HRV, baroreflex, and why 'vagal tone' is more nuanced than wellness apps suggest.
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.
HRV and 'vagal tone' claims live mostly in Tier 5 (consumer wellness). Apply the map before trusting any HRV product.
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.
Short patient encounters that test your judgment, not your recall. Pick the most defensible response, then reveal the rationale and a sample coaching script you could actually say at the bedside.
A 36-year-old patient texts you in alarm: their wearable shows HRV down 35% the morning after a friend's wedding (late dinner, three glasses of wine, ~5 hours of sleep). They ask if their vagus is 'damaged.'
A 50-year-old shows you their wearable's daily 'sympathovagal balance' score (LF/HF ratio) and asks whether a high number means too much stress.
A competitive amateur runner notices their RMSSD plateaued despite four weeks of harder training. They ask whether to increase training load to push HRV up.