Baroreflex
Also called baroreceptor reflex, baroreflex gain, baroreflex sensitivity
The reflex that steadies blood pressure from beat to beat by changing heart rate. Its built-in delay makes heart rate rise and fall about every 10 seconds, the 0.1 Hz rhythm in LF power.
Pressure sensors in the walls of the large arteries feel every rise and fall in blood pressure. When pressure rises, the brainstem presses the vagal brake and the heart slows; when it falls, the brake eases and the heart speeds up. That loop is the baroreflex. Researchers measure its strength as baroreflex gain: the change in heart rate per unit change in blood pressure.
The loop takes a few seconds to go round, so it overshoots and corrects in a regular wave. Shaffer, McCraty and Zerr (2014) put the resonance of this system at about 0.1 Hz, one cycle every 10 seconds.
Why it matters for HRV.
- LF power LF power The variance of the heartbeat intervals that falls in the 0.04 to 0.15 Hz band, in ms²: rhythms repeating every 7 to 25 seconds. Once read as sympathetic activity, which is now disputed. More about LF power . The same review concludes that at rest, the LF band (0.04 to 0.15 Hz) reflects baroreflex activity, not sympathetic Sympathetic nervous system The "fight or flight" branch of the autonomic nervous system. It speeds up the heart and makes it pump harder, but it acts too slowly to drive beat-to-beat HRV. More about Sympathetic nervous system nerve traffic to the heart. That is one reason the LF/HF ratio LF/HF ratio LF power divided by HF power, with no unit. It was proposed as a gauge of balance between the sympathetic and parasympathetic systems, but research shows it cannot measure that reliably. More about LF/HF ratio is not a stress meter.
- Slow breathing. Breathing at about 6 breaths a minute (0.1 Hz) lines the breathing wave up with the baroreflex wave, and the two add up. Lehrer and Gevirtz (2014) describe this resonance as the basis of HRV biofeedback HRV biofeedback A breathing practice in which you watch your heart rate live and breathe slowly, often near 6 breaths a minute, to make the heart's rise and fall with each breath as large as possible. More about HRV biofeedback and report large increases in baroreflex gain during it.
- Standing up. When you stand, blood pools in the legs and pressure dips. The baroreflex lifts heart rate within seconds, which is what an orthostatic test Orthostatic test A short morning test: lie or sit still for 2 minutes, then stand for 2 minutes, recording heart rate and HRV in both positions to see how your body handles standing up. More about Orthostatic test records.
Slide the breathing pace to 6 breaths a minute below and watch the swing in the gaps between beats grow as breathing and the reflex fall into step.
Breathe and watch your heart
A teaching model, not a measurement
- Heart rate
- 60 bpm
- RMSSD
- 46 ms
Last 30 seconds
This picture moves when JavaScript is on.
A breathing circle grows on each in-breath and shrinks on each out-breath. A strip of heartbeats below it shows the beats bunching closer together during the in-breath, which is shaded, and spreading apart during the out-breath. A line under the strip plots the gap before each beat: it dips on the in-breath and rises on the out-breath. A slider sets the pace from 4 to 15 breaths a minute, and readouts give heart rate and RMSSD for the last 30 seconds. The swing is largest at about 6 breaths a minute.
See also Mayer waves and the lesson The nervous system behind HRV.