Effect of digitalis on left ventricular function in exercising dogs.

The effect of ouabain on left ventricular function in nonfailing hearts was assessed in 14 chronically instrumented dogs during graded treadmill exercise. At rest, ouabain increased the maximum first derivative of the left ventricular pressure (dp/dtmax) and stroke volume by 38% and 16%, respectively. No change occurred in end-diastolic left ventricular diameter or peak systolic left ventricular pressure. During exercise, ouabain reduced maximum running speed and limited the increments in heart rate and systolic pressure but did not alter dp/dtmax, stroke volume, or end-diastolic diameter. When atropine and ouabain were given and severe exercise was performed, there were no differences from controls in running speed, heart rate, dp/dtmax, or other parameters. When ouabain and propranolol were given dp/dtmax increased at rest and during exercise, compared with results with propranolol alone. It is concluded that the inotropic effect of ouabain is negligible during strenuous physical activity because of the presence of high levels of sympathetic stimulation. However, during exercise in the presence of beta-adrenergic blockade, increases in myocardial contractility do occur in response to ouabain.