FUNCTIONAL ANATOMY OF CARDIAC PUMPINC 



775 



ventricular pressures are recorded with instruments of 

 sufficient sensitivity, the transfer of the atrial pressure 

 rise can be observed on the ventricular pressure trac- 

 ing, since atrium and ventricle form a common cavity 

 during atrial systole. Similarly, after the peak pressure 

 of atrial systole has been reached, the pressure drops 

 not only in the atrium but also in the ventricular 

 cavity. When the atrioventricular valves actually 

 close is still a matter of debate (see also later section). 

 It may well be that the large valve leaflets begin to 

 approximate each other at the moment when the 

 atrial pressure starts dropping and that they continue 

 to move toward each other because ventricular blood 

 flows into the large spaces behind the closing leaflets. 

 Therefore, the valves may start to close at a time 

 when the ventricular pressure is decreasing slightly. 

 Complete closure would then be achieved when the 

 dropping atrial and intraventricular pressures level 

 off (Z point in atrial pressure curve). 



As the heart rate becomes faster under sympathetic 

 stimulation or in exercise, the period of slow ventric- 

 ular filling is progressively shortened by an earlier 

 onset of the atrial systole [see Mitchell ct a/. (113)]. 

 At heart rates above 1 20 per min the phase of slow 

 ventricular filling is more or less abrogated (56). 

 Figure 17 illustrates how at a heart rate of 160 per 

 min the phase of rapid ventricular filling is directly 

 followed by the inflow due to atrial contraction. In 

 these curves one cannot discern the usual hump in the 

 upstroke of the filling curve which occurs when rapid 

 ventricular filling changes to slow ventricular filling 

 before the atrium adds its contribution. It is likely that 

 at still faster heart rates, the atrial component of the 

 curve blends completely with the inclined tracing 

 characteristic of rapid ventricular filling. At extreme 

 degrees of tachycardia even the phase of rapid ventric- 

 ular filling may be encroached upon. This would 

 explain why the stroke volume decreases at very- 

 rapid heart rates since there is not sufficient time 

 for adequate filling of the ventricle. 



The force of atrial contraction usually varies 

 concomitantly with that of ventricular contraction. 

 Therefore, the percentile contribution of atrial 

 systole to ventricular filling is lower under vagal 

 influence and higher under sympathetic excitation. 

 In extreme tachycardia, when atrial systole begins 

 during the phase of rapid ventricular filling, the 

 actual contribution of the atrium to the filling of the 

 ventricle may be as high as 30 to 40 per cent. The 

 atrial contraction would then serve to increase the 

 pressure difference between the atrium and ventricle 

 in the later part of the rapid filling phase and thereby 



0.5 sec 



Fig. 17. Phase relationships among aortic pressure, [eft 

 ventricular volume (dots), atrial volume (open circles), and 

 electrocardiogram (ECG), in an anesthetized normal dog with 

 a spontaneous heart rate of 160/min. The pressure tracings 

 were simultaneously recorded and correlated with the volume 

 measurement from the kinematographic frames. [Original 

 curves and labeling by courtesy of Pco Gribbe, Wenner-Grcn 

 Research Laboratory, Norrtull's Hospital, Stockholm, Sweden 

 (personal communication, 1 96 1 ).] 



produce a maximal velocity of inflow throughout the 

 entire, though brief, phase of rapid ventricular 

 filling. 



CORRELATION OF OTHER CARDIAC EVENTS 

 WITH THE CARDIAC CYCLE 



The time sequence of cardiac events originally 

 described by Wiggers (156) was based upon those 

 pressure changes in the circulatory system which were 

 measurable at the time. However, during the last 

 three decades our trend of thinking about cardiac 

 events has been greatly affected by the progress of 

 electrocardiography. At the very beginning of the 

 investigations of the field (about 1910-1920) the 

 electrocardiogram could only be correlated second- 

 arily with the time course of the more easily meas- 

 urable pressure events (138). Nowadays one can 

 record electrocardiograms with a higher degree of 

 time resolution than intracardiac pressures. For this 

 reason it is rather common to use the electrocardio- 

 gram as the basis or guideline for dividing the cardiac 

 cycle into phases and then to fit secondarily the 

 pressure and flow events into the patterns of the 

 electrical events (17, 157). However, there is a varying 

 time lag between electrical and mechanical events 

 under different experimental conditions [see Luisada 

 & Liu (104)], so that such a correlation system is not 

 entirely satisfactory. 



