Objective

This study was conducted to contrast effects of two β-adrenergic blockers on heart rate variability in dogs.

Materials & Methods

Twelve, healthy, trained beagle hounds randomly received placebo, carvedilol (nonspecific β-adrenergic blocker), and atenolol (β₁-adrenergic blocker), the beta blockers at single oral doses of 1.25 and 5 mg/kg respectively. After daily receiving the compounds for one week, ECG's were obtained for 15 minutes, 4 hours after dosing. Heart rate variability was expressed in both time and frequency domains.

Results

Atenolol increased RR interval, the standard deviation of differences between adjacent RR intervals (SDSD) and the square root of the mean of the sum of the squares of differences between adjacent RR intervals (RMSSD), but carvedilol increased only SDSD in the time domain. Both drugs increased the normalized high frequency, and decrease the normalized low frequency components in the frequency domain. Histograms of RR intervals demonstrated different patterns between animals receiving the two drugs. Carvedilol increased only the longest RR interval, but atenolol increased both shortest and longest RR intervals.

Conclusion

Carvedilol may enhance the parasympathetic activity and may modulate the sympathetic control to the heart in a way different from atenolol.