Average systolic impedance during ejection and myocardial contractility were evaluated as determinants of cardiac output in two studies. In group I, 10 dogs received separate infusions of isoproterenol and ouabain systemically, both before and after adjustments in afterload. In group II, five dogs received intracoronary infusions of calcium and/or isoproterenol. Data from both studies, as discussed, support the conclusion that independently or in combination both average systolic impedance and myocardial contractility act significantly to alter cardiac output.

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