(1) BACKGROUND: Aquatic exercise training is a relatively understudied exercise modality in patients with CAD; with the present study, we sought to compare the impact of short-term 14-day water- and land-based exercise training on heart rate variability (HRV). (2) METHODS: We randomized 90 patients after a recent CAD event (myocardial infarction and/or revascularization within 2 months prior to inclusion) to either (i) water-based or (ii) land-based exercise training (14 days, two 30 min sessions daily), or (iii) controls. Before and after the intervention period, all participants underwent 20 min 12-channel high-resolution ECG recordings with off-line HRV analysis, including conventional linear time- and frequency-domain analysis (using the Welch method for fast-Fourier transformation), and preselected non-linear analysis (Poincare plot-derived parameters, sample entropy, and the short-term scaling exponent alpha1 obtained by detrended fluctuation analysis). (3) RESULTS: Eighty-nine patients completed the study (mean age 60 +/- 8 years; 20 % women). We did not detect significant differences in baseline- or age-adjusted end-of-study HRV parameters, but aquatic exercise training was associated with a significant increase in the linear LF/HF parameter (from 2.6 (1.2 to 4.0) to 3.0 (2.1 to 5.5), p = 0.046) and the non-linear alpha1 parameter (from 1.2 (1.1 to 1.4) to 1.3 (1.2 to 1.5), p = 0.043). (4) CONCLUSIONS: Our results have shown that a short-term 14-day aquatic exercise training program improves selected HRV parameters, suggesting this mode of exercise is safe and may be beneficial in patients with CAD.

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