Different Action of Beta‐blockers on Daytime and Nighttime Heart Rate Variability

Abstract

Background: Beta‐blockers in patients surviving acute myocardial infarction (AMI) and in those with dilated cardiomyopathy have proven to be of beneficial effect, particularly for the sudden cardiac death rate. They are also used to control various forms of arrhythmias because of the strong correlation between cardiac arrhythmias and adrenergic reaction. Heart rate variability (HRV) variables provide valuable information related to the autonornic nervous system function. The present prospective study was undertaken to investigate the effects of beta‐blockers on 24‐hour HRV.Methods: We studied 60 patients, aged 39 to 76 years (mean 56 ± 15). Forty of the patients had survived a myocardial infarction 12 to 24 months previously (group I). Twenty patients did not have apparent cardiac heart disease (group II). Twenty‐four‐hour monitoring was performed at baseline and after 8 to 10 days of atenolol (100 mg/day, n = 35) or metoprolol (100 to 150 mg/day, n = 25) (BB). Measures of HRV in the time and frequency domains were calculated and printed for the the entire 24 hours and from 09:00 to 21:00 (daytime) and 23:00 to 6:00 (nighttime). The 24‐hour analysis of HRV shows an improvement over control values in indices of parasympathetic tone, but the results were statistically significant only for high frequency power (HF) in groups I (P < 0.01) and II (P < 0.05). A significant decrease of the coefficient of variance was noted in group II (P < 0.05). The analysis during the day and the night revealed a predominant action of beta‐blockers during the night with a high frequency increase in both groups from 64.5 ± 45 to 161 ± 111 ms2 in group I (P < 0.001) and from 99 ± 89 to 268 ± 348 ms2 in group II (P < 0.02). In group II, the daily high frequency power did not vary after beta‐blockers. The decrease of the coefficient of variance in group II disappeared in the daily and nightly analysisConclusions: Beta‐blockers enhance the HRV indexes reflecting the parasympathetic activity especially during the night in patients with and without ischemic heart disease. Although an indirect effect of beta‐blockers on respiration cannot be excluded, this effect could explain one of the beneficial effects of beta‐blockers on general survival in patients with and without myocardial infarction.