There are some ways to assess HRV. It can be described using time-domain methods (statistical and geometrical methods) and frequency-domain methods. Time domain methods answer the question: “How big is HRV?”. Frequency domain methods answers the question: “What is it (HRV) determined by?”.
Statistical methods – indices:
Heart rate (HR), mean RR (mRR) – indices that characterize the heart rate and mean value of R-R interval. They reflect the level of functioning of CVS.
SDNN – standart deviation of the N-N interval (N-N means “normal-to-normal” R-R interval, when it is assumed normal rhytm which is generated by sinoatrial node (it is manifested by normal P-Q-R-S-T complexes in ECG). It characterizes integral HRV without its division into sympathetic and parasympathetic components.
RMSSD – the square root of mean squared differences of successive NN interval. “HRV-index” = 20*Ln(RMSSD). pNN50% – the part (percent) of successive NN intervals, in which the value of difference exceeds 50 ms. These three indices (RMSSD, HRV and pNN50%) reflect “beat-to-beat” variability, characterizing mainly parasympathetic activity of ANS.
I. Based on histogramm building
There are two approaches concerning the choice of the value of width of histogramm pocket.
The first approach is based on 50 ms width of pocket. Usually calculated indices are:
Mo – mode – the most frequent value of R-R interval (the highest column in the histogramm).
AMo – amplitude of mode – number of R-R intervals included in the pocket corresponding to mode. Usually measured in percents (%).
SI – stress-index (s. index of strain of Roman Bayevskiy). SI = AMo(%) / (2*Mo*Var); [Mo] = s; Var = RRmax – RRmin; [Var] = s.
The higher AMo and SI, the higher is the level of strain of ANS (or, in other words, the higher is the level of stress).
The second approach is based on 1/128 s width of pocket.
Heart rate variability triangular index (HRVTi) – is ratio between N (total count of R-R intervals) and n (number of R-R intervals included in the pocket corresponding to mode). The higher is HRVTi, the higher is HRV, espesially conditioned by parasympathetic regulation.
WN1 – width of histogramm which is taken at the level of 1% of N
WN4 – width of histogramm taken at the level of 4% of N
WN5 – width of histogramm which is cut on the level of 10% of N
Scatter – plot is the plot based on two axises. The first axis (horizontal) corresponds to current R-R interval (RRi). The second axis corresponds to the next (following) R-R interval (RRi+1). We can assess the shape of cloud, its width (W), length (L), ratio between length and width (L/W). The higher is the square of the cloud, the higher is HRV. The higher is the ratio L/W, the higher is the level of neuro-emotional strain (or stress).