Heart rate variability and cardiovascular health RL, Conder AA. The number healtb adjacent NN intervals that differ hhealth each other by more than 50 ms NN50 requires a 2 min epoch. The wavelet entropy measures are calculated using a three-step procedure defined in the literature. Heart rate variability HRV Heart rate variability and cardiovascular health the Post-game nutrition for tennis phenomenon of variation in the ratte interval between heartbeats. Variabiligy is measured by cardiiovascular variation in Enhancing muscular endurance beat-to-beat interval. Other terms used include varjability Enhancing muscular endurance variability", "R—R varaibility where R is a point corresponding to the peak of the QRS complex of the ECG wave; and RR is the interval between successive Rsand "heart period variability". Methods used to detect beats include ECG, blood pressure, ballistocardiograms[1] [2] and the pulse wave signal derived from a photoplethysmograph PPG. ECG is considered the gold standard for HRV measurement [3] because it provides a direct reflection of cardiac electric activity.