220 THE CIRCULATION OF THE BLOOD 



When the aortic valve leaks the ventricle becomes overfilled with blood during diastole 

 so that the initial tension is elevated and in the subsequent systole the pressure at- 

 tained is sufficient to compensate for the regurgitation in the sense that a normal 

 mass movement of the blood is maintained (page 208). If the tonus of the myo- 

 cardium be depressed however, the initial pressure does not properly increase and 

 consequently the maximum pressure attained is inadequate, i.e., decompensation 

 occurs. Decompensation may also be the result of exhaustion of the reserve power 

 of the heart. 61 To understand what this means we must remember that the systolic 

 pressure and discharge are proportional to the initial pressure until this reaches a 

 certain limit beyond which with further increase the discharge, etc., decline. This 

 optimal initial pressure varies with the condition of the myocardium ; it is the index of 

 the reserve power. When it is low, decompensation soon occurs so that "back pressure 

 effects" become evident; when it is normal, the response of the ventricle is adequate 

 to compensate for the leak and the circulation may be maintained for a long time 

 without any pulmonary stasis or venous congestion. 



When the auriculoventricular valve leaks (mitral incompetence) the back pressure 

 in the auricle occurs mainly, not during the presphygmic period of ventricular systole 

 as might be expected but during the first part of the sphygmic period (Wiggers 

 and Feil). 



The Reserve Power of the Heart 



When the heart is well developed, as in a trained athlete, the increase in venous 

 inflow which occurs during exercise (see page 219) will stimulate the ventricle so 

 that the output per beat exactly corresponds to the inflow, and the rate is only 

 moderately increased. But when the musculature is feeble, as in a person of sedentary 

 habits, the dilatation must become considerably greater in order to call out sufficient 

 contractile power; the output per beat, therefore, only increases moderately and con- 

 siderable quickening has to occur so that the minute volume may be adequate to 

 meet the increased demands of the muscles for blood. This means that there must 

 be a certain optimal initial pressure in the ventricle and that when this is exceeded 

 the systolic discharge becomes less. The exact pressure which is optimal will vary 

 with the condition of the myocardium, being;, low when this is subnormal. When the 

 reserve power of the heart is very low, even extreme dilatation during diastole may 

 be insufficient to stimulate contractions that are powerful enough to empty the heart; 

 blood is therefore left over in it at the end of systole, and when the venous blood 

 becomes superadded during diastole, extreme dilatation occurs, the beat becomes very 

 rapid in the attempt to maintain an adequate output and "back pressure effects" 

 become evident. The maximal pulse rate that can ultimately be attained during 

 exercise is about 160 beats per minute. 



The Testing of Cardiac Efficiency 



The most practical method for testing the reserve power of the heart is to observe 

 the pulse rate and the systolic blood pressure at frequent stated intervals after a 

 prescribed amount of muscular work. Dumb-bell exercises are useful for this purpose. 

 The pulse rate should be back to normal in at least 4 minutes after the exercise. The 

 blood pressure behaves peculiarly; immediately after the exercise it falls below its 

 height during it, but soon rises again and finally returns to normal within at least 

 four minutes. If the pulse rate and blood pressure are not restored to normal in four 

 minutes after moderate exercise, the cardiac reserve is presumed to be low. 



