Other things being equal, slowing of the heart means improved supply of coronary blood, resulting in better nutrition and better recuperative power. It has been demonstrated by Stewart and Pike (1910) that the heart will not continue beating unless there is a certain intracoronary pressure.

The time of filling of the heart, i e., the diastole proper, depends upon the venous pressure, and is usually not much greater than the time of systole. The remainder of the diastolic pause, i. e., the diastasis, is the period during which food and oxygen reach the heart through the coronary arteries and during which the heart recuperates. If the period of diastasis is shortened, the heart beats more frequently, and its output per minute is increased. But if the shortening of the diastasis is too great, or if there is no diastasis, the heart soon fails for lack of a period of nutrition and rest. The maximum output occurs when the period of diastasis is just abolished, but under such conditions the heart cannot long maintain its efficiency. On the other hand, if the period of diastasis is too prolonged, the heart beats so few times in a minute that it cannot maintain adequate arterial pressure. Thus it is evident that failure of the circulation may result from too few beats per minute or from too many beats. And it may be assumed that for each heart there is an optimum rate, which is the rate that gives the greatest number of beats consistent with a proper resting period. This optimum rate is neither the maximum rate nor that which allows the greatest output of blood; so that the effect on the rate of the heart is not the criterion of efficiency for a circulatory drug.

Regardless of which control mechanism is utilized, the heart's action can practically be modified as regards its rhythm, its rate, its contractility, its tone, its irritability, and its conductivity. The rhythm is either regular, irregular, or intermittent, and may be influenced by changes in irritability and conductivity. If the rate is changed, it must be either slower or faster; if the contractility is changed, it must be either weaker or stronger. If there is an alteration in tone, the degree of relaxation in diastole must be either greater or less.