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EFFECTS OF NERVE STIMULATION AND HORMONES ON THE HEART 543 



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FIG. 6. If a reclining dog abruptly begins to exercise on a treadmill, the \entricular diameter 

 first diminishes as he stands up and then changes little during the subsequent exertion. If the animal 

 is standing during the control period, the ventricular dimensions are changed very little during 

 moderate treadmill exercise. [From Rushmer (33).] 



CARDIOVASCULAR RESPONSES TO EXERTION 



The nature of the changes in ventricular per- 

 formance in dogs during exertion depends to some 

 extent upon conditions during tlie control period. 

 For example, if the animal is reclining quietly on the 

 treadmill until the moment it starts, the ventricular 

 dimensions abruptly diminish as the animal stands, 

 and the diastolic, systolic, and stroke dimensions are 

 all reduced throughout the exercise and thereafter 

 until the animal reclines again (see fig. 6). On the 

 other hand if the animal is standing during the con- 

 trol period, the diastolic and systolic dimensions and 

 stroke deflections are already diminished when the 

 treadmill is started and change only slightly during 

 the exertion. Under these conditions the systolic 

 ejection may be somewhat more complete or diastolic 

 distention slightly greater, but the over-all change is 

 not impressive at moderate levels of exertion (fig. 6). 

 The rate of change of diameter and the peak rate 

 of flow are both greatly accelerated during exercise. 

 The duration of systole is reduced, the ventricular 

 systolic pressure is somewhat elevated, as a rule, and 

 the rise and fall of ventricular pressure are very much 

 steeper. The heart rate is accelerated promptly and 

 the tachycardia is well sustained through the exertion. 

 The cardiac output at the levels of exercise employed 



in these dogs is increased predominantly by ac- 

 celerated heart rate without much increase in stroke 

 volume. In other words, the changes in ventricular 

 performance during moderate e.xercise correspond 

 to the changes in contractility illustrated in figures i 

 and 2 but of somewhat lesser degree. 



During exertion the early diastolic pressure is 

 lower, corresponding to a more rapid early diastolic 

 filling — such as, a more rapid release of intrafascicular 

 tension at the onset of diastole {34). The very low 

 impedance to ventricular filling during early diastole 

 is termed "diastolic suction" by some authors (5, 20). 

 Under conditions of increased "contractility" a more 

 synchronous relaxation of the myocardial fibers may 

 permit a more simultaneous release of the total 

 intrafascicular tension with a further reduction in the 

 impedance to early diastolic filling. The increased 

 ventricular pressure at the end of the diastolic period 

 is clearly related to a more "powerful" atrial contrac- 

 tion, as though the "contractility" increased in both 

 atrial and ventricular myocardium. Although the 

 author previously reported that distensibility was 

 increased by administration of epinephrine to intact 

 dogs (38), this conclusion now seems unwarranted. 

 Since a difference of i or 2 mm Hg in filling pressure 

 may make a very great difference in ventricular 

 x'olume under some conditions, this question must be 



