BACKGROUND: Exaggerated sympathetic nervous system activity associated with low heart rate variability (HRV) is considered to trigger cardiac arrhythmias and sudden death. Regular exercise training is efficient to improve autonomic balance. OBJECTIVE: We aimed to verify the superiority of high-intensity interval training (HIIT) to enhance HRV, cardiorespiratory fitness and cardiac function as compared with moderate intensity continuous training (MICT) in a short, intense cardiac rehabilitation program. METHODS: This was a prospective, monocentric, evaluator-blinded, randomised (1:1) study with a parallel two-group design. Overall, 31 individuals with voluntary chronic heart failure (CHF) (left ventricular ejection fraction (LVEF) < 45%) were allocated to MICT (n = 15) or HIIT (n = 16) for a short rehabilitation program (mean (SD) 27 (4) days). Participants underwent 24-hr electrocardiography, echocardiography and a cardiopulmonary exercise test at entry and at the end of the study. RESULTS: High-frequency power in normalized units (HFnu%) measured as HRV increased with HIIT (from 21.2% to 26.4%, p < 0.001) but remained unchanged with MICT (from 23.1% to 21.9%, p = 0.444, with a significant intergroup difference, p = 0.003). Resting heart rate (24-hr Holter electrocardiography) decreased significantly for both groups (from 68.2 to 64.6 bpm and 66.0 to 63.5 bpm for MICT and HIIT, respectively, with no intergroup difference, p = 0.578). The 2 groups did not differ in premature ventricular contractions. Improvement in peak oxygen uptake was greater with HIIT than MICT (+21% versus +5%, p = 0.009). LVEF improved with only HIIT (from 36.2% to 39.5%, p = 0.034). CONCLUSIONS: In this short rehabilitation program, HIIT was significantly superior to the classical MICT program for enhancing parasympathetic tone and peak oxygen uptake.

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