Total power

Total power is the sum of the power in all the bands of the HRV spectrum, in ms². It is the variance of the interval series, seen from the frequency side.

Why this measure exists

A spectrum shares one total out among its frequencies, and the 1996 Task Force listed that total as a measure of its own: the variance of the NN intervals NN intervals NN interval An RR interval between two normal heartbeats, in milliseconds, after extra beats and measurement errors are removed. Most HRV metrics are calculated from NN intervals. More about NN interval over the recording, up to 0.4 Hz. It is the frequency-domain twin of SDNN SDNN SDNN The standard deviation of all NN intervals in a recording, in milliseconds. It measures overall variability, and its value depends strongly on how long the recording lasts. More about SDNN , which is the square root of the variance; over 24 hours Shaffer, McCraty and Zerr note that SDNN is highly correlated with ULF ULF ULF power The variance of the heartbeat intervals below 0.003 Hz, in ms²: rhythms slower than one cycle every 5 minutes, including the day-night cycle. It can only be measured from a 24-hour recording. More about ULF power and total power. Its main job is as a denominator: relative power divides each band by it, and the normalised units normalised units Normalised units LF or HF power as a percentage of their sum (LF + HF), rather than in ms². The two always add up to 100, so they carry one piece of information between them. More about Normalised units divide LF LF LF power The variance of the heartbeat intervals that falls in the 0.04 to 0.15 Hz band, in ms²: rhythms repeating every 7 to 25 seconds. Once read as sympathetic activity, which is now disputed. More about LF power and HF HF HF power The variance of the heartbeat intervals that falls in the 0.15 to 0.40 Hz band, in ms²: the range of normal breathing. It mainly reflects vagal activity when breathing is 9 to 24 breaths a minute. More about HF power by the total minus VLF VLF VLF power The variance of the heartbeat intervals in the 0.003 to 0.04 Hz band, in ms²: rhythms repeating every 25 seconds to 5 minutes. It needs at least a 5-minute recording and is best read over 24 hours. More about VLF power .

The formula

$$\mathrm{TP} = \mathrm{ULF} + \mathrm{VLF} + \mathrm{LF} + \mathrm{HF} = \int_{0}^{0.40} P(f)\,\mathrm{d}f$$

Add the four band powers together, or add up the power spectral density $P(f)$ over every frequency $f$ up to 0.40 Hz. For a short recording there is no ULF band, so the total is VLF + LF + HF.

Worked example

Take the rhythm apart

Five minutes of the teaching recording

Power at each frequency, Hz Breathing

VLFLFHF

0 0.1 0.2 0.3 0.4 0.5

ULF

needs 24 h

VLF

3 ms²

LF

120 ms²

HF

635 ms²

Total

758 ms²

LF nu

15.9

HF nu

84.1

LF/HF

0.19

Peak

0.250 Hz, HF

Rested, 15 breaths a minute (0.250 Hz), 5 minutes, Welch's method after detrending. The breathing peak sits at 0.250 Hz in the HF band, with the ~0.1 Hz blood-pressure wave as a smaller LF peak. LF/HF is 0.19.

Rested Stressed Drifting Ectopic beats

Breathing rate 15 breaths a minute Recording length 5 minutes

Welch (FFT) Lomb–Scargle

Detrend first (smoothness priors, λ = 500)

The controls work when JavaScript is on.

A line chart of power against frequency from 0 to 0.5 Hz with the VLF (0.003–0.04 Hz), LF (0.04–0.15 Hz) and HF (0.15–0.4 Hz) bands shaded and a marker at the breathing frequency. Readouts give ULF, VLF, LF, HF and total power in ms squared, LF and HF in normalised units, the LF/HF ratio and the peak frequency, followed by a sentence explaining what changed. Controls: a breathing-rate slider from 5 to 20 breaths a minute, a recording-length slider, a detrending checkbox, Welch or Lomb–Scargle buttons, and four preset buttons for a rested, stressed, drifting or ectopic recording.

The five minutes of the teaching recording give 3 + 120 + 635 = 758 ms². The SDNN of the same intervals is 28.7 ms, and 28.7² is about 826 ms²: the two agree in size but not exactly, because the spectrum was detrended and windowed and SDNN was not. Turn detrending detrending Detrending Removing the slow drift from a series of beat intervals before a spectrum is calculated, so the drift does not pile up as false power in the lowest frequencies. More about Detrending off on the drifting preset and watch the total swell with the VLF band.

What moves it

Everything that moves any band: breathing, the baroreflex baroreflex Baroreflex The reflex that steadies blood pressure from beat to beat by changing heart rate. Its built-in delay makes heart rate rise and fall about every 10 seconds, the 0.1 Hz rhythm in LF power. More about Baroreflex , slow drifts, and, over a day, activity and sleep. The Task Force’s table gives 3,466 ± 1,018 ms² for a stationary stationary Stationarity The assumption that the rhythms in a recording stay the same from start to finish. Frequency measures such as LF and HF need it; a recording that changes partway through breaks it. More about Stationarity supine 5-minute recording, and warns that its normal values came partly from small studies and are only approximate; like every absolute power it is skewed, so papers often report its log.

When it misleads

It depends on the recording length as SDNN does, and on the program’s detrending: remove more of the slow trend and the total falls. Two programs will give different totals from the same beats. A total says nothing about which band the power sits in, which is the whole reason the bands exist.

Where you will see it

The Task Force’s name is total power, in ms². Kubios HRV uses it as the denominator of its “relative power” column and reports it for each method. What do LF, HF, VLF and nu mean in my HRV app? decodes a typical app screen.