Vagal tone and slow breathing

Brief reference section on sympathetic and parasympathetic physiology, companion to the Why 6 BPM? doc.

What "vagal tone" actually means

The vagus nerve is the tenth cranial nerve and the primary highway of the parasympathetic nervous system. It runs from the brainstem to the heart, lungs, gut, and other viscera. "Cardiac vagal tone" refers to the ongoing inhibitory signal the vagus sends to the sinoatrial node of the heart: the more vagal tone, the slower the resting heart rate, the larger the beat-to-beat variability, and the faster heart rate recovers after stress.

It's not a binary "on/off" state. It's a continuous output level. Higher tonic vagal output means greater parasympathetic dominance: lower resting HR, higher HRV, faster recovery, lower inflammatory markers, and better emotional regulation.

The two oscillating branches

The autonomic nervous system has two branches that act in opposition, though not in a strict zero-sum way:

• Sympathetic (SNS): "Fight or flight." Norepinephrine release, vasoconstriction, heart rate up, pupils dilate, digestion suppressed. Active in stress, exercise, alertness.
• Parasympathetic (PNS): "Rest and digest." Acetylcholine release via the vagus, heart rate down, blood pressure down, digestion engaged, recovery and tissue repair. Active in sleep, after meals, during slow breathing.

In healthy people, these aren't locked at fixed levels. They oscillate, often within a single breath cycle. On inhalation, vagal outflow is briefly inhibited and heart rate rises. On exhalation, vagal outflow resumes and heart rate falls. This is respiratory sinus arrhythmia (RSA), and it's the largest natural HRV signal in healthy adults.

How slow breathing intervenes

1. The longer exhale phase gives the vagus a longer window of unopposed activity each cycle, producing larger phasic acetylcholine release.
2. Pulmonary stretch receptors during deep inhale activate the Hering-Breuer reflex, which independently increases vagal output.
3. Phase coherence between respiration, blood pressure oscillations, and heart rate at about 0.1 Hz amplifies HRV through the resonance mechanism.
4. Baroreflex sensitivity, the gain of the BP-to-HR feedback loop, increases.
5. Over weeks of practice, tonic vagal tone rises: the baseline level of parasympathetic output, not just the during-session effect.

Important caveat: it's not a strict seesaw

Older models treated SNS and PNS as a simple seesaw: one up, the other down. Modern autonomic physiology has largely abandoned this. Sympathetic and parasympathetic activity can be independently elevated, independently suppressed, or co-activated depending on context. The frequently cited "LF/HF ratio" as a measure of sympathovagal balance has been heavily criticized in recent literature. What HRV reliably indexes is cardiac vagal tone, not a simple SNS/PNS balance.

So: slow breathing reliably increases vagal tone. Whether it decreases sympathetic activity depends on context and is harder to measure cleanly.

HRV metrics you'll see referenced

• RMSSD (root mean square of successive differences): the cleanest time-domain index of cardiac vagal tone. Higher means more vagal activity.
• HF power (0.15-0.40 Hz): frequency-domain index of vagal activity at normal breathing rates (9-24 BPM).
• LF power (0.04-0.15 Hz): contains the resonance peak around 0.1 Hz. During slow-paced breathing below 9 BPM, increases in LF power are almost entirely vagally mediated (Kromenacker et al. 2018), not sympathetic, despite older interpretations.
• SDNN: overall HRV across all frequencies. Coarser than RMSSD but useful for longer recordings.
• LF/HF ratio: historically interpreted as sympathovagal balance. Largely discredited as such. Use with caution.

Primary sources

Cardiac vagal tone and HRV

• Laborde, S., Mosley, E., & Thayer, J. F. (2017). Heart Rate Variability and Cardiac Vagal Tone in Psychophysiological Research: Recommendations for Experiment Planning, Data Analysis, and Data Reporting. Frontiers in Psychology, 8, 213. frontiersin.org
• Shaffer, F., McCraty, R., & Zerr, C. L. (2014). A healthy heart is not a metronome: an integrative review of the heart's anatomy and heart rate variability. Frontiers in Psychology, 5, 1040.

Slow breathing increases vagal tone

• Laborde, S., Allen, M. S., Borges, U., et al. (2022). Effects of voluntary slow breathing on heart rate and heart rate variability: A systematic review and a meta-analysis. Neuroscience & Biobehavioral Reviews. Confirms vagally mediated HRV increases during, immediately after, and following multi-session slow breathing interventions.
• Zaccaro, A., Piarulli, A., Laurino, M., et al. (2018). How Breath-Control Can Change Your Life: A Systematic Review on Psycho-Physiological Correlates of Slow Breathing. Frontiers in Human Neuroscience, 12, 353.

LF power and sympathovagal balance

• Kromenacker, B. W., Sanova, A. A., Marcus, F. I., Allen, J. J. B., & Lane, R. D. (2018). Vagal Mediation of Low-Frequency Heart Rate Variability During Slow Yogic Breathing. Psychosomatic Medicine, 80(6), 581-587. Pharmacological autonomic blockade showed LF-band increases during slow breathing are almost entirely vagal, not sympathetic.
• Billman, G. E. (2013). The LF/HF ratio does not accurately measure cardiac sympatho-vagal balance. Frontiers in Physiology, 4, 26.
• Heathers, J. A. J. (2014). Everything Hertz: methodological issues in short-term frequency-domain HRV. Frontiers in Physiology, 5, 177.

Resonance, single-session effects, and outcomes

• Sevoz-Couche, C., & Laborde, S. (2022). Heart rate variability and slow-paced breathing: when coherence meets resonance. Neuroscience & Biobehavioral Reviews. sciencedirect.com
• You, M., Laborde, S., Salvotti, C., et al. (2021). Benefits from one session of deep and slow breathing on vagal tone and anxiety in young and older adults. Scientific Reports, 11.
• Thayer, J. F., Yamamoto, S. S., & Brosschot, J. F. (2010). The relationship of autonomic imbalance, heart rate variability and cardiovascular disease risk factors. International Journal of Cardiology, 141(2), 122-131.
• Wulsin, L. R., Horn, P. S., Perry, J. L., Massaro, J. M., & D'Agostino, R. B. (2015). Autonomic Imbalance as a Predictor of Metabolic Risks, Cardiovascular Disease, Diabetes, and Mortality. Journal of Clinical Endocrinology and Metabolism, 100(6), 2443-2448.

Clinician explainer

• Dr. Pradip Jamnadas, MD, FACC, FSCAI, FCCP: interventional cardiologist at Cardiovascular Interventions, P.A. in Orlando, practicing since 1990. His channel covers vagal tone, autonomic regulation, slow breathing, fasting, and cardiometabolic disease from a practicing-cardiologist perspective. YouTube channel

The honest summary

Slow-paced breathing at about 6 BPM with extended exhale, practiced about 15 min/day, reliably:

• Increases cardiac vagal tone during the session.
• Increases HRV amplitude, especially in the LF band, almost entirely vagally mediated.
• Modestly reduces blood pressure over weeks of practice.
• Acutely reduces subjective anxiety and stress.

What it does not clearly do, despite popular claims:

• Produce dramatic shifts in a single "sympathovagal balance" score.
• Replace medication for established hypertension.
• Cure conditions.
• Work better than other evidence-based slow-breathing techniques for most outcomes. The specific rate and ratio matter more than the specific tradition.

It's a real, well-evidenced intervention with modest-but-meaningful effects. The science is sound. The marketing around it is often louder than the evidence supports.