Assessment of oral quinidine effects on sinus node function in sick sinus syndrome patients.

The effects of therapeutic doses of orally administered quinidine sulfate on sinus rhythmicity and automaticity were observed in 11 patients with sick sinus syndrome (SSS). Evaluation of sinus node (SN) function was undertaken by assessing sinus nodal recovery time (SNRT), treadmill exercise testing, and 24-hour ambulatory ECG monitoring before and after quinidine administration (25 mg/kg) (range 800 to 1600 mg daily). Corrected SNRT ranged from 100 to 1320 msec (average 551) before quinidine and was not significantly (p greater than 0.05) altered after quinidine to 346 to 660 msec (average 481). Further, quinidine did not induce accelerated infrasinus pacemaker activity. Spontaneous sinus rate evaluated with ambulatory monitoring revealed average rate of 57 bpm (range 53 to 63) before quinidine without significant increase to average 59 bpm (range 52 to 80) after quinidine therapy. Similarly, the maximal SN response to exercise was not significantly affected by quinidine (average 129 bpm before and 129 bpm after drug therapy). It is concluded that therapeutic doses of quinidine do not exert adverse effects on SN function in SSS patients. Chronic oral quinidine therapy can therefore be used safely with caution in patients with chronic SN disease when indicated for control of tachyarrhythmias.