HR and RR intervals (iR-R) were recorded using a telemetric cardiac monitor (Polar¯ S810i, Finland). An elastic band (Polar T31 transmitter) was placed around the patient's thorax at the level of the lower third of the sternum, while the patient was in a sitting position; signals were continuously transmitted to the receiving unit by electromagnetic field. Recorded data were then transferred to Kubios HRV¯ analysis software (version 2.2, Finland) for subsequent analysis.
The signal processing was carried out as follows: at rest (5 min), during the 6MWT (we discarded the initial 60 s of data and selected the most stable signal, corresponding to the latter portion of the test), and post-exercise recovery (5 min) with and without EPAP. The HRV signal collected in the time domain provided mean RR, STD RR, mean HR, STD HR, RMSSD, and RR tri index, in the frequency domain were LF, HF, and LF/HF ratio, and nonlinear analysis provided the ApEn and Shannon entropy indices.
The signal processing was carried out as follows: at rest (5 min), during the 6MWT (we discarded the initial 60 s of data and selected the most stable signal, corresponding to the latter portion of the test), and post-exercise recovery (5 min) with and without EPAP. The HRV signal collected in the time domain provided mean RR, STD RR, mean HR, STD HR, RMSSD, and RR tri index, in the frequency domain were LF, HF, and LF/HF ratio, and nonlinear analysis provided the ApEn and Shannon entropy indices.