77° 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



became the deciding guidelines for characterizing the 

 phases of the cycle. The generally adopted sub- 

 divisions of Wiggers (156) stem from this era. Since 

 other landmarks of cardiac activity such as flow, 

 volume changes, or biochemical processes were 

 difficult to record adequately, they were only corre- 

 lated with the pressure curves at a later date. 



It is still impossible to subdivide the cardiac cycle 

 according to the most important physiological 

 events : the blood flow into and out of the cavities. 

 The approximate beginning and end of systolic 

 ejection can be determined from simultaneous pres- 

 sure tracings in a ventricle and in an arterial outflow 

 tract. However, the precise timing of flow is only 

 possible through direct recording of flow either at the 

 root of the aorta or at the pulmonary artery [see also 

 Moscovitz & Wilder (117)]. The recent advent of 

 refined flowmeters will probably necessitate some 

 adjustments in the original Wiggers scheme of the 

 cardiac cycle. For the time being it is still preferable 

 to retain the well-established scheme and to fit 

 modifications into it, rather than to advocate a com- 

 pletely new one [see also Horowitz (80)]. 



Figure 14 [modified from Wiggers (156, 159)] 

 illustrates in schematic form the sequence of pressure 

 events during the cardiac cycle in the left ventricle, 

 left atrium and aorta, and the volume changes in the 

 combined ventricles [from Henderson (69)]. For 

 time correlation, tracings of the heart sounds and 

 of the electrocardiogram are added. This composite 

 chart is mainly based on curves obtained in animal 

 experiments. 



The cycle is divided into two periods, systole and 

 diastole. The former begins with the rise of ventricular 

 pressure caused by ventricular contraction (fig. 1 4, 1 ) 

 and ends at the onset of myocardial relaxation, 4, 

 at the point when ejection actually ceases. This point 

 then also represents the beginning of the diastole. 

 The systolic period is subdivided into 1-2, isovolu- 

 metric ventricular contraction (50 msec); 2-3, 

 maximum ventricular ejection (90 msec); and 3-4, 

 reduced ventricular ejection (130 msec). The diastolic 

 period is subdivided into 4-5, isovolumetric ventricular 

 relaxation (120 msec), which includes a phase 

 occurring just prior to the incisura and formerly 

 called protodiastole (40 msec), plus the phase formerly 

 known as isometric relaxation (80 msec); 5-6, rapid 

 ventricular filling (no msec); 6-7, slow ventricular 

 filling or diastasis (190 msec); and 7-1, ventricular 

 filling by atrial contraction (60 msec). 



Numerous other cyclical events occur with each 



fig. 14. Scheme of the cardiac cycle. Time, totaling 1 sec, 

 on upper margin. Numbers under lower margin indicate be- 

 ginning and end of phases. Period of- ventricular systole lasts 

 from 1 to 4, period of ventricular diastole lasts from 4 to 1 . 

 Detailed description in text. [Figure (but not numbers in text) 

 slightly modified from Wiggers (159).] 



heart beat. They are correlated timewise with the 

 phases of the pressure-volume cycle as follows. 



1-2: Isovolumetric ventricular contraction. During this 

 phase the myocardium builds up tension and this 

 gives a fast rise of intraventricular pressure without 

 change in the volume of blood contained in the 

 ventricular cavity. The intraventricular pressure 

 must rise to the level of the diastolic pressure pre- 

 vailing in the aorta (or pulmonary artery) before 

 blood can be ejected from the ventricles during the 

 next two phases. The term "isovolumetric contrac- 

 tion" suggested by Rushmer (139) should supersede 

 the older term "isometric contraction," since at the 

 beginning of this phase there is an actual shortening 



