The quality of evidence is downgraded by inconsistency (unexplained variabiliy in results).
A Cochrane review on antiarrhythmics for maintaining sinus rhythm after cardioversion of atrial fibrillation[Abstract] 1 included 59 studies with a total of 20 981 subjects. Metoprolol was included in 3 of the studies (n=697), and 2 of them compared metoprolol with placebo.
Metoprolol seemed to reduce recurrence of atrial fibrillation compared with placebo, but the difference was not statistically significant (RR 0.83, 95% CI 0.68 to 1.02; 2 studies, n=562; statistical heterogeneity I2 =59%). Recurrence rates were 72.0% in people receiving placebo and 59.7% (49.0% to 73.4%) in people receiving metoprolol.
Metoprolol increased proarrhythmia compared to placebo due mainly to symptomatic bradyarrhythmias (94% of all proarrhythmic events; RR 18.14, 95% CI 2.42 to 135.66; 2 studies, n=562). Proarrhythmic events were reported in no participants in the placebo group and in 60 participants per 1000 people in the metoprolol group; NNTH was 19 (95% CI 2 to 235) participants treated for 1 year to have one additional bradyarrhythmia.The risk of withdrawing due to adverse events was more than 3 times higher among people receiving metoprolol than people receiving placebo (RR 3.47, 95% CI 1.48 to 8.15; 2 studies, n=562). This represented 21 per 1000 people receiving placebo withdrawing compared with 74 (95% CI 31 to 173) per 1000 people receiving metoprolol; NNTH was 19 (95% CI 7 to 99). Metoprolol showed no significant difference in mortality (RR 2.02, 95% CI 0.37 to 11.05; 2 studies, n=562) compared to placebo.
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