The Effect of Positive and Negative Intrathoracic Pr.es- Sure on Cardiac Output Axd Vex-ous Pressure in the Dog1

It has been shown (1) in the dog that the development of a large negative intrathoracic pressure is associated with a marked fall in superior vena caval pressure and a slight fall in jugular venous pressure, while the development of a large positive intrathoracic pressure is associated with a rise in peripheral venous and superior vena caval pressure. In a recent, paper (2) it was shown that when the intrathoracic pressure of the barbitalized dog was increased the right auricular and peripheral venous pressure increased, and that when the intrathoracic pressure was decreased right auricular pressure decreased but the peripheral venous pressure did not change. Evidence was presented that the peripheral venous pressure remained constant when the auricular pressure decreased because the veins became partially collapsed just before entering the chest and increased the resistance to the flow of blood from the periphery to the right auricle. However, the possibility that the peripheral venous pressure remained constant, when right auricular pressure was decreased, as a result of an increased rate of flow of blood along the veins, was not ruled out. l