Comparison of the Peak Inotropic Effects of a Catecholamine and a Digitalis Glycoside in the Intact Canine Heart

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Abstract
The relative peak effects of isoproterenol and ouabain on myocardial contractility and cardiac output were compared by infusing increasing amounts of these two drugs into seven open-chest anesthetized dogs until toxicity developed. Just prior to the development of toxicity isoproterenol increased contractile force an average of 149% and the peak rate of force development (df/dt) an average of 278% of control values, compared to an increase of only 49% and 35%, respectively, with the administration of ouabain. Cardiac output and stroke volume were also significantly greater with the catecholamine than the glycoside. The combination of isoproterenol and ouabain produced essentially the same contractile force and stroke volume achieved by isoproterenol alone. Suppression of ouabain-induced arrhythmias by ventricular pacing allowed additional glycoside to be infused until ventricular fibrillation terminated the study. With pacing and ouabain, contractile force increased 131% above control, a level similar to that achieved by isoproterenol; peak df/dt increased to 200% above control, a value significantly lower than that obtained with isoproterenol. However, stroke volume decreased despite a substantial increase in left ventricular end-diastolic pressure. It is concluded that maximal doses of isoproterenol produce significantly greater increases in myocardial contractility and cardiac output compared to ouabain, even when the toxicity produced by the latter is suppressed by electrical stimulation.