VLF power: definition

VLF power is the amount of heartbeat-interval variance that falls between 0.003 and 0.04 Hz, in ms²: waves that take 25 seconds to about 5 minutes to repeat.

Why this measure exists

The 1996 Task Force set the band’s edges and gave it a name, but was careful about its meaning: in short recordings VLF is “a dubious measure” that should be avoided when interpreting 5-minute spectra. The interest came from 24-hour studies. Bigger and colleagues followed 715 people after a heart attack and found that the two slowest bands, 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 VLF, were more strongly linked to death than 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 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 , and that VLF was the only measure more strongly linked to arrhythmic death than to death from all causes. Shaffer, McCraty and Zerr’s 2014 review reports that VLF has stronger associations with all-cause mortality than LF and HF.

What makes the rhythm is less settled. Candidates in the reviews are thermoregulation, the renin–angiotensin hormone system, physical activity and the heart’s own intrinsic nervous system; parasympathetic Parasympathetic nervous system The "rest and digest" branch of the autonomic nervous system. It slows the heart, mainly through the vagus nerve, and drives most of the beat-to-beat variation in HRV. More about Parasympathetic nervous system blockade almost abolishes it, and sympathetic Sympathetic nervous system The "fight or flight" branch of the autonomic nervous system. It speeds up the heart and makes it pump harder, but it acts too slowly to drive beat-to-beat HRV. More about Sympathetic nervous system activity seems to set its size.

The formula

\mathrm{VLF} = \int_{0.003}^{0.04} P(f)\,\mathrm{d}f

Add up the power spectral density $P(f)$ , in ms² per Hz, over every frequency $f$ from 0.003 to 0.04 Hz. In a 5-minute recording the band holds only a few frequency bins, so the estimate is coarse.

Worked example

Take the rhythm apart

Five minutes of the teaching recording

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.

Breathing rate 15 breaths a minute Recording length 5 minutes

Detrend first (smoothness priors, λ = 500)

The controls work when JavaScript is on.

The power spectrum of five minutes of the teaching recording: how much of the variation happens at each speed, with the Task Force bands shaded. The tall peak is breathing. Slow the breathing below 9 a minute and watch the peak cross from the HF band into the LF band; switch detrending off on the drifting recording and watch the VLF band fill.

The five minutes of the teaching recording give a VLF power of 3 ms²: the recording’s slow drift component is removed by 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 before the transform. Switch to the drifting preset and turn detrending off: VLF reads 179 ms², nearly all of it the slide in the mean leaking into the lowest bins; turn it back on and it drops to 2 ms². Which of those is “right” depends on whether the drift was a rhythm or a trend, and in 5 minutes no program can tell.

What moves it

Over 24 hours: activity, meals, temperature regulation, hormones and sleep. In a short recording: mostly how much the heart rate drifted during it, and the analysis settings. Shaffer and Ginsberg say VLF needs at least 5 minutes and is best monitored over 24 hours; a 270-second ultra-short minimum has been proposed.

When it misleads

In any recording shorter than 5 minutes, where a slow wave cannot complete even one cycle. In any short recording without detrending, where it is largely the trend. And whenever a program’s detrending cutoff sits inside the band: the smoothness-priors method (Tarvainen and colleagues, 2002) that Kubios applies by default works like a high-pass filter, and Kubios’s guide says its default setting removes most of the power below 0.04 Hz on purpose. With the λ = 500 setting common in HRV software and 4 Hz resampling, the filter’s cut-off works out near 0.035 Hz. VLF from one program is not VLF from another.

Where you will see it

The Task Force’s name is VLF. Kubios HRV prints its peak frequency Peak frequency The frequency, in Hz, at which a band of the HRV spectrum has its highest power. The HF peak is usually the breathing rate; the LF peak is usually near 0.1 Hz. More about Peak frequency , absolute power, log power and relative power; consumer apps rarely show it. Over 24 hours the SDNN index SDNN index The mean of the SDNN of each 5-minute segment of a 24-hour recording, in milliseconds. It keeps the variability inside each segment and drops the slow drift between them. More about SDNN index tracks much the same thing. What do LF, HF, VLF and nu mean in my HRV app? decodes a typical app screen.