Respiratory Sinus Arrhythmia (RSA)
On the rhythmic dance between breathing and heartbeat, and on vagal health and resilience
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Place your hand on your chest, or feel your pulse, while breathing calmly in and out. If you pay close attention, you may notice that your heart rate subtly speeds up as you inhale and slows down as you exhale. This rhythmic pattern, entirely natural yet usually unnoticed, is called respiratory sinus arrhythmia, abbreviated RSA.
The term may sound technical, even alarming (“arrhythmia”1 [1] means “no (= a) rhythm” and suggests a heart rhythm disorder), but RSA is actually a sign of good health. It reflects the smooth cooperation between your breathing and your heart function, with the vagus nerve as the connecting link: the long, branching nerve that runs from your brainstem downward and innervates2 your heart, lungs, and many other organs.
Where heart rate variability (HRV) offers a broad window onto the total variability in your heart rate, respiratory sinus arrhythmia is more specific and more precise. RSA measures the direct influence of the ventral vagus: the fast, myelinated pathway of the vagus nerve that, in mammals, sits at the center of a system for social engagement, flexibility, and resilience.
In this article I’ll take you through the world of RSA: what it is, how it arises, why it matters so much for your health and wellbeing, how it’s measured, and how it sits at the heart of polyvagal theory, the framework that has transformed our understanding of trauma, connection, and recovery.
RSA in a nutshell
In this section I tell the story of RSA in an accessible form, suitable and sufficiently detailed for most readers: what it is, how it arises, what it says about your health, and how you can strengthen it. Readers who are eager to know more about the technical and scientific background will find that further on, in the deep-dive section.
What is respiratory sinus arrhythmia?
The definition: breathing and heartbeat in harmony
RSA is the natural, rhythmic variation in heart rate that runs in sync with the breathing cycle. During inhalation, your heart rate speeds up slightly. During exhalation, it slows down again. This rise and fall is not a coincidence or a malfunction, but a beautiful example of how your body continuously optimizes and synchronizes itself.
The word “arrhythmia” in the term can be confusing. In medical usage, arrhythmia denotes a (sometimes problematic) irregularity in heart rhythm. But the “sinus arrhythmia” in RSA refers to a healthy, functional variation. The heartbeat simply originates in the sinoatrial node, the heart's natural pacemaker, but its tempo is rhythmically adjusted via the vagus nerve, in time with breathing. The word “sinus” signals precisely that the beats continue to come from that pacemaker as normal, and it is that fact which distinguishes this harmless variation from a true rhythm disorder. RSA is therefore not a disorder at all, but a sign of a well-functioning, flexible autonomic nervous system.
Making the pattern felt
Try it yourself: sit or lie down quietly, place your hand on your chest or feel your pulse, and breathe slowly and deeply. Inhale for about four to five counts, exhale for about six to eight. You’ll notice that your heart rate subtly but perceptibly speeds up as you inhale and slows down as you exhale. That’s your RSA in action.
The amplitude of that fluctuation, the difference between the fastest and the slowest heart rate within a breathing cycle, averaged across several breaths, says something about the strength and flexibility of your vagal regulation. A larger amplitude points to more flexible vagal tone, a measure of the “fitness” of your nervous system. A small amplitude, or the absence of RSA, can point to reduced vagal function.
How does RSA arise?
RSA is not a passive byproduct of breathing; it is the result of a finely tuned interplay in the brainstem that couples breathing and heart function. Two elements are enough to grasp the essentials: the role of breathing and the role of the ventral vagus. The brainstem network that fine-tunes this coupling is described in the deep-dive section toward the end of this article.
The role of breathing
During inhalation, your chest expands and the pressure in your chest cavity drops. Besides drawing air in, this also pulls more blood from the veins toward the heart, giving the heart more blood to work with. At the same time, sensors in the lungs and blood vessels send a signal to the brainstem: the heart rate may rise slightly. During exhalation this reverses: pressure rises again, less blood flows back, and the heart rate falls.
At first glance this looks purely mechanical. But the real control lies in the nervous system. Not in the nerve fibers themselves; a nerve fiber doesn’t generate the signal itself, it conducts it: the rhythm originates in neurons located in the brainstem and reaches the heart via the vagus nerve.
