Clinical cardiac electrophysiological assessment of indoramin.

Indoramin is a selective postsynaptic alpha-adrenoceptor antagonist used for the treatment of hypertension. Animal experiments have shown that indoramin has some antiarrhythmic activity, but whether this is due to its alpha-blocking effect or some other mechanism is not known. Fifteen patients (10 men) underwent electrophysiological investigations before and 15 min after intravenous indoramin injection (0.20-0.50 mg/kg). The plasma level of indoramin was measured and the patients were divided into two groups: group I (eight patients), whose plasma level was less than 98 micrograms/ml (average 75 micrograms/ml), and group II (seven patients), whose plasma level was greater than 98 micrograms/ml (average 151 micrograms/ml). In both groups, there was a significant drop in the systolic blood pressure after indoramin administration (129 +/- 22 to 111 +/- 23 mm Hg; p less than 0.001). There was a marked improvement in the sinus node recovery time in group I only (271 +/- 94 to 147 +/- 30 ms; p greater than 0.01). Similarly, there was a decrease in the AH interval during fixed-rate atrial pacing (128 +/- 33 to 100 +/- 37 ms; p less than 0.05) and a significant decrease in the Wenckebach cycle length (372 +/- 85 to 347 +/- 74 ms; p less than 0.05) after indoramin in group I only. At the atrial level, there were no significant effects in either group; however, there was a significant increase in the ventricular effective refractory period in group II (231 +/- 35 to 264 +/- 64 ms; p less than 0.05) but not in group I.(ABSTRACT TRUNCATED AT 250 WORDS)