LF
LF — low-frequency power — is the heart-rate-variability power in the band 0.04–0.15 Hz, centred around oscillations of about 10 seconds (~0.1 Hz). These "Mayer waves" arise from the baroreflex — the feedback loop that keeps blood pressure stable. LF power reflects a mixture of sympathetic and parasympathetic influences and, according to current evidence, mainly the baroreflex modulation of autonomic outflow rather than pure sympathetic tone.
Physiology
Blood pressure is controlled by baroreceptors in the aorta and carotid arteries. When pressure rises, they signal the brainstem to slow the heart (vagally) and relax vessels (by reducing sympathetic tone); when it falls, the opposite happens. Because the loop has a delay of a few seconds, blood pressure and heart rate oscillate with a period of about 10 seconds. These oscillations appear as the LF peak in the heart-rate spectrum. In the past LF power was interpreted as a direct index of cardiac sympathetic activity, but this has not been confirmed: blocking the vagus nerve markedly reduces LF power, and LF power does not correlate with direct measures of cardiac sympathetic innervation. LF is therefore best regarded as an index of baroreflex function and of combined autonomic modulation.
Measurement and units
- Absolute LF (ms²) varies widely between people and falls with age; it is best followed as a personal trend or as ln LF.
- Normalised LF (LF n.u. = LF / (LF + HF) × 100) expresses LF relative to the sum of LF and HF — see the separate article on nLF.
- Slow breathing at about 6 breaths per minute (0.1 Hz) shifts respiratory oscillations into the LF band and greatly increases LF power — this is the basis of "resonance breathing" and HRV biofeedback, not a sign of sympathetic activation.
Interpretation
Low LF
- Reduced baroreflex function and overall autonomic modulation
- Heart failure (LF may almost disappear in advanced stages), diabetic autonomic neuropathy, after myocardial infarction
- Ageing, lack of fitness, severe stress or exhaustion
High LF
- Standing up, mental or emotional load (often with a relative rise in LF n.u.)
- Slow breathing around 6 breaths per minute, HRV biofeedback
- Artefacts, ectopic beats, non-stationary recordings
LF alone does not diagnose disease. If spectral and other HRV indices remain low, or if you experience dizziness on standing, fainting, palpitations or unstable blood pressure, consult a doctor — these may reflect impaired baroreflex or autonomic function.
References
- 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
- 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
- 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
- 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
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 1 July 2019