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 |