The Low-Frequency Power Ratio, often expressed as the LF/HF ratio (Low Frequency to High Frequency), is a critical metric derived from the frequency domain analysis of Heart Rate Variability (HRV). This ratio is widely interpreted in clinical physiology as an index of sympathovagal balance, reflecting the relative contribution of the sympathetic and parasympathetic nervous systems to cardiac regulation. A higher ratio generally indicates a dominance of sympathetic tone, suggesting a state of heightened stress, fight-or-flight activation, or reduced parasympathetic withdrawal capacity. Monitoring this ratio is essential for assessing allostatic load and its impact on hormonal health.
Origin
This frequency domain analysis originated from engineering signal processing techniques adapted for biological signals, gaining formal standardization in the 1990s by the Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. The low-frequency band (LF: 0.04–0.15 Hz) is thought to reflect a mix of sympathetic and vagal modulation, while the high-frequency band (HF: 0.15–0.40 Hz) is purely vagal. The ratio simplifies this complex interaction into a single, clinically relevant number.
Mechanism
The mechanism involves spectral analysis of the interbeat interval data, decomposing the heart rate fluctuations into their component frequencies. The LF component is primarily influenced by baroreflex activity, which is mediated by both branches of the autonomic nervous system. When the LF/HF ratio is elevated, it signifies a shift toward sympathetic predominance, which is associated with chronic release of catecholamines and sustained HPA axis activation. This sustained neuroendocrine state contributes directly to systemic inflammation and hormonal dysregulation, emphasizing the ratio’s utility as a biomarker of chronic stress.
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