Shunt, Lung Volume and Perfusion during Short Periods of Ventilation with Oxygen

Twenty patients requiring ventilation for acute respiratory failure were studied to determine whether intrapulmonary shunt fraction (Qs/Qt) measured at an inspired oxygen concentration (FIO2) of 1.0 differs from Qs/Qt measured at the clinically indicated FIO2 and, if so, the mechanism by which this occurs. Qs/Qt increased from 15.5 +/- 1.8 per cent (mean +/- SE) at the clinically indicated inspired oxygen fraction (FIO2 0.3-0.6) to 21.7 +/- 2.1 per cent after 20 minutes at FIO2 1.0. Functional residual capacity (FRC) decreased by 6 +/- 6 per cent and total compliance (CT) by 10 +/- 6 per cent. Mean pulmonary arterial pressure fell from 21 +/- 2 to 17 +/- 2 mm Hg, whereas pulmonary capillary wedge pressure (PCWP) and cardiac output remained unchanged. Mixed venous oxygen tension increased from 37 +/- 1 to 45 +/- 2 mm Hg with 100 per cent oxygen. At 90 per cent oxygen, Qs/Qt increased from the value at low FIO2, but FRC and CT did not change. Simultaneous application of 100 per cent oxygen and a positive end-expiratory pressure (6 cm H2O) increased FRC, CT and Qs/Qt. Patients with increased PCWP showed smaller increases in Qs/Qt with 100 per cent oxygen. These findings suggest two mechanisms responsible for the increase in Qs/Qt: 1) redistribution of blood flow to nonventilated areas, resulting from the vasodilating effect of an increased oxygen tension in the vessels of hypoxic lung segments; 2) resorption atelectasis. Of the total change in Qs/Qt observed during ventilation with oxygen, 63 per cent was calculated to be due to factors other than a decrease in FRC. (Key words: Ventilation, positive end-expiratory pressure; Oxygen, pulmonary shunt and; Lung, compliance; Lung, shunts.)