Heart rate variability in the clinical assessment of patients with chronic liver disease

Table 1 Most common measurements of heart rate variability

Variable	Description	ANS interpretation
Time domain measurements
SDNN (ms)	The standard deviation of all the NN intervals	SNS and PSNS
RMSSD (ms)	The square root of the mean of the sum of the squares of differences between the adjacent NN intervals	PSNS
NN50	The number of adjacent NN intervals that differ by more than 50 ms	PSNS
pNN50	The NN50 count divided by the total number of all NN intervals	PSNS
Frequency domain measurements
LF (ms2)	Power in the low frequency range (0.04–0.15 Hz)	SNS and PSNS
HF (ms2)	Power in the high frequency range (0.15–0.4 Hz)	PSNS
LF/HF (nu)	Ratio LF/HF	SNS and PSNS
Nonlinear measurements
SD1/SD2	Quotient between standard deviation 1 and 2 from the Poincaré plot	SNS and PSNS
ApEn	Approximate entropy, measures signal complexity and regularity	SNS and PSNS
DFA	Detrended fluctuation analysis from RRnvs RRn+1 correlations; short (α1) and long (α2) fluctuations	SNS and PSNS

ANS: Autonomic nervous system; SNS: Sympathetic nervous system; PSNS: Parasympathetic nervous system; ApEn: Approximate entropy; LF/HF: Low frequency/high frequency ratio DFA: Detrended fluctuation analysis; SDNN: Standard deviation of “NN” intervals; RMSSD: Root mean square of successive differences between normal heartbeats pNN50: Percentage of adjacent NN intervals that differ from each other by more than 50 ms.