The ventral vagus: the fast pathway to the heart
The vagus nerve is the tenth cranial nerve, one of the longest and most extensively branching nerves in the body. Within that nerve, fibers of two different origins run intertwined3. The ventral vagus consists of myelinated (insulated) fibers that originate in the nucleus ambiguus and conduct quickly; they innervate the heart, the larynx, and other structures above the diaphragm. The dorsal vagus consists of slower, unmyelinated fibers and mainly innervates organs below the diaphragm, such as the stomach, intestines, liver, and pancreas.
RSA arises through the ventral vagus. During exhalation, the neurons in the nucleus ambiguus fire most strongly, applying the vagal brake to the heart and slowing the heart rate. During inhalation, that vagal brake eases, a phenomenon known as “inspiratory gating,” allowing the heart rate to rise. RSA is therefore not a mechanical reflex but an actively regulated rhythm that finely coordinates breathing and heart function.
RSA and HRV: what’s the difference?
Heart rate variability (HRV) is a broad concept: all variation over time between successive heartbeats, regardless of cause. Breathing, blood pressure regulation, temperature, hormones, and emotion all contribute to it. Think of it as the total sound of an orchestra. Respiratory sinus arrhythmia is one specific component within that: the breathing-linked variation, which mainly reflects regulation by the ventral vagus. If HRV is the orchestra, RSA is the solo violin. For the full picture on HRV, the metrics involved, and its meaning for our health, see my in-depth Relaxicon article on HRV.
Why does this distinction matter? A general measure like HRV bundles all those influences (the whole orchestra) into a single number, without separating them out. A high value can point to healthy vagal flexibility, but it can just as easily reflect other factors. RSA isolates the ventral vagal component and so provides more targeted information about how smoothly that ventral vagal system specifically is working, the system that supports social engagement, safety, and resilience.
RSA and polyvagal theory
Polyvagal theory (Porges, 1995, 2007) places RSA within an evolutionary and neurobiological framework. Its core idea is that the autonomic nervous system consists not of two, but of three hierarchically ordered systems.
The oldest is the dorsal vagal complex, which supports metabolic recovery and rest and, under overwhelming threat, can lead to immobilization or “shutdown.” Above that sits the sympathetic nervous system, which supports mobilization: being active, or fight-and-flight behavior under threat. In the theory’s layered structure, this system is younger than the dorsal vagus and older than the ventral vagus. The newest, in terms of the integration seen in mammals, is the ventral vagal complex, which supports social engagement, communication, and co-regulation, and which generates RSA.
These systems operate according to a simple logic. When the body feels safe, the ventral system dominates: you are socially accessible, your voice carries melody, your face is expressive. This is sometimes called the “green zone.” When the ventral system withdraws under threat, sympathetic mobilization takes over, the “yellow zone” of action and alertness. If that doesn’t help either, for example under overwhelming threat with no way out, the dorsal system comes to dominate: immobilization and dissociation, the “red zone.”
RSA as a marker of safety
Within this hierarchy, RSA functions as a real-time signal of ventral vagal activity. The amplitude of that fluctuation (see above) reflects the strength and flexibility of your vagal regulation.
Low or absent RSA points to reduced activity in that system and to a more defensive or mobilized state. This carries implications for therapy, education, and parenting: someone showing low RSA is, physiologically, not in the best possible state to connect or to learn. Interventions that foster safety and ventral vagal activation can help shift that.
Does RSA still serve a function in us?
Here comes a surprising fact that may challenge your intuition a little. The breathing-linked variation in heart rate is ancient, predating mammals by hundreds of millions of years. In animals without fully separated pulmonary and systemic circulation, such as amphibians and most reptiles, and in fish with their gill-based breathing, RSA does important work: the fluctuation helps direct blood efficiently past the respiratory organs (gills or lungs) with each breath.
Mammals no longer need that efficiency gain, because our hearts have fully separated chambers for the pulmonary and systemic circulations. In mammals, then, RSA is not really an evolutionary innovation, nor a functional mechanism anymore, but a holdover from a time when the rhythm still had real work to do. I’ve explored this in depth in the article “How Old Is That Ventral Vagus, Really?”
