Vagal tone

Vagal tone refers to the ongoing activity of the vagus nerve (the tenth cranial nerve, running from the brainstem through the chest and abdomen) on the heart and other organs. The vagus nerve is the primary outflow of the parasympathetic nervous system, the branch of the autonomic nervous system (the system that controls automatic body functions such as heart rate, breathing, and digestion) associated with rest, recovery, and calm. At rest, the vagus nerve slows the heart by releasing acetylcholine at the sinoatrial node (the heart's natural pacemaker); higher vagal tone means that parasympathetic influence on the heart is stronger at baseline. See Wikipedia: Vagal tone for an overview, and Laborde et al. (2017) in Frontiers in Psychology (PMC5316555): Heart Rate Variability and Cardiac Vagal Tone in Psychophysiological Research, which describes cardiac vagal tone as the contribution of the parasympathetic nervous system to cardiac regulation.

Because the vagus nerve cannot be measured directly without invasive procedures, vagal tone is most often estimated indirectly from heart rate variability (HRV), the natural variation in the time between heartbeats. One component of HRV, respiratory sinus arrhythmia (RSA, the way the heart rate rises slightly on inhalation and falls on exhalation), is driven largely by vagal activity and is the most widely used non-invasive proxy for vagal tone. HRV is also shaped by breathing rate and depth, age, physical fitness, posture, and time of day; it is therefore a proxy estimate of vagal activity, not a direct measurement. See Hasan (2018), PMC5974730: Probing heart rate variability to determine parasympathetic dysfunction, which notes that HRV parameters traditionally associated with parasympathetic tone do not always reliably measure parasympathetic tone.

Vagal tone is estimated in practice from HRV metrics such as RMSSD (the root mean square of successive differences between adjacent beat intervals) or the high-frequency power band of the HRV spectrum (roughly 0.15 to 0.4 Hz), both of which reflect vagally driven beat-to-beat fluctuation. Higher values on these metrics serve as a proxy for higher resting vagal tone. In observational and experimental research, higher resting vagal tone (as estimated by HRV) is associated with calmer baseline physiology, a greater capacity to shift smoothly between rest and active states (sometimes called autonomic flexibility), and better emotional regulation. These are associations, not demonstrations of a causal pathway; any individual HRV reading is also shaped by breathing pattern, posture, fitness level, and age. See Hasan (2018), PMC5974730: Probing heart rate variability to determine parasympathetic dysfunction. Note: vagal tone as a physiological measure is distinct from polyvagal theory. Vagal tone describes a measurable quantity (the level of parasympathetic input to the heart); polyvagal theory is a separate, contested theoretical framework about how the vagus nerve structures human social behaviour and threat response. For context, see the polyvagal theory glossary term and the sound healing pillar.