Variations of Intrathoracic Pressure. 53 



secondarily upon its relation to the body. It may be shown that 

 under certain conditions, the head is the only part affected. 



31 (848) 



The respiratory and cardiac variations of intrathoracic pressure 

 and their significance in cardiac contraction. 



By Carl J. Wiggers. 



[From the Physiological Laboratory, Cornell University Medical 

 College, Neiv York City.] 



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 I : 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 vibra- 

 tion and then drops sharply until it reaches a turning point, after 

 which the curve follows the reverse of the contour of the intra- 

 ventricular 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. 