But if RSA has lost its original function in us, why is it still considered a marker of health? Because for us, its value lies not in what it does, but in what it reveals. The variation arises because the heart is continuously fine-tuned, moment to moment, via the fast ventral vagal fibers. The magnitude of that fluctuation is therefore a window onto how smoothly that regulation is working. High variability points to a flexible, responsive system; low variability points to a system faltering under stress, illness, or age. In mammals, RSA is no longer a functional mechanism, but it remains a useful and measurable signal, and that is precisely where its value lies.
RSA across the lifespan
RSA is measurable even before birth, in fetal heart rate patterns. The fetus is not yet breathing air, but does make rhythmic breathing movements, and RSA is visible in the heart rate during those episodes4 (Divon et al., 1985). Reed, Ohel, David, and Porges (1999) showed that low RSA, a sign of low ventral vagal tone, precedes and is associated with vulnerability to life-threatening bradycardia during labor; strong ventral vagal tone, by contrast, appears protective. In newborns, baseline RSA predicts autonomic reactivity and developmental outcomes; premature infants consistently show lower RSA. Interventions such as skin-to-skin contact and a soft, melodic voice can raise it.
In children, higher RSA is associated with better attention, emotion regulation, and social skills. A healthy “RSA suppression,”5 in which RSA temporarily drops during a challenge and then recovers, points to flexibility; a failure to recover can point to rigidity.
Respiratory sinus arrhythmia declines with age. That decline is not linear: on average it is steepest in early adulthood and levels off afterward. This decline can be influenced by behavior: people who exercise regularly, sleep well, manage stress, and maintain social connections retain more autonomic flexibility later in life.
RSA and health
RSA carries signal value across a wide range of contexts. In cardiology, low RSA predicts elevated risk of cardiovascular disease and a poorer prognosis after a heart attack, while RSA recovery after rehabilitation is a favorable sign.
In mental health care, reduced RSA is commonly seen in anxiety, depression, and post-traumatic symptoms. This fits with a chronically “withdrawn” ventral system and a less flexible autonomic state that is less accessible to connection and relaxation. Treatments that raise vagal tone, from psychotherapy to movement and breathwork, are often accompanied by an increase in RSA and by symptom improvement (Lehrer et al., 2020; Goessl et al., 2017). Reduced RSA has also been described in functional conditions such as irritable bowel syndrome, chronic fatigue, and fibromyalgia (Meeus et al., 2013; Escorihuela et al., 2020).
For athletes, RSA helps with dosing training and recovery: a high value points to recovery and capacity for load, a declining value to possible overtraining. Faster RSA recovery after exertion is a sign of good autonomic flexibility and cardiovascular fitness.
Strengthening RSA: what can you do?
The good news is that RSA can be influenced. The most direct route is slow, deep breathing. Breathing at your so-called resonance frequency improves your RSA amplitude. That frequency differs from person to person but for most people lies around five to six breaths per minute (it depends partly on body build and on the timing of your baroreflex). A simple exercise: inhale for four to five seconds through your nose and exhale for six to eight seconds, repeated for five to ten minutes a day. The longer exhale strengthens vagal activity and directly raises RSA. HRV biofeedback, in which you guide your breathing rhythm using real-time feedback, helps you find your personal resonance frequency and is especially useful for anxiety and autonomic dysregulation6.
Regular, moderate exercise also helps, provided it’s balanced with recovery, since overtraining actually lowers RSA. Meditative practices such as mindfulness and attentive relaxation raise RSA, as does good sleep. Social connection plays a notable role: in safe, connected interactions, the heart rhythms and RSA of two people become more aligned (Feldman et al., 2011; Palumbo et al., 2017): their ventral vagal systems tune to one another. A calm, well-regulated partner can in this way help the other person settle. This is called co-regulation (Porges, 2022), and it is not only emotionally valuable but also physiologically restorative.
Voice and hearing play a part too: singing, humming, and speaking melodiously activate the ventral vagus via the nucleus ambiguus, which also controls the larynx. And according to polyvagal theory, the experience of safety, so-called neuroception of safety, is a basic precondition: calm, predictable, socially supportive environments support RSA, while threat and unpredictability lower it.
