THE EFFECT OF POSITIVE AND NEGATIVE INTRATHORACIC PRESSURE ON CARDIAC OUTPUT AND VENOUS PRESSURE IN THE DOG

Abstract

Cardiac output was measured by the direct Fick method and by a modification of Stewarts method in dogs breathing O2 and air at a pressure of 8 and 16 cms. of water below atmospheric, 8 and 16 cms. of water above atmospheric and at atmospheric pressure. Peripheral venous and right auricular pressures were measured simultaneously in dogs breathing air from a chamber in which the pressure varied as above. The control cardiac output detns. with the Fick method showed considerable variation while the control detns. with the modified Steward method showed little variation. When air under a positive pressure of 16 cms. of water was breathed the cardiac output (Stewarts method) was decreased 33% of the control, while the pressure fall from peripheral vein to right auricle was decreased 72% of the control on the average. When air under a negative pressure of 16 cms. of water was breathed there was little change in the cardiac output while the pressure fall from peripheral vein to right auricle increased 244% on the avg. Since the cardiac output of the dog changes very little when air under a negative pressure of 16 cms. of water is breathed it would appear that the maintenance of a high peripheral venous pressure, when right auricular pressure is greatly decreased, is due to the fact that the veins become partially collapsed just before entering the chest and increase the resistance to the flow of blood to the right auricle.