Abstract
A condition of temporary alternation, such as can be induced in dogs' hearts which are normal, according to every criterion we are able to apply, was studied by apparatus recording pressure and volume curves by optical projection.
It was found that temporary alternation induced by the application of premature stimuli to a ventricle or an auricle, by temporary vagus inhibition or by a temporary a-v block, invariably involves both ventricles but does not affect the auricles. Neither the permanence nor duration of the alternation is affected by the volume of venous return or the height of the arterial pressure. Alternation appears to be conditioned entirely by the rate of beat, and cannot be induced in the normal dog's heart when beating at rates below a critical level of about 140 per minute.
The alternating beats resemble those of a more permanent nature induced in other ways. In both cases they differ from the normal, not only in their amplitude of contraction and volume of systolic discharge, but also in the contour of contraction and relaxation and in the duration of their phases. Thus, in the smaller beats the pressure rises more gradually, the isometric contraction phase is prolonged, while systolic ejection and total systole are abbreviated. The isometric relaxation also occurs more slowly so that the interval between the end of systole and the beginning of ventricular inflow is prolonged.
The alternate beats begin with different diastolic volumes and as a rule also with corresponding changes in initial tension. Thus, just before the smaller contraction begins the diastolic size is less and the initial tension is lower. The reverse obtains in the case of the larger beat.
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