Inhibitory effect of acute and chronic administration of digitalis on the sick sinus node.

We have determined the effects of the acute administration of intravenous (i.v.) digitalis administration (0.8 mg of digoxin in 5-10 min) on the electrophysiological variables of 12 normal subjects (group 1), 12 patients with asymptomatic sinus node bradycardia (group 2) and 34 patients with symptomatic sinus node dysfunction (SSS) (group 3). The injection of digitalis induced: an increase of sinus node cycle length (SNCL) in all groups; a 10% increase of sinus node recovery time (SNRT) in group 1, no changes in group 2 and a 25% increase in group 3; a tendency to increase, without reaching statistical significance, of corrected sinus node recovery time (CSNRT) and sino-atrial conduction time (SACT) in all groups. In 16 patients of group 3 the effects of oral chronic treatment with digoxin (0.25 mg daily for 7 days) was also determined. The results show that chronic treatment did not further affect SNCL, SNRT or CSNRT, but caused a greater increase of SACT than did the acute i.v. administration. Chronic treatment significantly reduced the sinus rate and increased the number of premature supraventricular and ventricular beats, measured by means of 24 hour-Holter monitoring. These data suggest that digitalis should be used with great caution in patients with SSS.