Heart rate variability and Chagas heart disease.

We believe that the present study is relevant, which investigated the effect of exercise training on heart rate variability (HRV) in patients with Chagas heart disease[1]. However, there are some issues in this study, which should be further discussed.

The use of amiodarone in approximately 80% of patients may have decreased their autonomic response[2], affecting the validity of HRV parameters.

Furthermore, low-ejection fraction (mean, 37%) may have acted as a confounding factor, and this finding may need to be investigated by inclusion of a control group without Chagas heart disease but with similar ejection fraction; inclusion of a control group with Chagas heart disease but with ejection fraction close to normal; or better yet, the inclusion of both groups.

A small sample size (37 subjects divided into two groups) masks potential differences; for a power of 80% and a two‑tailed alpha of 0.05, we estimate that the effect size ("d") of a large magnitude (d = 0.95) would be required to be detectable.

In fact, even when calculating the sample size, underpowering has been one of the major obstacles in clinical studies[3]. Although we did not consider a very high standard deviation (which would lead to greater difficulties), the effective post hoc power to detect intergroup differences considering a SDNN value of 0.15 would be only 7.3%, according to our calculations.

Moreover, we believe that, instead of the separate use of paired tests and tests for independent samples to answer the original question, other models (e.g., panel data or mixed models) are better adjusted to the experimental design and to the proposed objectives.