FUNCTIONAL ANATOMY OF CARDIAC PUMPING 



77 1 



of the fibers of the papillary muscles and trabeculae 

 carneae which results in a tension of the chordae 

 tendineae, and an approximation of the atrioventri- 

 cular valves (139)- Simultaneously, there is a passive 

 stretching of the other still relaxed myocardial layers, 

 mainly those of the outer walls of the heart [see also 

 Hawthorne (67), Anzola (4), and Burton (29)]. 

 The older term ''isometric contraction'' had the 

 misleading implication that all myocardial fibers 

 contract simultaneously and isometrically from the 

 very start. Since in fact some muscle fibers shorten 

 whereas others are passively lengthened during this 

 phase, while the intraventricular volume remains 

 constant, the term isovolumetric contraction provides 

 a more accurate description than isometric con- 

 traction. Apparently instrumentation has not yet been 

 refined sufficiently to decide whether or not there is 

 in this phase a brief "latent relaxation" of cardiac 

 muscle fibers as there exists in skeletal muscle fibers. 



The shortening of the ventricle in the longitudinal 

 axis results in a descent of the atrioventricular 

 junction which in turn expands the atrial cavities. 

 This leads to a precipitous lowering of the atrial 

 pressure (fig. 14) which is often observed even before 

 ' the ventricle ejects blood. The ventricular muscle 

 fibers contract in a successive order, probably follow- 

 ing the same time sequence as their depolarization 

 (75, 142). As a consequence the blood contained in 

 the ventricular cavity is pushed from the apex region 

 toward the center of the ventricle and moves thereby 

 closer to the outflow tract. The subsequent ejection 

 from the ventricles can be looked upon as a con- 

 tinuation of the intraventricular movement of blood 

 which already starts before the semilunar valves open. 

 At the same time the ventricular cavity changes from a 

 cylindrical to a more spherical shape, which from 

 the energy standpoint represents a more economical 

 way of discharging the ventricular content, once the 

 aortic diastolic pressure is overcome. As pointed out 

 by Rushmer (139), the asynchronous contraction of 

 the ventricular myocardium readily explains the 

 brief upward deflection at the beginning of iso- 

 volumetric contraction in the ventricular volume 

 curve described by Wiggers (156) in fig. 14. 

 This was formerly interpreted as an artifact in the 

 recording. 



Some arbitrariness is invoked in determining 

 accurately the start of isovolumetric contraction. In 

 all pressure tracings the upward movement begins 

 slowly in the form of a rounded curve. There is no 

 abrupt beginning, inflection, or break. This becomes 

 especially evident if one records the pressure events 



by drawing out the time axis with fast moving paper 

 as can be easily done today with electrical recording 

 apparatus. The rounded beginning of the upward 

 limb results from the combined effect of a) the 

 contraction of the papillary muscles, and b) the 

 simultaneous passive distention of some of the muscle 

 fibers in the ventricular wall. Whenever the trans- 

 figuration of the ventricle causes a detectable rise of 

 intraventricular pressure, then by convention the 

 ventricular isometric contraction is said to begin. The 

 fact that the different strands of myocardial fibers 

 contract in sequence rather than simultaneously may 

 also explain the great variability of the slopes of the 

 pressure tracings in the early part of isovolumetric 

 contraction. 



The steepness of the slope during isovolumetric 

 contraction is predominantly determined by the 

 forcefulness of the fiber contraction. If the difference 

 between the end-diastolic ventricular and end- 

 diastolic aortic pressure remains unchanged, the 

 duration of the ventricular isovolumetric contraction 

 is shortened by sympathetic or sympathomimetic 

 stimulation and lengthened by agents or conditions 

 which depress the sympathetic control of the heart 

 [Cotton & Maling (35), Gleason & Braunwald 

 (54); see also Reeves et al. (133)]. Thus in forcefully 

 contracting ventricles, the slope will be steeper than 

 in feebly contracting preparations. 



The atrioventricular valves close approximately at 

 the beginning of isovolumetric contraction; the 

 opening of the semilunar valves marks the end of this 

 phase. The precise moment of the valve actuation is 

 difficult to establish experimentally (discussed in the 

 section on heart valves). In the interval between 

 closure of the atrioventricular valves and opening of 

 the aortic and pulmonary valves, the blood contained 

 in the ventricular cavities is temporarily isolated from 

 the fluid columns in the atria and arteries. However, 

 the ventricular content does not remain still (10). 

 In fact the blood which rushed into the ventricles at 

 high velocity during diastole may aid in expanding 

 the ventricular cavities. Since the inflow is primarily 

 directed toward the apex, it is this part of ventricular 

 wall which could be preferentially expanded. As the 

 papillary muscles and trabeculae carneae begin to 

 contract, the movement of the blood is deviated 

 toward the outflow tract. This change in direction of 

 flow is favored anatomically by the fact that the axis 

 of the inflow tract and that of the outflow tract form 

 an angle. In other words, the inflowing blood prob- 

 ably does not come to a complete standstill in order to 

 reverse its direction of flow for ejection into the 