In closing
Respiratory sinus arrhythmia is more than just a technical measurement. It is a window onto a deeply rooted system that supports safety, social engagement, and resilience. In mammals, the breath-linked fluctuation in heart rhythm may have lost its original function, but as a signal it remains valuable: it shows how smoothly the heart is being fine-tuned, moment to moment, via the ventral vagus.
High RSA points to a body that feels safe enough to relax and connect; low RSA points to withdrawal and self-protection. And because RSA can be influenced through breathing, movement, co-regulation, and safety, it also offers a foothold for recovery.
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Deep dive: for those who are keen to know more
This section is for readers who want to know more about the technical and scientific background underlying RSA. The general section above is likely sufficient for most readers; what follows is supplementary, not required reading.
The brainstem as conductor: the cardiopulmonary oscillator system
In short: a network in the brainstem couples breathing, heart rate, and voice, and actively generates the RSA rhythm.
In the early 1990s, Richter and Spyer described a brainstem circuit that coordinates breathing, heart rate, and laryngeal activity (Richter & Spyer, 1990). Three structures work together here. The pre-Bötzinger complex in the ventrolateral medulla (the structure located toward the front and side of the medulla oblongata, part of the brainstem) generates the basic breathing rhythm. The nucleus ambiguus regulates vagal output to the heart and larynx. The nucleus tractus solitarius (NTS) integrates sensory information, among other things from pressure receptors in the blood vessels, from oxygen and CO2 sensors, and from lung receptors, and relays it onward, including to the nucleus ambiguus and the pre-Bötzinger complex7.
These structures therefore synchronize their activity. The pre-Bötzinger complex rhythmically modulates vagal output in phase with breathing: during inhalation that output is suppressed, during exhalation it is facilitated again. The result is a centrally generated, rhythmic pattern in which breathing and heart rate are coordinated with one another. RSA is thus the output of an evolutionarily conserved system that integrates breathing, heart rate, and even vocalization.
A brief history of RSA
In short: RSA has been known since the mid-nineteenth century; Porges turned it into a usable measure in the twentieth century.
The connection between breathing and heart rate was already noted in 1733 by Stephen Hales. The first proper description of RSA is credited to Carl Ludwig, who recorded the phenomenon with his kymograph in 1847. Shortly after, Donders (1852) and Einbrodt (1860), among others, showed that breathing-induced pressure changes alone could not account for the observed fluctuations, implying that a neural reflex had to be involved. In the 1930s, Anrep and colleagues mapped the mechanism in detail (Anrep et al., 1936), and earlier still, in 1915, Eppinger and Hess linked elevated vagal tone to clinical presentations.
In the latter half of the twentieth century, Stephen Porges turned RSA into a workable measure. As a young researcher he noticed that the breathing-linked rhythm correlated with performance on reaction-time and attention tasks, and he set out to find a way to measure it cleanly (Porges, 1972). The problem was that ordinary spectral analysis ran into trouble because breathing is never perfectly regular and physiological signals are constantly changing. Together with mathematician Robert Bohrer, he developed a method that removes slow trends from the signal and then filters within the respiratory frequency band. This Porges-Bohrer method has since been used in hundreds of studies and proved sensitive enough to measure RSA in premature infants, children, and adults, even under unstable conditions.
How exactly is RSA measured?
In short: you record heart rate (and ideally breathing as well) and then isolate the breathing-linked component.
The most precise method is an electrocardiogram (ECG). The R-peak in that signal marks each heartbeat, and the time between two R-peaks forms the basis for analysis. Chest straps that record an ECG signal are suitable; optical wrist sensors are generally too imprecise for accurate RSA determination. It is useful to record breathing simultaneously, for example with a respiratory belt, because the correct frequency band varies by person and situation.
The Porges-Bohrer method works in a few steps. First, the time intervals between successive heartbeats are arranged in sequence. Then the slow, gradual changes in that signal are filtered out, leaving only the fast variation that moves with breathing. The RSA amplitude is then a measure of how large that variation is. Simpler approaches also exist, such as the difference between the fastest and slowest heart rate per breath, or isolating the breathing-linked frequency through spectral analysis. These are easier to calculate but more sensitive to disturbance. Validation studies point to the Porges-Bohrer method as the most reliable (Lewis et al., 2012).
