THE CONTROL OF THE FUNCTION OF THE HEART 495 



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Consloni Coronory Flow 



Heart RQte = ll2/min 



FIG. 4-.^, B. AoP = aortic pressure; SV = stroke volume; LVED = left ventricular end diastolic 

 pressure; MSL (ED) myocardial segment length at end of diastole. 



increased work was not taking place from a longer 

 initial fiber length. The finding of a decreased myo- 

 cardial extensibility under circumstances of increased 

 coronary perfusion pressure is consonant with the 

 recent evidence obtained by Horres et al. (51) and 

 the intimations put forth by Salisbury and col- 

 leagues (93), indicating that a more tumescent 

 myocardium is less extensible. 



Figure ^B shows the range over which these phe- 

 nomena can occur even when coronary flow is not 

 allowed to vary appreciably as the result of changing 

 aortic pressure. The run labeled I was conducted 

 with mean aortic pressure held approximately con- 

 stant and stroke work increased by increasing stroke 

 volume as indicated in the two bottom panels. Then, 

 at each of four different stroke volumes (runs II, 

 III, IV, and V), stroke work was increased solely 

 by increasing aortic pressure. Figures to left of the 

 curves indicate the stroke volumes performed. In 

 runs III, IV, and V there was a substantial range of 

 stroke work which could be accomplished with little 

 change in LVED pressure. It was of interest to note 

 that the larger the value at which stroke volume was 



held constant, the steeper was the function curve 

 when aortic pressure was increased. 



The transient phenomena that occur immediately 

 after the increase and subsequent decrease of re- 

 sistance to left ventricular ejection are shown in figure 

 5 (upper left). The heart rate was held constant at 

 160 per min. Stroke volume was ii.i ml during the 

 low aortic resistance and 10.3 ml during the high 

 aortic resistance. Left coronary flow was constant. 

 There are four distinct phases. In phase i, immedi- 

 ately after increasing the aortic resistance, LVED 

 pressure rises along with the elevation of aortic pres- 

 sure. The beginning of pha.se 2 (first arrow) occurs 

 shortly thereafter and is signaled by the decline in 

 L\"ED pressure while aortic pressure continues to 

 rise. This decline in LVED pressure continues until 

 the new equilibrium level is reached (phase 3). When 

 the imposed aortic resistance is suddenly removed, 

 LVED pressure drops sharply (second arrow) and to 

 the lowest level observed in the sequence (beginning 

 of phase 4); thereafter, in phase 4, it gradually re- 

 turns to the level which obtained prior to the increase 

 in aortic resistance. 



