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Purpose: Heart rate variability (HRV) decreases after an acute myocardial infarction (AMI) due to changes in cardiac autonomic balance. The purpose of the present study, therefore, was to evaluate the effects of a progressive exercise protocol used in phase I cardiac rehabilitation on the HRV of patients with post-AMI., Material and Methods: Thirty-seven patients who had been admitted to hospital with their first non-complicated AMI were studied. The treated group (TG, n=21, age=52±12 years) performed a 5-day programme of progressive exercise during phase I cardiac rehabilitation, while the control group (CG, n=16, age=54±11 years) performed only respiratory exercises. Instantaneous heart rate (HR) and RR interval were acquired by a HR monitor (Polar®S810i). HRV was analysed by frequency domain methods. Power spectral density was expressed as normalised units (nu) at low (LF) and high (HF) frequencies, and as LF/HF., Results: After 5 days of progressive exercise, the TG showed an increase in HFnu (35.9±19.5 to 65.19±25.4) and a decrease in LFnu and LF/HF (58.9±21.4 to 32.5±24.1; 3.12±4.0 to 1.0±1.5, respectively) in the resting position (p<0.05). No changes were observed in the CG., Conclusions: A progressive physiotherapeutic exercise programme carried out during phase I cardiac rehabilitation, as supplement to clinical treatment increased vagal and decreased sympathetic cardiac modulation in patients with post-AMI., (© 2011 Informa UK, Ltd.)