Heart rate variability analysis methods
Heart rate variability is typically assessed by calculating various parameters that describe the magnitude or nature of the interbeat interval variability. These HRV analysis methods can be divided into time-domain, frequency-domain and nonlinear parameters.
Time-domain HRV analysis methods
Time-domain HRV analysis methods are calculated from the interbeat intervals between successive heartbeats also known as the RR intervals (time intervals between successive R-peaks of the ECG). One commonly used time-domain parameter is the root mean square of successive RR interval differences (RMSSD) which measures beat-to-beat variability and is strongly associated with the parasympathetic nervous system activity. Another commonly used time-domain measure is the standard deviation of RR intervals (SDNN) which is a measure of overall HRV, and thus, reflects both sympathetic and parasympathetic nervous activity.
Frequency-domain HRV analysis methods
Frequency-domain HRV analysis methods are calculated from the power spectrum of RR interval data and describe the distribution of heart rate variability into different frequency components. Short-term HRV recordings are typically divided into very low frequency (VLF: 0-0.04 Hz), low frequency (LF: 0.04-0.15 Hz), and high frequency (HF: 0.15-0.4 Hz) components. Powers of the frequency components are typically presented both in absolute units as well as in normalized units. When using frequency-domain analyses of HRV, one should make sure that the respiratory rate is within the HF band. In case of exercise recordings, the upper limit of the HF band should usually be raised because respiratory rate can even rise up to 1 Hz (equivalent to 60 breaths/min).
Nonlinear HRV analysis methods
Nonlinear HRV analysis methods are sometimes used to evaluate nonlinear mechanisms, complexity or chaotic behavior of heart rate variability. One commonly used nonlinear HRV analysis method is the Poincaré plot, which is a graphical representation of the correlation between RR intervals. Another quite often used nonlinear HRV parameter is the detrended fluctuation analysis (DFA), which measures the fractal behavior of HRV.