Autonomic Nervous System & 'Vagal Tone'
HRV, baroreflex, and why 'vagal tone' is more nuanced than wellness apps suggest.
What's covered
- 01Parasympathetic vs sympathetic — beyond the oversimplification
- 02Baroreflex and respiratory sinus arrhythmia (RSA)
- 03HRV metrics: RMSSD, HF-HRV, SDNN, pNN50, LF/HF (and why LF/HF is contested)
- 04Vagal brake and autonomic flexibility
- 05Confounders: breathing rate, posture, fitness, sleep, meds, age, fever, hydration
- 06Standardization: Task Force 1996, eCardiology updates
What this means for you
Heart rate variability (HRV) is a useful signal, but it isn't a 'vagus score'. Many things change it — your breathing, sleep, posture, fitness, even fever or alcohol the night before. A single HRV reading rarely means anything on its own; trends over weeks are more useful.
Teach HRV interpretation in clinical context. Avoid using single HRV readings for diagnosis. Discuss measurement standardization (Task Force 1996). RMSSD and HF-HRV are the cleanest vagal proxies; LF/HF ratio should be interpreted cautiously. Document context (breathing rate, posture, time of day, recent activity) with every reading.
RSA gating is an active area: vagal outflow is dynamically modulated within the respiratory cycle. Saccharine 'higher = better' framing collapses this. Consider time-domain vs frequency-domain vs nonlinear (DFA, sample entropy) approaches and their respective limitations.
Low HRV means trauma or a 'damaged' vagus nerve.
HRV is a context-dependent marker; low HRV correlates with many conditions but is not diagnostic of any single one. It can be lowered by acute illness, dehydration, alcohol, poor sleep, or simply standing up.
What the data says
Test yourself
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Optional deeper dive
- An overview of heart rate variability metrics and norms — Shaffer & Ginsberg, Frontiers in Public Health 2017↗
- Heart rate variability: standards of measurement, physiological interpretation, and clinical use — Task Force of the ESC/NASPE, Circulation 1996↗