The respiratory and cardiac variations of intrathoracic pressure and their significance in cardiac contraction

Abstract

When intrathoracic pressure is recorded by a trocar connecting with a calibrated Frank's segment capsule it is found that the intrathoracic pressure does not change smoothly with inspiration and expiration, but each respiratory variation consists of a series of negative and positive cardiac changes. The ratio between the cardiac and the respiratory variations range from 1 : 3 to 1 : 6. Thus, in an animal whose entire intrathoracic variation was 36 mm. of water, the cardiopneumatic changes were equal to 11 mm. during an apnea period, increased to 15 mm. in inspiration and fell to 9 mm. during expiration. A comparison with simultaneous intraventricular pressure curves shows that the negative pressure decreases slightly during the period of rising intraventricular tension; then, as the ejection period begins, gives a sharp vibration and then drops sharply until it reaches a turning point, after which the curve follows the reverse of the contour of the intraventricular pressure curve.

Are these variations in whole or in part responsible for the inspiratory fall of arterial and intraventricular pressures that occur when cardiac rhythm is regular? It is conceivable that the more negative pressure during inspiration might do this either by directly counteracting the cardiac systole or by diminishing its vigor through a decrease in the initial intraventricular tension at the beginning of systole. In either case the steepness of the isometric rise of the curve should show a decrease. That this is so is shown in experiments where considerable negative pressure is applied to the heart by a cardiometer over the top of which the pericardium was tied.

When extracardial pressures equal to those previously existing in the closed thorax are applied, however, the records so far obtained show no alteration in the steepness of the curve, nor is any difference discernible when the cardiometer is left open to the air or in communication with a tambour within which a pressure equal to 15 mm. of water develops during systole.