REDISTRIBUTION OF PULMONARY BLOOD-FLOW INDUCED BY POSITIVE END-EXPIRATORY PRESSURE AND DOPAMINE INFUSION IN ACUTE RESPIRATORY-FAILURE

Abstract

The mechanism by which mechanical ventilation (MV) with positive end-expiratory pressure (PEEP) improves hypoxemia in patients with acute respiratory failure (ARF) is unclear, and may be attributed in part to a decrease in cardiac output inducing by itself a reduction of the shunt. Using the multiple inert gas elimination technique were evaluated, the effects of PEEP on ventilation-perfusion (.ovrhdot.VA/.ovrhdot.Q) distribution were evaluated in 8 patients while cardiac output was maintained at control value by means of a dopamine infusion. In each patient, evaluation was performed during MV without PEEP (control) then with PEEP (17 .+-. 2 cm H2O) and dopamine. After application of PEEP, PaO2 [arterial partial pressure], P.hivin.vO2 [mixed-venous partial pressure] and O2 transport (TO2) increased significantly: venous admixture decreased from 37.5 .+-. 5 to 17 .+-. 2% (P < 0.01). Comparison of .ovrhdot.VA/.ovrhdot.Q distribution during PEEP and zero end-expiratory pressure documented a redistribution of pulmonary blood flow; the shunt decreased markedly from 30 .+-. 4 to 13 .+-. 2% (P < 0.001); the fraction of cardiac output distributed to normal .ovrhdot.VA/.ovrhdot.Q ratio units (0.1-10) increased from 62-78.5% (P < 0.001). Dead space increased slightly with PEEP, from 44-49% (P < 0.01) of total ventilation. The pattern of ventilation distribution was essentially unaltered; specifically, no additional high .ovrhdot.VA/.ovrhdot.Q mode was observed during PEEP. Cardiac output maintenance with dopamine infusion during PEEP does not suppress the beneficial effects of PEEP on gas exchange, but induces a redistribution of pulmonary blood toward the main .ovrhdot.VA/.ovrhdot.Q ratio.