TABLE OF CONTENTS The baseline variability of the heart rate time series is determined by many factors including age, gender, activity, medications, and health [46]. However, not only does the mean beat-to-beat interval (the heart rate) change on many scales, but the variance of this sequence of each heartbeat interval does so too. On the shortest scale, the time between each heartbeat is irregular (unless the heart is paced by an artificial electrical source such as a pacemaker, or a patient is in a coma). These short-term oscillations reflect changes in the relative balance between the sympathetic and parasympathetic branches of the ANS, the sympathovagal balance. This heart rate irregularity is a well-studied effect known as heart rate variability (HRV) [47]. HRV metric values are often considered to reflect the competing actions of these different branches of the ANS on the sinoatrial (SA) node. [12] Therefore, RR intervals associated with abnormal beats (that do not originate from the SA node) should not be included in a HRV metric calculation and the series of consecutive normal-to-normal (NN) beat intervals should be analyzed. [13]
It is important to note that, the fiducial marker of each beat should be the onset of the P wave, since this is a more accurate marker than the R peak of the SA node stimulation (atrial depolarization onset) for each beat. Unfortunately, the P wave is usually a low-amplitude wave and is therefore often difficult to detect. Conversely, the R wave is easy...