How do we know RSA reflects the ventral vagal system?
In short: pharmacological studies, direct nerve recordings, and the timing argument all point to a vagal origin.
The vagal basis of RSA has been demonstrated in several ways. When you administer atropine, a substance that blocks cholinergic transmission from the vagus to the heart, RSA disappears completely while breathing remains unchanged. Give a beta-blocker instead, which inhibits sympathetic activity, and RSA remains largely intact. Block both at once, and what remains is a nearly flat, even heart rhythm. RSA is therefore a vagal phenomenon, not a sympathetic one.
Direct recordings from the vagus nerve further show that cells in the nucleus ambiguus fire in time with breathing, driving the fluctuation in heart rate. What you record in the nerve is the conducted signal traveling along their axons, the vagal fibers. Sympathetic neurons do not show this pattern. There is also a timing argument. RSA requires fine-tuning of the heart at every single beat, within a fraction of a second. Sympathetic transmission runs through slow cascades that take seconds to tens of seconds, far too slow. Parasympathetic transmission via acetylcholine, by contrast, works within milliseconds, because it directly opens coupled potassium channels. Only the fast, myelinated ventral vagus can deliver that speed.
Is RSA really mammal-specific?
The evolutionary debate
In short: the building blocks of RSA are not unique to mammals; what is new is how mammals integrated them.
For a long time, RSA, and the fast myelinated vagal pathway underlying it, was presented as distinctly mammalian. That claim has come under pressure from comparative physiology. Paul Grossman and Edwin Taylor (Grossman & Taylor, 2007) already raised questions about interpreting RSA as a direct, exclusive measure of the mammalian vagus, partly because RSA is strongly influenced by breathing rate and depth. Taylor, Wang, and Leite (2022) subsequently showed that the building blocks, myelinated cardiac vagal fibers, a division of brainstem nuclei, and a breathing-linked heart rhythm, were already present in sharks, lungfish, frogs, and reptiles hundreds of millions of years before mammals existed. In rattlesnakes, for instance, a breathing-linked component in heart rate has been demonstrated that closely resembles mammalian RSA (Campbell et al., 2006).
Porges has responded that breathing is not a confound but a functional part of the same system, because the coupling between breathing and heart is centrally organized in the brainstem (the shared cardiopulmonary oscillator). Filtering out breathing, on this view, does not remove noise but cuts away part of the very system one is trying to measure.
The most reconciling perspective on this debate, in my view, is that of exaptation: old components taking on a new function in a new context. The building blocks are ancient and shared, but the integration of heart, breathing, voice, and gaze into one coherent system, with the mobile, expressive face as the mammal-specific element within it, is genuinely new. If you’d like to read more on this, I recommend my article “How Old Is That Ventral Vagus, Really?” There I also work out why RSA lost its original function in mammals and became a window onto vagal regulation instead, as touched on above.
Looking ahead
In short: measurement technique, linkage to other biomarkers, and molecular research are making RSA increasingly useful.
Research into RSA continues to develop. Machine learning, for instance, is now being used to recognize patterns and make individual predictions. RSA is increasingly combined with other measures, such as skin conductance, cortisol levels, pupil size, and brain activity, to build a multidimensional picture of autonomic and emotional regulation. Wearables are becoming more sophisticated, which may eventually enable reliable measurement and just-in-time interventions. Finally, molecular and transcriptomic research may map the genetic basis of vagal specialization, including genes involved in myelination within the nucleus ambiguus.
In the area of RSA, and how we measure and interpret it, interesting developments lie ahead, ones that matter for polyvagal theory as well.
