Effects of Chronic Digoxin Administration on Left Ventricular Performance in the Normal Conscious Dog

The effects of chronic digoxin administration on left ventricular (LV) function were studied in six healthy conscious dogs, instrumented with high-fidelity LV manometers and endocardial ultrasonic diameter gauges. Studies were performed at an average of eight days after a therapeutic serum level of digoxin had been achieved by daily intramuscular injections. In the resting state, at matched heart rates and at unchanged LV end-diastolic pressures, LV systolic pressure increased after digoxin by a mean of 9 mm Hg (P < 0.008), while peak LV dP/dt was augmented by 38% (P < 0.006). Mean and maximal velocity of circumferential fiber shortening (VCF) increased by 22% and 21%, respectively (both P < 0.02), and LV systolic diameter excursion (&Dgr;LVD) increased by 10% (P < 0.05) above control values. Intravenous propranolol did not alter these responses significantly. Elevation of LV systolic pressure to an average of 206 mm Hg by phenylephrine infusion did not change the previous basal augmentation of LV dP/dt and VCF produced by digoxin, but &Dgr;LVD exceeded the control values by 25% as compared with the increase of 10% produced by digoxin alone (P < 0.02). Thus, chronic digoxin administration in the normal conscious dog exerts a potent positive inotropic effect in the resting state and markedly improves cardiac reserve as evidenced by enhanced LV performance during acute pressure overloading.