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CLINICAL STUDY: ARRHYTHMIAS

Rate-dependent effect of verapamil on atrial refractoriness

^* Division of Cardiology, Department of Internal Medicine, University of Michigan Medical Center, Ann Arbor, Michigan, USA

Manuscript received June 24, 2002; revised manuscript received October 3, 2002, accepted October 17, 2002.

^* Reprint requests and correspondence: Dr. Fred Morady, B1 F245, University of Michigan Medical Center, 1500 East Medical Center Drive, Box 0022, Ann Arbor, Michigan 49109-0022 USA.
fmorady{at}umich.edu

OBJECTIVES: The purpose of this study was to determine whether verapamil has rate-dependent effects on the atrial effective refractory period (AERP).

BACKGROUND: Block of calcium current (I_Ca) and rapid component of the delayed rectifier potassium current (I_Kr) by verapamil is frequency-dependent. This may result in variable effects of verapamil on the AERP, depending on the rate.

METHODS: The subjects of this study were 30 adults with a mean age of 45 ± 13 years who did not have structural heart disease. In 20 subjects, the AERP was measured at basic drive cycle lengths (BDCLs) of 650 to 250 ms, in 50 ms decrements, before and after infusion of 0.1 mg/kg verapamil. The effective refractory periods (ERPs) were measured in the setting of autonomic blockade in 10 subjects and without autonomic blockade in 10 subjects. Ten subjects served as a control group and received a saline infusion instead of verapamil.

RESULTS: Verapamil significantly prolonged the AERP at BDCLs of 650 to 500 ms (p < 0.01 or p < 0.05) and significantly shortened the ERP at BDCLs of 300 and 250 ms (p < 0.01). In the control group, there were no significant differences between the baseline and post-saline measurements of ERP.

CONCLUSIONS: Verapamil prolongs AERP at slow rates and shortens AERP at rapid rates. These findings are consistent with a predominant effect on I_Ca at rapid rates and a predominant effect on I_Kr at slow rates.

Abbreviations and Acronyms   AERP   atrial effective refractory period   AF   atrial fibrillation   AV   atrioventricular   BDCL   basic drive cycle length   ERP   effective refractory period   ICa   calcium current   IKr   rapid component of the delayed rectifier potassium current

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