Left Ventricular Ejection in Conscious Dogs:

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Abstract
Some factors affecting stroke volume were investigated in conscious dogs in which left ventricular outflow was measured with an electromagnetic flowmeter. Stimulation of the heart at constant heart rate, by intracoronary injections of calcium gluconate and isopropylnorepinephrine, or postectopic potentiation failed to produce any increase in stroke volume. By contrast, alterations of posture at constant heart rate with the presumed associated changes in end diastolic dimensions, produced large changes in stroke volume. This resulted from a change in late systolic flow.