In estimating the degree of shunt that is created by one-lung ventilation when it is performed in the lateral decubitus position, on average, 40% of cardiac output perfuses the nondependent lung and the remaining 60% perfuses the dependent lung (Fig. 1).15 Mechanisms that tend to decrease the percent of cardiac output perfusing the nondependent, nonventilated lung are passive (e.g., mechanical-like gravity, surgical manipulation, amount of pre-existing lung disease) or active (e.g., hypoxic pulmonary vasoconstriction).15 The normal response of the pulmonary vasculature to atelectasis is an increase in pulmonary vascular resistance (in the atelectatic lung), and the increase in atelectatic lung resistance is almost entirely caused by hypoxic pulmonary vasoconstriction. Hypoxic pulmonary vasoconstriction is a protective reflex mechanism that diverts blood flow away from the atelectatic lung. With an intact hypoxic pulmonary vasoconstriction response, the transpulmonary shunt through the nondependent lung decreases to approximately 23% of the cardiac output (see Fig. 1).
Anesthesiology Clinics of North America
Volume 19, Issue 3, 1 September 2001, Pages 435-453