A comparison of the effects of digoxin and digitoxin on systolic time intervals and colour vision

Animal studies suggest that for the same inotropism hydrophilic cardiac glycosides produce greater depression of atrioventricular (AV) conduction than lipophilic ones. This has been explained on the basis of a greater vagomimetic effect with hydrophilic agents and a greater sympathomimetic effect with lipophilic agents. In this randomized, cross-over study we investigated the effects of placebo, digoxin (relatively hydrophilic), and digitoxin (relatively lipophilic) in twelve healthy volunteers. For both drugs steady-state serum concentrations in the mid-therapeutic range were achieved. Both drugs produced the same positive inotropic effect as measured by systolic time intervals (QS2c). There was a trend for digoxin to have a greater effect on AV conduction than digitoxin. After atropine or propranolol there was no difference between the effect of the two cardiac glycosides on AV conduction. No significant effects on colour vision were seen. We conclude that, there do not seem to be pharmacodynamic differences between digoxin and digitoxin at mid-therapeutic serum concentrations.