THE CONTROL OF THE FUNCTION OF T HF, HEART 



519 



activity of the central nervous system in relation to 

 acutely induced changes, two concise statements now 

 appear to be appropriate for the heart operating at 

 any given rate and aortic pressure, and in the absence 

 of abnormal conditions such as hypoxia and acidosis. 

 / ) If the eflective catecholamine stimulus remains 

 constant, the contraction of the ventricle varies 

 directionally with its end diastolic pressure and fiber 

 length; if the end diastolic pressure and fiber length 

 remain constant, the contraction of the ventricle 

 varies directionally with the effective catecholamine 

 stimulus. 2) The central nervous system has direct 

 neural connections to the heart by means of which it 

 can vary the left ventricular end diastolic pressure 

 and fiber length while keeping the effective catechol- 

 amine stimulus constant (atrial systole), means by 

 which it can vary the effective catecholamine stimu- 

 lus, or both. 



III. INFLUENCE OF THE CAROTID SINUS ON THE 



PERFORM.-KNCE CH.^R.\CTERISTICS OF THE HE.\RT 



The conventional view of the reflex function of the 

 carotid sinus has been that it primarily influences 

 heart rate and peripheral arteriolar resistance (49). 

 That the carotid sinus can influence the pressure- 

 volume relation of visceral veins was established by 

 Alexander (2), whose position was supported by 

 experiments with peripheral venous micro-balloons 

 (94, 95, 102) and the definitix'e experiments of 

 Bartelstone (8). 



More recently, attention lias iDeen focused on the 

 changes in the contractilit\ of the ventricle and 

 atrium which can be reflexK induced by carotid 

 sinus stimulation (106). 



A. Carotido-]'en!tuular Reflex 



W'hen either the right or left carotid sinus nerve is 

 stimulated while heart rate is held constant, results 

 like those shown in figure 24 are obtained. Bilateral 

 cervical vagotomy had been done, therefore the 

 changes observed can be attributed to a reflex altera- 

 tion in sympathetic activity. The heart rate was held 

 constant at i 78 per min by left atrial pacing, and both 

 carotid sinus nerves were stimulated at 3.5 volts, 25 

 per sec, and 5 msec impulse duration during the 

 interval indicated at the bottom. During stimulation, 

 mean left atrial pressure rises while aortic pressure, 

 stroke volume, and calculated stroke work falls. The 

 PA-LA pressure difference narrows. The directional 



25 r 



R.A, 

 cm 



HgO 



250 



A. P. 

 mm 



Hg 





 25 



umau. 



wm 



mi-i 



L.A. 

 cm 





 50 



P. A. 



cm 



HgO 



ii 



J0 



i»wiii>«iiM»ffWkiiiiim 



^'miimi^iii0il» 



FIG. 24 



A.F. 

 L./min 



I - 



\- 



^_1:0_^: 



4 



changes observed are similar to those seen when 

 stellate stimulation is withdrawn. 



Using a preparation in which carotid artery pres- 

 sure can be varied independently of systemic pressure 

 (106), an examination can be made of the effects of 

 changing carotid artery pressure on myocardial 

 contractility while heart rate is held constant. In this 

 study both vagi are sectioned in the neck, and both 

 stellate ganglia are intact. In figure 25/I the panel to 

 the left {A, B, C, D) shows the hemodynamic effects 

 of acutely changing carotid perfusion pressure (CSP). 

 The changes in aortic flow which occur in the first 

 30 sec after the pump induced cliange in carotid 

 pressure are not reliable. Initially, when the carotid 

 pressure is high and pulse pressure large, aortic flow, 

 aortic pressiu'e, and left ventricular stroke work are 

 low in the presence of an elevated left atrial pressure. 

 When carotid perfusion pressure is lowered, aortic 

 flow and pressure and left ventricular work rise 

 substantially while mean atrial pressures fall. It is 

 noteworthy that the increase in the left \entricular 

 stroke work, even at a lower filling pressure, is several 

 times the observed increase in peripheral vascular 

 resistance. This result is a reproducible one and, as 

 shown in the segmented panel at the right in figure 

 25.4 (E, F, G, H, and /), is a gradable effect. That is, 

 the relation between filling pressure and external 



