Pharmacokinetics and pharmacodynamics of intravenous digoxin and digitoxin

Summary Healthy volunteers received single 1.0-mg doses of intravenous digoxin (n=10) or digitoxin (n=12). Glycoside pharmacokinetics were determined from multiple plasma samples drawn over the 48 hours (for digoxin) or 14 days (for digitoxin) after the dose. Electrocardiogram, echocardiogram, and blood pressure were recorded at multiple time points 24 h after the dose. To control for nonspecific cardiovascular changes, pharmacodynamic measurements were repeated on a second occasion for 8 hours after an intravenous injection of saline. Mean (±S.E.) kinetic variables for digoxin were: volume of distribution (Vd), 8.3 (±0.6) l/kg; elimination half-life (t1/2), 49 (±5) h; clearance 2.1 (±0.2) ml/min/kg. Changes in blood pressure, ventricular rate, and corrected QT-interval attributable to digoxin were small. However, echocardiographically-determined mean rate of circumferential fibre shortening (mVcf) and ejection fraction (EF) increased significantly following digoxin when compared to saline infusion. Changes were maximal at 4–6 h after dosage, and were highly correlated with plasma digoxin concentration. mVcf and EF returned to baseline by 24 h post-dosage. Mean kinetic variables for digitoxin were: Vd, 0.63 (±0.03) l/kg; t1/2, 7.3 (±0.4) days; clearance, 0.043 (±0.003) ml/min/kg. Like digoxin, digitoxin infusion produced minimal change in blood pressure, ventricular rate, or QT-interval. However, mVcf and EF increased significantly when compared to saline control. Changes were maximal at 4–8 h after infusion, and were correlated with plasma digitoxin concentration; at 24 h post-dosage, mVcf and EF were still increased over baseline. Thus, digoxin and digitoxin significantly increase myocardial contractility in healthy humans, but without important change in heart rate and blood pressure. Changes in contractility are of slow onset, probably due to slow distribution of glycoside to sites of pharmacologic activity.Supported in part by Grant Oc 10/4 from Deutsche Forschungsgemeinschaft, Bonn-Bad Godesberg, FRG; and by Grant MH-12279 from the United States Public Health Service