After right heart catheterization and baseline measurements, 0.5 to 0.6 mg of ouabain were administered intravenously and responses measured an hour later. No, one would not expect digitalis to have a positive inotropic effect in a normal, non- failing, heart. No, in fact, the effects of cardiac index and forearm blood flow suggest that digitalis exerted a negative inotropic effect. Yes, as indicated by the effects on mean arterial pressure. Yes, as indicated by the effects on arterial pulse pressure and stroke volume index.
Digitalis directly depresses sinus node automaticity.
All of the above contribute to the effect of digitalis on heart rate. Yes, digitalis increases the refractory period and slows conduction through the AV node. Yes, digitalis usually converts atrial fibrillation to normal sinus rhythm. Yes, digitalis directly decreases atrial automaticity. No, the greater decreases of heart rate in these patients was probably related to the higher initial heart rate.