INFLUENCE OF ALPHA-ADRENERGIC BLOCKADE ON VENTRICULAR ARRHYTHMIAS, QTC INTERVAL AND HEART-RATE-VARIABILITY IN PHEOCHROMOCYTOMA

There are strong experimental evidences that alpha(1)-adrenergic stimu lation significantly influences cardiac arrhythmogenesis, especially d uring myocardial ischaemia and reperfusion. However, anti-arrhythmic e ffects of alpha-blockade in humans were scarcely utilised. Thus the pu rpose of our study was to assess these effects in patients with phaeoc hromocytoma. In 22 patients simultaneous 24h ECG and blood pressure (B P) monitoring, as well as estimation of the urinary 24 h free catechol amine excretion, were performed twice: before and during the treatment with the non-selective alpha-blocker, phenoxybenzamine. The heart rat e variability was measured during four 5 min periods, at 10.00, 16.00, 22.00 and 04.00. During alpha-blockade systolic blood pressure (SEP) decreased from 137.6+/-23.8 to 126.5+/-15.7 mm Hg (P< 0.01), heart rat e increased from 83.0 +/- 9.9 to 88.5 +/- 10.0/min (P< 0.02) and durat ion of QTc interval unsignificantly increased. Incidence of frequent o r repetitive ventricular arrhythmias was significantly higher before t reatment (in 9 vs 3 of 22 patients, P< 0.05). Heart rate variability s ignificantly decreased during the treatment, with regard to all parame ters, 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 pat ients with phaeochromocytoma, although the lack of QTc interval shorte ning suggests that the effect of class I drugs may participate in the anti-arrhythmic effects of phenoxybenzamine. Moreover, non-selective a lpha-blockade in phaeochromocytoma significantly diminishes vagal acti vity.