LFHF
LF/HF — the ratio of low-frequency to high-frequency power — compares the slow (~0.1 Hz, baroreflex-related) oscillations of heart rate with the respiratory (vagal) oscillations. It has traditionally been called the "sympatho-vagal balance", but modern research shows that it does not measure sympathetic and parasympathetic activity directly; it is best used as a descriptive index of the shape of the heart-rate spectrum, interpreted with care.
Recording requirements
To evaluate the high-frequency component of the heart rhythm, a recording of about 1 minute is needed, whereas analysis of the low-frequency component requires about 2 minutes. For reliable LF/HF values the Task Force recommends standardised recordings of 5 minutes, at rest, in a stable position and with regular breathing.
Why the classical interpretation is limited
The idea behind LF/HF was that LF reflects sympathetic and HF parasympathetic activity, so their ratio would show the "balance". However:
- LF power is substantially vagal: it falls by at least half after blockade of the vagus nerve, and it reflects baroreflex modulation rather than sympathetic tone.
- Sympathetic and vagal activity do not always change in opposite directions; both can rise or fall together.
- Breathing rate and depth change HF power independently of autonomic tone; slow breathing moves respiratory power into the LF band and raises LF/HF.
- In large populations the ratio is very variable and skewed, so a single value has limited meaning.
Practical interpretation
| LF/HF | Spectral pattern | Typical situations |
|---|---|---|
| < 1 | HF predominates | Lying at rest, deep sleep, relaxation, strong respiratory sinus arrhythmia |
| ≈ 1–3 | Common at rest | Healthy adults in short recordings |
| > 3–4 | LF predominates | Standing, mental load, stress, REM sleep, slow breathing; reduced vagal modulation |
The ratio is most informative when it is followed over time or compared between standardised conditions (for example lying versus standing) in the same person, together with absolute LF and HF power, RMSSD and heart rate.
LF/HF is not a diagnostic test on its own. Consult a doctor if HRV indices remain abnormal over a long period together with palpitations, dizziness on standing, high blood pressure, insomnia or chronic fatigue.
References
- Billman GE. The LF/HF ratio does not accurately measure cardiac sympatho-vagal balance. Front Physiol. 2013;4:26. doi:10.3389/fphys.2013.00026
- Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. Heart rate variability: standards of measurement, physiological interpretation and clinical use. Circulation. 1996;93(5):1043–1065. doi:10.1161/01.CIR.93.5.1043
- Nunan D, Sandercock GRH, Brodie DA. A quantitative systematic review of normal values for short-term heart rate variability in healthy adults. Pacing Clin Electrophysiol. 2010;33(11):1407–1417. doi:10.1111/j.1540-8159.2010.02841.x
- Goldstein DS, Bentho O, Park MY, Sharabi Y. Low-frequency power of heart rate variability is not a measure of cardiac sympathetic tone but may be a measure of modulation of cardiac autonomic outflows by baroreflexes. Exp Physiol. 2011;96(12):1255–1261. doi:10.1113/expphysiol.2010.056259
This article is for general information and does not replace a consultation with a doctor. HRV values depend on the device, recording length and conditions.
Published on 29 June 2019