Clinical cardiac electrophysiology of indoramin, a post-synaptic alpha blocker

Indoramin is a selective post-synaptic alpha blocker. Animal experiments had shown that it has antiarrhythmic effects, but whether this is due to its alpha blocking effect or some other mechanism is not known. Fifteen patients (10 males) underwent electrophysiological investigations before and 15 minutes after intravenous indoramin injection (0.2-0.5 mg kg-1). The plasma level of indoramin was measured and the patients were divided into two groups: group 1 (8 patients) whose plasma level was less than 100 micrograms ml-1 (average 72 micrograms ml-1) and group 2 (7 patients) whose plasma level was more than 100 micrograms ml-1 (average 151 micrograms ml-1). In both groups there was a significant drop in the systolic blood pressure after indoramin (129 +/- 22 to 111 +/- 23 mmHg, P less than 0.001). There was a marked improvement in the sinus node recovery time in group 1 only (253 +/- 92 to 163 +/- 40 ms, P less than 0.01). Similarly there was a decrease in AH interval during fixed rate atrial pacing in group 1 only (128 +/- 33 to 100 +/- 37 ms, P less than 0.05) and a significant decrease in the Wenckebach cycle length after indoramin in group 1 only (372 +/- 85 to 347 +/- 74 ms, P less than 0.05). At the atrial level there were no significant effects in either group but there was a significant increase in the ventricular effective refractory period in group 2 (231 +/- 35 to 264 +/- 64 ms, P less than 0.05) but no change in group 1.(ABSTRACT TRUNCATED AT 250 WORDS)