The Importance of α-Adrenergic Stimulation of Cardiac Tissue and its Contribution to Arrhythmogenesis During Ischemia

Electrophysiologic mechanisms of the long QT interval syndromes and torsade de pointes

PURPOSE

To review the current understanding of the mechanisms and treatment of the long QT interval syndromes and torsade de pointes.

DATA SOURCES

Personal databases of the authors and a search of the MEDLINE database from 1966 to 1994.

STUDY SELECTION

Experimental and clinical studies and topical reviews on the electrophysiologic mechanisms and treatment of torsade de pointes were analyzed.

RESULTS

The long QT interval syndromes have been classified into acquired and hereditary forms, both of which are associated with a characteristic type of life-threatening polymorphic ventricular tachycardia called torsade de pointes. The acquired form is caused by various agents and conditions that reduce the magnitude of outward repolarizing K+ currents, enhance inward depolarizing Na+ or Ca2+ currents, or both, thereby triggering the development of early afterdepolarizations that initiate the tachyarrhythmia. The hereditary form appears to result from an abnormal response to adrenergic or sympathetic nervous system stimulation. At least some cases of the hereditary long QT interval syndromes may result from a single gene defect that alters the intracellular regulatory proteins responsible for the modulation of K+ channel function. Treatment of the acquired form is primarily directed at identifying and withdrawing the offending agent, although emergent therapy using maneuvers and agents that favorably modulate transmembrane ion currents can be lifesaving. In torsade de pointes associated with the hereditary long QT interval syndromes, early diagnosis leading to treatments designed to both shorten the QT interval and block the beta-adrenergic-induced instability of the QT interval is essential.

CONCLUSIONS

The long QT interval syndromes and torsade de pointes are potentially life-threatening conditions caused by various agents, conditions, and genetic defects. The mechanisms responsible for these conditions and available treatment options for them are reviewed.

Alpha 1-adrenergic blockade and sudden cardiac death

INTRODUCTION

The primary goal of the present study was to test whether selective pharmacologic blockade of alpha 1 receptors, and specifically of the subtype alpha 1a, could prevent ventricular fibrillation (VF) during acute myocardial ischemia.

BACKGROUND

The development of new autonomic interventions is of clinical interest in view of the failure of traditional antiarrhythmic drugs to prevent sudden death. Experimental evidence indicates that alpha 1 receptors, and in particular the subtype alpha 1a, may be involved in the genesis of malignant arrhythmias during acute myocardial ischemia and reperfusion. Despite this evidence, questions have been raised about the actual antifibrillatory efficacy of alpha-adrenergic blockade in the acutely ischemic myocardium. The effects of prazosin and of abanoquil (UK 52,046), a highly selective alpha 1a receptor blocker, were tested and compared with propranolol in a conscious animal preparation for sudden death.

METHODS AND RESULTS

Ten dogs with a 1-month-old anterior wall myocardial infarction were studied. These dogs had all developed, in control conditions, VF during a 2-minute occlusion of the circumflex coronary artery while exercising (n = 9) or lying on the table (n = 1). Afterwards, the dogs underwent additional tests with the following intravenously administered drugs: abanoquil (n = 10; 1 micrograms/kg), prazosin (n = 9; 0.1 mg/kg), and propranolol (n = 10; 1 mg/kg). Internal control analysis was used. All dogs tested had recurrence of VF with both alpha-adrenergic blockers. Propranolol significantly reduced heart rate during ischemia and prevented VF in 5 of 10 dogs tested (P < 0.05). When heart rate was kept constant by atrial pacing (n = 3), 2 of the 3 animals remained protected by propranolol. Just prior to onset of VF, heart rate was not significantly different in the control and in the abanoquil tests (237 +/- 45 and 253 +/- 34 beats/min, respectively), whereas it was higher (P < 0.05) with prazosin (288 +/- 40 beats/min).

CONCLUSIONS

Alpha 1 and alpha 1a receptor blockers do not prevent VF secondary to acute myocardial ischemia in the presence of elevated sympathetic activity and heart rate. In the same setting, beta-adrenergic blockade prevents the reflex heart rate increase due to ischemia and provides a significant protection.

Clinical relevance of cardiac arrhythmias generated by afterdepolarizations. Role of M cells in the generation of U waves, triggered activity and torsade de pointes

Recent findings point to an important heterogeneity in the electrical behavior of cells spanning the ventricular wall as well as important differences in the response of the various cell types to cardioactive drugs and pathophysiologic states. These observations have permitted a fine tuning and, in some cases, a reevaluation of basic concepts of arrhythmia mechanisms. This brief review examines the implications of some of these new findings within the scope of what is already known about early and delayed afterdepolarizations and triggered activity and discusses the possible relevance of these mechanisms to clinical arrhythmias.