The Acute Effects of Atelectasis on the Pulmonary Circulation*

Abstract

The response to acute pulmonary atelectasis was studied in 14 lightly anesthetized dogs breathing spontaneously with chest wall intact. Portions of lung were made atelectatic by endobronchial occlusion. Acute atelectasis produced a two to fourfold increase in respiratory minute volume, a decrease in intrathoracic pressure, and an initial fall in pulmonary arterial pressure usually followed by a slight rise. The percentage of cardiac output shunted during acute atelectasis was greater than the percentage of lung tissue collapsed. In contrast, unilateral ventilation with 100% nitrogen or 92.5% N and 7.5% CO2 reduced the amount of blood flow through the affected lung. Except for a variation in the time of response, there was no difference whether the lung was filled with room air or 100% oxygen before acute collapse. Constant flow perfusions of innervated pulmonary lobes (negative intrathoracic pressure breathing) revealed a sudden fall in perfusion pressure with acute collapse of the perfused area in 5 of 6 preparations. Vagotomy and sympathectomy did not eliminate the response. Direct measurement of pulmonary venous outflow confirmed the increase in blood flow through the acutely collapsed lobe. After acute endobronchial occlusion in the intact dog, blood flow is initially increased through atelectatic lung as a result of mechanical factors. Mechanisms of this effect are discussed.