Glossary
Respiratory sinus arrhythmia
Also called RSA, sinus arrhythmia
The normal rise in heart rate as you breathe in and fall as you breathe out. It is driven by the vagus nerve and makes up much of resting HRV.
Despite the name, respiratory sinus arrhythmia (RSA) is not a heart problem. “Sinus” means the beat starts in the heart’s normal pacemaker, the sinus node, and “arrhythmia” here only means the rhythm is not perfectly even. You can feel it yourself: take a slow breath in and your pulse quickens slightly, breathe out and it slows.
The vagus nerve Vagus nerve The main nerve of the parasympathetic nervous system, running from the brainstem to the heart, lungs and gut. Its fast signals to the heart drive most beat-to-beat HRV. More about Vagus nerve drives this. Breathing in eases the vagal brake on the heart, and breathing out puts it back. In a short recording at rest, RSA is the main source of beat-to-beat variation, which is why it sits at the center of most HRV measures. It shows up as the regular wave in a tachogram Tachogram A line chart of heartbeat intervals in the order they happened, one point per beat. It shows HRV as a wavy line instead of a single number. More about Tachogram and as power in the HF HF power The share of heart rate variability in fast rhythms of 0.15 to 0.40 Hz, the range of normal breathing. It mainly reflects vagal activity when breathing is 9 to 24 breaths a minute. More about HF power band. Slow, deep breathing makes the wave larger.
RSA is often used as an index of cardiac vagal tone Vagal tone The ongoing influence of the vagus nerve on the heart, which slows it down. It is usually estimated from HRV measures such as RMSSD and HF power. More about Vagal tone , but Grossman and Taylor list several cautions. Breathing rate and depth change RSA on their own. So does physical activity during the measurement. And the link between RSA and vagal tone is usually stronger within one person over time than between different people.
That gives two practical rules. First, the way you breathe during a reading affects the result, so breathe normally unless the protocol says otherwise. Second, compare readings with your own past readings rather than with someone else’s.
The lesson The nervous system behind HRV shows how RSA connects breathing, the vagus nerve and your HRV number.