Respiratory Sinus Arrhythmia: How Breathing Shapes Your Heart Rhythm

What is respiratory sinus arrhythmia?

Respiratory sinus arrhythmia (RSA) is the natural rise and fall of heart rate with each breath: the heart speeds up slightly as you breathe in and slows as you breathe out [S2] [S4]. "Sinus" means the beats still start in the heart's normal pacemaker; "respiratory" names the driver. The word "arrhythmia" here is a historical label for a varying rhythm, not a diagnosis — RSA is the rhythmic fluctuation of beat-to-beat intervals seen in healthy people at rest [S5].

Researchers recommend keeping the term for exactly this breath-linked speeding and slowing of the heart [S3]. It is the most visible part of heart rate variability — the beat-to-beat variation in the heart's rhythm — and this page is about that one component: what produces it, what changes its size and what it can and cannot tell you.

How does it work?

What is established. RSA is carried mainly by the vagus nerve: it is the respiration-driven speeding and slowing of the heart via vagal regulation [S1]. The mechanisms are understood to involve the brain's breathing drive and stretch reflexes from the lungs acting on the vagal outflow to the heart, and RSA becomes larger as that vagal influence on the heart grows [S7].

What is still a model or a debate. How exactly breathing and circulation combine to generate RSA is not settled. One long-running debate asks whether a central respiratory centre in the brainstem or the baroreflex — the blood-pressure feedback loop — predominantly generates it [S4]. One view holds that the coupling of blood pressure and heart intervals at breathing frequencies mostly reflects breathing acting on both, rather than baroreflex physiology [S5]; others give the baroreflex a larger role. The broadly accepted middle ground is that central, peripheral and mechanical elements interact to produce it [S4]. In practice, the page you are reading treats the vagal pathway as established and the relative weight of each generating mechanism as open.

Why slow breathing makes RSA so much larger — the link to resonance and the baroreflex — is a mechanism question of its own, planned for a separate page on breathing and HRV.

How is it measured?

RSA is read from the intervals between heartbeats recorded alongside breathing, or estimated from heart rate variability itself. In short resting recordings it is the primary source of the variation, especially with slow, paced breathing [S1] — which is why a breathing exercise visibly moves an HRV number. Among time-domain metrics, RMSSD typically gives a better assessment of RSA than related measures [S1]. The general rules for comparing such numbers — same device, same metric, comparable conditions, your own baseline rather than someone else's value — apply here as well.

What affects it?

• Breathing rate. RSA is small at fast breathing rates and large at slow ones [S5] [S4]. Typical resting breathing sits around 12–20 breaths per minute; as breathing slows, RSA grows, and it tends to peak around about 5.5–6 breaths per minute (around 0.1 Hz) — a value that varies from person to person [S4]. Finding your own rate is a practical question covered in find your resonance breathing rate.
• Breathing depth. A larger breath increases RSA, more so at slow rates [S4], and in a classic laboratory study its size rose in proportion to breathing volume [S6].
• Body position and state. RSA is largest during sleep, relaxation, slow deep breathing and when lying down, and smaller during exercise and anxiety [S4].
• Age. On average, HRV tends to fall with age — in our night-time RMSSD table the median drops by about 4–8 ms from one age band to the next [S1]; RSA is part of that picture, and a classic laboratory study found smaller low-frequency RSA in older participants up to about the mid-thirties [S6]. Individuals vary widely.

What does the evidence show?

Established. RSA is a normal feature of the resting heart [S5], carried mainly by vagal regulation [S1] [S7]. It is larger with slow and deep breathing and smaller with fast breathing [S4] [S5], and it is the main source of heart rate variability in short resting recordings [S1].

Context-dependent. The size of the effect of posture, state and age comes from laboratory and review data with different protocols [S4] [S6]; how much it changes for a given person is not something a population study can predict.

Debated. Whether a central respiratory rhythm generator or the baroreflex is the main generator of RSA [S4] [S5].

Unknown — and the hypotheses. Why the body has RSA has not been fully worked out [S4]. One hypothesis, supported by an experiment in anesthetized dogs, is that RSA improves the efficiency of gas exchange in the lungs by matching heartbeats to the phases of breathing [S7]. A later modelling study found gas exchange gains unrelated to RSA and proposed instead that RSA reduces the work the heart has to do while keeping carbon dioxide in its normal range [S8]. Both are hypotheses; neither is established, and they partly compete.

What it does not tell you

• It is not a rhythm disorder. Despite the name, RSA is a normal feature of a healthy heart at rest [S5]. A heart rhythm that is irregular for other reasons is a different matter and belongs with a doctor.
• It is not a clean readout of vagal tone. The framing matters. Vagal tone cannot be measured directly; HRV measures such as RMSSD reflect vagally mediated changes in heart rate [S3]. RSA is strongly shaped by breathing itself: respiratory parameters can confound its relation to cardiac vagal tone [S9], and the two can dissociate in specific conditions [S1] [S9]. A larger RSA during slow breathing reflects the breathing pattern at least as much as any lasting change.
• It is not a health score or a diagnosis. a single low HRV reading does not by itself mean you are stressed or unwell [S1], and the same holds for the size of RSA: a large or small value on its own does not establish fitness, health or illness.
• It is not a practice guide. How to breathe slowly, and why it may help, is covered in coherent breathing, the resonance breathing guide and the resonance breathing tool.

In ONDA

ONDA's practices are built around slow, paced breathing, the condition in which RSA is largest. With the iPhone camera, ONDA shows live pulse and a breathing-rate estimate derived from the breathing-linked rhythm of the pulse; with an Apple Watch it adds a live coherence score — ONDA's own rhythm-regularity feedback, not a clinical measurement of RSA or of vagal tone. ONDA does not diagnose anything. See what ONDA measures.