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Sources and further reading
Anrep, G. V., Pascual, W., & Rössler, R. (1936a). Respiratory variations of the heart rate: I. The reflex mechanism of the respiratory arrhythmia. Proceedings of the Royal Society of London B, 119(813), 191–217. https://doi.org/10.1098/rspb.1936.0005
Anrep, G. V., Pascual, W., & Rössler, R. (1936b). Respiratory variations of the heart rate: II. The central mechanism of the respiratory arrhythmia and the inter-relations between the central and the reflex mechanisms. Proceedings of the Royal Society of London B, 119(813), 218–230. https://doi.org/10.1098/rspb.1936.0006
Campbell, H. A., Leite, C. A. C., Wang, T., Skals, M., Abe, A. S., Egginton, S., Rantin, F. T., Bishop, C. M., & Taylor, E. W. (2006). Evidence for a respiratory component, similar to mammalian respiratory sinus arrhythmia, in the heart rate variability signal from the rattlesnake, Crotalus durissus terrificus. Journal of Experimental Biology, 209(14), 2628–2636. https://doi.org/10.1242/jeb.02278
Divon, M. Y., Yeh, S. Y., Zimmer, E. Z., Platt, L. D., Paldi, E., & Paul, R. H. (1985). Respiratory sinus arrhythmia in the human fetus. American Journal of Obstetrics and Gynecology, 151(4), 425–428.
https://doi.org/10.1016/0002-9378(85)90262-5
Escorihuela, R. M., Capdevila, L., Castro, J. R., Zaragozà, M. C., Maurel, S., Alegre, J., & Castro-Marrero, J. (2020). Reduced heart rate variability predicts fatigue severity in individuals with chronic fatigue syndrome/myalgic encephalomyelitis. Journal of Translational Medicine, 18, 4. https://doi.org/10.1186/s12967-019-02184-z
Feldman, R., Magori-Cohen, R., Galili, G., Singer, M., & Louzoun, Y. (2011). Mother and infant coordinate heart rhythms through episodes of interaction synchrony. Infant Behavior and Development, 34(4), 569–577. https://doi.org/10.1016/j.infbeh.2011.06.008
Grossman, P., & Taylor, E. W. (2007). Toward understanding respiratory sinus arrhythmia: Relations to cardiac vagal tone, evolution and biobehavioral functions. Biological Psychology, 74(2), 263–285. https://doi.org/10.1016/j.biopsycho.2005.11.014
Lehrer, P. M., Kaur, K., Sharma, A., Shah, K., Huseby, R., Bhavsar, J., Sgobba, P., & Zhang, Y. (2020). Heart rate variability biofeedback improves emotional and physical health and performance: A systematic review and meta analysis. Applied Psychophysiology and Biofeedback, 45(3), 109–129. https://doi.org/10.1007/s10484-020-09466-z
Lewis, G. F., Furman, S. A., McCool, M. F., & Porges, S. W. (2012). Statistical strategies to quantify respiratory sinus arrhythmia: Are commonly used metrics equivalent? Biological Psychology, 89(2), 349–364. https://doi.org/10.1016/j.biopsycho.2011.11.009
Meeus, M., Goubert, D., De Backer, F., Struyf, F., Hermans, L., Coppieters, I., De Wandele, I., Da Silva, H., & Calders, P. (2013). Heart rate variability in patients with fibromyalgia and patients with chronic fatigue syndrome: A systematic review. Seminars in Arthritis and Rheumatism, 43(2), 279–287.
https://doi.org/10.1016/j.semarthrit.2013.03.004
Palumbo, R. V., Marraccini, M. E., Weyandt, L. L., Wilder-Smith, O., McGee, H. A., Liu, S., & Goodwin, M. S. (2017). Interpersonal autonomic physiology: A systematic review of the literature. Personality and Social Psychology Review, 21(2), 99–141.
https://doi.org/10.1177/1088868316628405
Porges, S. W. (1972). Heart rate variability and deceleration as indexes of reaction time. Journal of Experimental Psychology, 92(1), 103–110. https://doi.org/10.1037/h0032181
Porges, S. W. (1995). Orienting in a defensive world: Mammalian modifications of our evolutionary heritage. A Polyvagal Theory. Psychophysiology, 32(4), 301–318.
https://doi.org/10.1111/j.1469-8986.1995.tb01213.x
Porges, S. W. (2007). The polyvagal perspective. Biological Psychology, 74(2), 116–143.
https://doi.org/10.1016/j.biopsycho.2006.06.009
Porges, S. W. (2022). Polyvagal Theory: A Science of Safety. Frontiers in Integrative Neuroscience, 16, 871227.
