EFFECTS OF NERVE STIMULATION AND HORMONES ON THE HEART 



537 



A Functional Description of "Increased Contractility'^ 



To illustrate the salient features of increased con- 

 tractility, the cardiac responses to administration of 

 /-epinephrine (20 tig rapidly injected into a 14-kg dog) 

 are shown at right in figure i . This particular example 

 was selected because the typical bradycardia was not 

 present (36, 39, 53); the heart rate was regular at 

 about 140 beats per min. In brief, the various 

 recorded parameters were affected as follows: The 

 effective left ventricular pressure reached peak values 

 about 30 per cent higher than those during the control 

 interval. Visual inspection indicates that ventricular 

 pressure rose and fell more rapidly than during the 

 control. This impression is confirmed by the larger 

 upward and downward deflections in the next lower 

 record, which shows the rate of change of ventricular 

 pressure (dP/dt). The instantaneous rate of aortic 

 blood flow reached higher peak values during systolic 

 ejection under the influence of epinephrine. This 

 increase in ejection velocity was not necessarily 

 reflected in an increase in the volume ejected during 

 each systolic interval. Often, the aortic flow per stroke 

 was increased only slightly or remained unchanged 

 because the duration of systole was reduced by an 

 amount that was roughly equivalent to the increase in 

 ejection velocity. As a result, the area under the 

 aortic flow curves was not significantly changed. The 

 shortening of ventricular systole by epinephrine is 

 quite apparent in all the records. 



The left ventricular diameter was slightly 

 diminished during both diastole and systole. The 

 stroke deflection was not appreciably altered, a 

 finding which confirms that stroke volume was not 

 significantly changed by the action of the epinephrine. 

 The rate of change of diameter resembled the aortic 

 flow. To the extent that the diameter is related to the 

 ventricular volume, the rate of change of diameter 

 must be related to the outflow pattern. However, a 

 sharp downward deflection at the onset of systole 

 did not appear in the record of aortic flow. The 

 abrupt increase in diameter has previously been 

 explained in terms of a sudden change in diameter 

 without a corresponding change in ventricular volume 

 during so-called "isovolumetric" contraction (31). 

 The abrupt downward deflection was also prominent 

 in the record of myocardial "power"'. The negative 

 "power" deflection represented work being done on 

 the myocardial sample between the diameter crystals 

 as affected by contracting myocardium elsewhere in 

 the ventricle (for example, papillary muscles, trabec- 



ular carneae). The peak "power" was increased 

 very markedly by epinephrine, an increase reflecting 

 the accelerated rate at which the myocardium per- 

 forms work during very rapid ventricular ejection. 

 The steps representing stroke work were only slightly 

 higher than those during the control period in spite 

 of the very much higher rate of "power" develop- 

 ment. This point illustrates again that the shortening 

 of the systolic interval very nearly compensates for 

 the much higher deflection, leaving the area under 

 the curve only slightly increased. However, the 

 accumulated stroke work over each 5 sec was very 

 much higher because the tachycardia increased the 

 number of steps per unit time. 



During the control record the heart rate was quite 

 irregular because of sinus arrhythmia. (The indica- 

 tion of heart rate always lags by one cardiac cycle 

 because the rate meter responds to the length of a 

 completed cycle.) During the response to epinephrine, 

 the heart rate was faster and more regular than it 

 had been during the control period. This is an atypical 

 response to epinephrine in the intact animal, but the 

 record was selected because it closely resembles those 

 obtained during sympathetic stimulation. The re- 

 sult of the changes in heart beat was a marked increase 

 in both the accumulated work per unit time and the 

 accumulated flow per unit time (2 sec). 



A unifying generalization emerged from examina- 

 tion of many records: increased ventricular con- 

 tractility is expressed primarily by large changes in 

 "rates" and small changes in quantities (fig. 2). The 

 increase in ventricular systolic pressure is not covered 

 by this statement, but this increased pressure directly 

 reflects the increased rate of ventricular ejection. The 

 rate of change of pressure is very greatly accelerated. 

 The quantity of blood ejected into the aorta per stroke 

 (stroke volume) increases little or not at all, but the 

 rate of ejection is much faster. The diastolic and 

 systolic dimensions of the ventricle are not very 

 different, but the rate at which the diameter changes 

 during systole is increased. The slight increase in 

 stroke volume is commonly accompanied by increased 

 systolic ejection rather than by greater diastolic 

 filling, although both may occur. The computed 

 "work per stroke" changes relatively little although 

 the peak "power" (rate of doing "work") is con- 

 sistently increased. 



Changes in the "physiological condition" of the 

 ventricular myocardium like those illustrated in 

 figures I and 2 are obser\'ed during treadmill exercise 

 by intact dogs and during a number of procedures 



