Feature extraction

Heart rate variability features

Features are based on standards of heart rate variability (HRV) measurement and interpretation described in Task Force of the European Society of Cardiology (1996) and Shaffer & Ginsberg (2017).

Time domain

Group identifier: hrv-time

All time domain HRV features are either derived from normal-to-normal (NN) intervals, from successive differences between NN intervals (SD), or from the Poincaré plot (PP).

Note

SleepECG expects time in seconds to compute features. Since many features like RMSSD are typically specified in milliseconds in the field of heart rate variability research, the resulting features need to be manually rescaled before comparing to millisecond-based features.

Feature	Description	Signal
`meanNN`	average normal-normal (NN) interval	NN
`maxNN`	maximum NN interval	NN
`minNN`	minimum NN interval	NN
`rangeNN`	difference between the longest and shortest NN interval	NN
`SDNN`	standard deviation of the NN intervals	NN
`RMSSD`	square root of the mean of the squares of differences between adjacent NN intervals	SD
`SDSD`	standard deviation of differences between adjacent NN intervals	SD
`NN50`	number of pairs of adjacent NN intervals differing by more than 50 ms	SD
`NN20`	number of pairs of adjacent NN intervals differing by more than 20 ms	SD
`pNN50`	percentage of pairs of adjacent NN intervals differing by more than 50 ms	SD
`pNN20`	percentage of pairs of adjacent NN intervals differing by more than 20 ms	SD
`medianNN`	median of the NN intervals	NN
`madNN`	median absolute deviation of the NN intervals	NN
`iqrNN`	interquartile range (IQR) of the NN intervals	NN
`cvNN`	coefficient of variation of the NN intervals	NN
`cvSD`	coefficient of variation of differences between adjacent NN intervals	SD
`meanHR`	average heart rate (HR = 60 / NN)	NN
`maxHR`	maximum heart rate	NN
`minHR`	minimum heart rate	NN
`stdHR`	standard deviation of the heart rate	NN
`SD1`	standard deviation along the short axis of the ellipse (corresponding to short term variability, equivalent to `RMSSD`)	PP
`SD2`	standard deviation along the long axis of the ellipse (corresponding to long term variability)	PP
`S`	area of the ellipse	PP
`SD1_SD2_ratio`	ratio of `SD1` to `SD2`	PP
`CSI`	cardiac sympathetic index	PP
`CVI`	cardiac vagal index	PP

Frequency domain

Group identifier: hrv-frequency

For calculating frequency domain HRV features, the RR time series is resampled at regular intervals, after which the power spectral density (PSD) is estimated using Welch's method.

Feature	Description	Frequency range
`total_power`	variance of NN intervals over the temporal segment	≤ 0.4 Hz
`VLF`	power in very low frequency (VLF) range	[0.0033, 0.04) Hz
`LF`	power in low frequency (LF) range	[0.04, 0.15) Hz
`HF`	power in high frequency (HF) range	[0.15, 0.4) Hz
`LF_norm`	LF power in normalized units (\(\frac{\text{LF}}{\text{LF}+\text{HF}}\cdot100\))	[0.04, 0.15) Hz
`HF_norm`	HF power in normalized units (\(\frac{\text{HF}}{\text{LF}+\text{HF}}\cdot100\))	[0.15, 0.4) Hz
`LF_HF_ratio`	ratio of LF to HF	–

Metadata features

Group identifier: metadata

Feature	Description
`recording_start_time`	time at which the recording was started in seconds (`0` corresponds to `00:00:00` and `86399` to `23:59:59`)
`age`	age of the subject in years
`gender`	`0` (female) or `1` (male)
`weight`	weight of the subject in kg