Pharmacodynamic distinctions between ouabain, digoxin and digitoxin.

Current pharmacologic texts recognize no significant pharmacodynamic differences between the various cardiac glycosides. To reconsider this concept, a special recording device was constructed so that electrocardiograms and phonocardiograms could be obtained in small mammals without anesthesia or premedication, and a spectrum of cardiac glycosides was studied. Utilizing guinea-pigs, cardiac rate reduction of 20% was sought and achieved with 0.07 mg/kg ouabain, 0.34 mg/kg digoxin and 1.12 mg/kg digitoxin. With comparable rate reduction, digitoxin produced significantly greater shortening of electro-mechanical systole than did ouabain or digoxin (P less than 0.05). Other authors have shown that cardiac glycosides produce slowing of cardiac rate prior to onset of positive inotropic effect. Therefore it is probable that for a given amount of vagal effect (sinoatrial slowing) digitoxin possesses greater positive inotropic effect (abbreviation of electromechanical systole) in guinea-pigs than do ouabain or digoxin. A review of the literature suggests that the same holds true for humans.