Data series used in HRV analysis are time series containing beat-to-beat intervals extracted from ECG signals. Temporal locations of beats are frequently based on the R-wave because it is often the easiest wave to distinguish. R waves typically have the largest amplitudes compared to surrounding P, Q, S, and T waveforms. Thus a beat-to-beat interval can be defined as the time difference between consecutive R peaks (RR interval).

Because the R wave is not the only temporal marker for beat locations, e.g. QRS complex, some use the term IBI as a generalization to represent any beat-to-beat intervals.

Additionally, RR intervals originating from normal sinus rhythms are sometimes referred to as NN (normal-to-normal) intervals. Thus, standard nomenclature of “NN” is used in place of IBI or RR to indicate IBI’s containing no ectopic intervals. Many authors, including this research, interchangeably use IBI, RR, or NN (normal-to-normal) to represent IBI series assuming ectopic beats have been corrected.

Figure below shows a hypothetical ECG and how IBI’s are determined based on R waves. IBI(1) and IBI (2) represent the first and second data point of the IBI time series signal. The IBI time series of an ECG segment containing N beats is given by the following equation where beat(n) is the time location of the nth beat.

k_{n+1} = n^2 + k_n^2 - k_{n-1}

Figure 2.1 – Determination of IBI. Simulated ECG containing three beats with arbitrary units of time and amplitude. Time intervals corresponding to the IBI are indicated by IBI(1) and IBI(2). ECG morphology is shown by five characteristic waves P, Q, R, S, and T.