https://doi.org/10.3389/fnint.2022.871227
Porges, S. W. (2023). The vagal paradox: A polyvagal solution. Comprehensive Psychoneuroendocrinology, 16, 100200. https://doi.org/10.1016/j.cpnec.2023.100200
Reed, S. F., Ohel, G., David, R., & Porges, S. W. (1999). A neural explanation of fetal heart rate patterns: A test of the polyvagal theory. Developmental Psychobiology, 35(2), 108–118.
https://doi.org/10.1002/(SICI)1098-2302(199909)35:2%3C108::AID-DEV4%3E3.0.CO;2-N
Richter, D. W., & Spyer, K. M. (1990). Cardiorespiratory control. In A. D. Loewy & K. M. Spyer (Eds.), Central Regulation of Autonomic Functions (pp. 189–207). Oxford University Press.
Taylor, E. W., Wang, T., & Leite, C. A. C. (2022). An overview of the phylogeny of cardiorespiratory control in vertebrates with some reflections on the ‘Polyvagal Theory’. Biological Psychology, 172, 108382. https://doi.org/10.1016/j.biopsycho.2022.108382
“Arrhythmia” in Dutch is spelled with one “r” less in Dutch (“aritmie”) than in English.
Innervation, modulation, and regulation are often used as if they were synonyms, but the three terms each point to something slightly different. Innervation refers to the anatomical presence of nerve fibers reaching an organ: the wiring, without which neither modulation nor regulation would be possible. Modulation means adjusting an already-running process in strength, rhythm, or timing, as when the heart rhythm generated by the sinoatrial node is slowed via the vagus nerve and breathing-linked variability (RSA) is layered on top of it. Regulation is the broader, goal-directed concept: maintaining a balance or set point (homeostasis), with vagal modulation as one of the mechanisms involved, alongside hormonal or behavioral adjustments, for example.
There are actually three types of vagal fibers. The ventral and dorsal vagal fibers are “outgoing” (efferent, motor) fibers and together make up roughly 20% of all vagal fibers; they originate in the nucleus ambiguus and the dorsal motor nucleus (DMNX). The remaining 80% are “sensing” (afferent, sensory) fibers, which carry information from the body to the brain. Unlike the motor fibers, these do not originate in the brainstem but in a ganglion at the base of the skull (the nodose ganglion), and they enter the brain via yet another nucleus: the nucleus tractus solitarius.
In fact, a breathing-linked rhythm is visible in the heart rate even without those movements, because the central respiratory rhythm generator in the brainstem already modulates the heart. This underscores that RSA is a centrally generated coupling, not merely a mechanical consequence of breathing in and out, precisely the point that resurfaces in the more in-depth discussion of the cardiopulmonary oscillator.
Behind this pattern lies what Porges calls the “vagal brake.” At rest, strong vagal influence keeps the heart rate slower than its own intrinsic tempo (slower, because the vagus is an inhibitory nerve). When something is asked of the body, such as exertion or concentration, it can quickly raise the heart rate simply by releasing that brake, without immediately needing the slower sympathetic “gas pedal.” A temporary drop in RSA is therefore not in itself a sign of stress, but an efficient way to mobilize briefly. An example: a child working on a difficult puzzle or attention task often shows a dip in RSA during the task and recovers afterward. Children who show this flexible pattern tend, on average, to do better on measures of attention, emotion regulation, and cooperative play. It resembles smooth gear-shifting: briefly releasing the brake to gain speed, then reapplying it to settle back down. If the dip fails to occur, that quick mobilization is missing. If the recovery fails to occur, the system remains stuck in gear, so to speak. Both point to reduced flexibility.
A well-known, accessible variant of HRV biofeedback is heart coherence training, popularized by HeartMath. You breathe slowly, around six breaths per minute, while a device gives real-time feedback on your heart rhythm, which increases RSA amplitude. Two caveats: heart coherence training usually adds an emotional component, namely deliberately evoking a positive feeling such as appreciation, and “coherence” is strictly speaking a specific property of the heart rhythm signal (a high, narrow peak around 0.1 Hz), not a synonym for your personal resonance frequency. The breathing- and HRV-based core of this training is well supported.
The NTS does a great deal more than this, but that (even 😉) goes beyond the scope of this article.