Baseline imbalance and heterogeneity are present in meta-analyses of randomized clinical trials examining the effects of exercise and medicines for blood pressure management

Randomized clinical trials attempt to reduce bias and create similar groups at baseline to infer causal effects. In meta-analyses, baseline imbalance may threaten the validity of the treatment effects. This meta-epidemiological study examined baseline imbalance in comparisons of exercise and antihypertensive medicines. Baseline data for systolic blood pressure, diastolic blood pressure, and age were extracted from a network meta-analysis of 391 randomized trials comparing exercise types and antihypertensive medicines. Fixed-effect meta-analyses were used to determine the presence of baseline imbalance and/or inconsistency. Meta-regression analyses were conducted on sample size, the risk of bias for allocation concealment, and whether data for all randomized participants were presented at baseline. In one exercise comparison, the resistance group was 0.3 years younger than the control group (95% confidence interval 0.6 to 0.1). Substantial inconsistency was observed in other exercise comparisons. Less data were available for medicines, but there were no occurrences of baseline imbalance and only a few instances of inconsistency. Several moderator analyses identified significant associations. We identified baseline imbalance as well as substantial inconsistency in exercise comparisons. Researchers should consider conducting meta-analyses of key prognostic variables at baseline to ensure balance across trials.