist-control mode. From the Ppao tracing, the end-expiratory and mid-point values were recorded; the latter was measured halfway between end-expiration and the nadir due to inspiratory triggering. The Ppao was then re-measured after administration of a non-depolarizing muscle relaxant. Setting: Medical intensive care unit of a university-affiliated teaching hospital. Measurements and results: The difference between the pre-relaxation end-expiratory Ppao and the relaxed Ppao was larger than the difference between the pre-relaxation mid-point Ppao and the relaxed Ppao (11 ± 5 vs 3 ± 3 mm Hg, p < 0.01). In 21 of 24 (88%) cases, the relaxed Ppao was more closely approximated by the mid-point Ppao than by the end-expiratory Ppao. The difference between the end-expiratory Ppao and the relaxed Ppao increased as the amount of respiratory excursion increased (r = 0,51; p < 0.01). Conclusions: In mechanically ventilated patients whose respiratory muscles produce large excursions in the Ppao, the end-expiratory Ppao is often much higher than the Ppao measured after muscle relaxation. The pre-relaxation mid-point Ppao and the relaxed Ppao are usually similar, but this may not be true in individual patients. In this setting, the Ppao measured after muscle relaxation probably provides the most clinically reliable estimate of left heart filling pressure.