RSA
RSA — respiratory sinus arrhythmia — is the natural, rhythmic speeding up of the heart during inhalation and slowing down during exhalation. It is produced by the vagus nerve and is the main source of short-term heart-rate variability. Its size is widely used as a non-invasive index of cardiac vagal tone; a pronounced RSA is a sign of a healthy, adaptable heart, especially in young people.
Physiology
During inhalation, activity of the cardiac vagus nerve is briefly inhibited by signals from the brainstem respiratory centre and lung stretch receptors; the heart rate rises. During exhalation vagal activity returns and the heart slows. The baroreflex and changes in intrathoracic pressure add to this effect. RSA is thought to improve the efficiency of gas exchange by matching blood flow through the lungs to ventilation, and it is present in all healthy mammals.
How RSA is measured
- Peak–valley method: the difference between the longest interval during exhalation and the shortest interval during inhalation, averaged over several breaths.
- HF power: spectral power of heart-rate variability in the respiratory band (0.15–0.40 Hz).
- Expiration/inspiration (E/I) ratio: longest/shortest interval during slow deep breathing (about 6 breaths per minute) — a standard bedside test of autonomic function.
- RMSSD and SD1: time-domain indices that closely follow RSA.
RSA depends strongly on breathing: it increases with slower and deeper breaths and reaches its maximum at about 6 breaths per minute, when respiratory and baroreflex oscillations resonate. This principle is used in HRV biofeedback, which trains people to breathe at their resonance frequency and has been studied for anxiety, depression, hypertension and other conditions.
Interpretation
Reduced RSA
- Weak vagal regulation: ageing, physical inactivity, stress, anxiety, depression
- Diabetic and other autonomic neuropathies (a reduced E/I ratio is an early sign)
- Heart failure, after myocardial infarction, some medicines (anticholinergic drugs)
Pronounced RSA
- Normal in children, adolescents and young adults
- Strong vagal tone in trained people, during rest and sleep
- Slow, deep breathing; in young people sinus arrhythmia visible on the ECG is usually harmless
Sinus arrhythmia that varies with breathing is a normal finding and does not need treatment. Irregularity that does not follow breathing — for example atrial fibrillation or frequent extrasystoles — is different and should be evaluated by a doctor.
Consult a doctor if RSA and other vagal indices are persistently very low, especially with diabetes, dizziness on standing or fainting, or if your pulse is irregular independently of breathing.
References
- Grossman P, Taylor EW. Toward understanding respiratory sinus arrhythmia: relations to cardiac vagal tone, evolution and biobehavioral functions. Biol Psychol. 2007;74(2):263–285. doi:10.1016/j.biopsycho.2005.11.014
- Lehrer PM, Gevirtz R. Heart rate variability biofeedback: how and why does it work? Front Psychol. 2014;5:756. doi:10.3389/fpsyg.2014.00756
- Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. Heart rate variability: standards of measurement, physiological interpretation and clinical use. Circulation. 1996;93(5):1043–1065. doi:10.1161/01.CIR.93.5.1043
This article is for general information and does not replace a consultation with a doctor.
Published on 7 August 2022