Effects of digitalis on sinus nodal function in patients with sick sinus syndrome.

The effect on sinus rhythmicity and automaticity of complete digitalization in a 24 hour period was observed in 14 patients with sick sinus syndrome. Sinus nodal function was evaluated in these patients by assessing sinus nodal recovery time and by treadmill exercise testing and 24 hour Holter monitoring, before and after digoxin administration. Corrected sinus nodal recovery times ranged from 240 to 2,065 msec (average 714) before digoxin and were shortened to 250 to 1,260 msec (average 565) after the glycoside. Further, digoxin induced accelerated infra sinus escape pacemaker activity in five patients: junctional and ventricular in one and atrial in four. Spontaneous sinus rate evaluated with Holter monitoring revealed an average of 56 beats/min (range 43 to 69) before digitalis that was unchanged (average 58 beats/min; range 48 to 74) after digoxin therapy. Similarly, the sinus nodal response to exercise was unaffected after digitalization (average 118 beats/min both before and during digitalis therapy). It is concluded that digoxin does not exert adverse effects on sinus nodal function in patients with sick sinus syndrome. The glycoside can be used safely in these patients when indicated for cardiac pump dysfunction or for control of tachyarrhythmia.