Heart rate variability
PREMATURE BEATS, EVENTS
The beat-by-beat series of R-R intervals were extracted from 1 ECG channel of each PSG through a derivative-and-threshold algorithm, selecting a segment of at least 10-min duration without respiratory events in the supine position during non-REM stage 2 (N2) sleep for the HRV analysis. Premature beats were visually identified and manually removed, obtaining series of normal-to-normal intervals (NNI). Time-domain indexes of HRV were the root-mean-square of successive differences (RMSSD) and the percentage of beats with NNI value at least 50 ms longer or shorter than NNI of their preceding beat (pNN50)