^ 



INTRACARDIAC PRESSURE 57 



THE CARDIAC CYCLE 



Intracardiac Pressure 



When the heart is beating at its normal rate — 72 beats 

 per minute — the complete cycle of changes occupies about 

 0-8 sec. and consists of three phases — 



Auricular systole , . .0*1 sec. 

 Ventricular systole . . .0-3 sec. 

 Diastole .... 0-4 sec. 



The pressure changes occurring in the heart during the 

 cycle have been investigated by the direct introduction 



A 



I 



D 



Fig. 10. — Piper's manometer (from Starling's Principles of Physiology). 



into the chambers of specially-constructed manometers. 

 Of the many forms of these which have been invented, the 

 one which most effectively eliminates instrumental error is 

 Pij)ers (Fig. 10). It consists of a cannula B, fitted with a 

 trocar A. At one side of the cannula, at E, is an elastic 

 membrane, upon which is fixed a mirror F. C is a tap 

 which when open admits the passage of the trocar. The 

 manometer is inserted direct into the desired chamber of 

 the heart, the point of the trocar piercing through the wall. 

 The trocar is then withdrawn and C closed. Changes in 

 pressure in the chamber cause alternate stretching and 

 slackening of the membrane, these movements being 

 recorded in a magnified form by hght thrown on the 

 mirror. The results obtained when manometers are thrust 



