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Paired stimulation of the isometrically contracting papillary muscle of the cat caused it to develop 80 to 100% of the tension it develops in a high calcium, low sodium medium. The isovolumic left ventricle of the dog responded to paired stimuli with a significant increase in developed pressure and in maximal rate of rise of pressure (dP/dt max) at all ventricular volumes. During stimulation of the isovolumic left ventricle, there was little or no further potentiation induced by norepinephrine or calcium gluconate infusion. Paired stimulation is thus capable of inducing a maximal or near-maximal contractility of ventricular muscle at any given ventricular volume. Using dP/dt max as an index of left ventricular contractile force which is relatively uninfluenced by aortic afterloading, paired stimulation was found to cause significant potentiation in the ejecting left ventricle (64% increase above control dP/dt max). Unlike the papillary muscle and the isovolumic ventricle, the combination of paired stimulation and norepinephrine infusion caused a further increase in dP/dt max of the ejecting left ventricle. This may be explainable by the increase in ventricular size caused by norepinephrine, but in some instances norepinephrine added to the inotropic effects of paired stimulation.

Paired Electrical Stimulation and the Maximal Contractile Response of the Ventricle