Heart rate variability (HRV) is often used as a measure of the sympathetic nervous system. One way to quantify HRV is by calculating the Inter-beat interval (IBI), also referred to as the RR-interval. The RR-interval refers to the time between two R-peak of a traditional ECG heart-beat waveform (see Figure 1).
Figure 1: Two subsequent heart-beats with component names. The RR interval is the time between two subsequent R-peaks. Picture taken from: http://lifeinthefastlane.com/ecg-st-segment-evaluation/
However, in a review article by Shaffer, McCraty and Zerr (2014) they refer to normal-to-normal (NN) intervals instead. This extends to the statistical measures of the RR intervals, such as SDRR, pRR50 (Urooj, Gupta, Sp & Tandon, 2014), i.e., SDNN and pNN50, respectively (Shafer et al., 2014). What is the difference between RR-intervals and the NN-intervals?
Urooj, M., Gupta, S., Sp, V., & Tandon, M. (2014). REFERENCE RANGE OF HEART RATE VARIABILITY AND VALIDATION IN SUBJECTS WITH ASYMPTOMATIC ELEVATED LIVER FUNCTION ENZYMES. International Journal of Current Pharmaceutical Research, 6(4), 49-52.