Influence of alpha-adrenergic blockade on ventricular arrhythmias, QTc interval and heart rate variability in...

There are strong experimental evidences that alpha 1-adrenergic stimulation significantly influences cardiac arrhythmogenesis, especially during myocardial ischaemia and reperfusion. However, anti-arrhythmic effects of alpha-blockade in humans were scarcely utilised. Thus the purpose of our study was to assess these effects in patients with phaeochromocytoma. In 22 patients simultaneous 24 h ECG and blood pressure (BP) monitoring, as well as estimation of the urinary 24 h free catecholamine excretion, were performed twice: before and during the treatment with the non-selective alpha-blocker, phenoxybenzamine. The heart rate variability was measured during four 5 min periods, at 10.00, 16.00, 22.00 and 04.00. During alpha-blockade systolic blood pressure (SBP) decreased from 137.6 +/- 23.8 to 126.5 +/- 15.7 mm Hg (P < 0.01), heart rate increased from 83.0 +/- 9.9 to 88.5 +/- 10.0/min (P < 0.02) and duration of QTc interval unsignificantly increased. Incidence of frequent or repetitive ventricular arrhythmias was significantly higher before treatment (in 9 vs 3 of 22 patients, P < 0.05). Heart rate variability significantly decreased during the treatment, with regard to all parameters, representing both the sympathetic and parasympathetic activity. We conclude that non-selective alpha-blockade significantly decreases the incidence of frequent or repetitive ventricular arrhythmias in patients with phaeochromocytoma, although the lack of QTc interval shortening suggests that the effect of class I drugs may participate in the anti-arrhythmic effects of phenoxybenzamine. Moreover, non-selective alpha-blockade in phaeochromocytoma significantly diminishes vagal activity.