Sinus Arrhythmia
Sinus Arrhythmia
Sinus rhythm whose rate rises and falls with the breath: over 120 ms of R-R variation, and a normal finding.
On the trace
Read the strip in this order.
- Notice that the spacing is uneven.
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The R-R intervals run 692 to 975 ms, so the rate wanders between 62 and 87 bpm on a single strip. The first instinct is that something is wrong. - Check the P waves before concluding that.
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One before every QRS, upright, 0.15 mV, and the same shape on every beat. Same shape means same origin. The sinus node is producing all of these; it is only changing its mind about when. - Measure the PR.
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175 ms, fixed, however much the spacing moves. The variation is in the node, not in conduction. - Measure the variation and compare it with the threshold.
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Longest minus shortest is 283 ms. Over 120 ms is what earns this the name, and a normal sinus strip from the same heart varies by only 56. - Look at the shape of the variation.
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It is not random. The intervals lengthen and shorten smoothly and repeat about every four seconds, which is a breath. Random irregularity is a different problem, and this is not it.
How to recognise it
| Feature | Value | Notes |
|---|---|---|
| R-R variation | Over 120 ms | 283 ms here. The diagnostic criterion, and the only number that matters. |
| The pattern | Cyclical, tracking the breath | Faster on inspiration, slower on expiration, about four seconds a cycle here. |
| P wave | Constant shape, one per QRS | Same origin throughout. This is what excludes a wandering pacemaker. |
| PR interval | Fixed, 175 ms here | The node varies, conduction does not. |
| Rate | Normal on average | 73 here. The mean is unremarkable; only the spacing is not. |
| Bedside test | Disappears on breath-holding | The most useful confirmation there is, and it needs no equipment. |
The strip
Mechanism
Vagal tone is not constant through the breathing cycle. Inspiration inhibits vagal outflow to the sinus node, so the rate rises. Expiration restores it, so the rate falls. The effect is mediated through stretch receptors in the lungs and through the baroreflex, and it arrives at the node within a beat or two, which is why the trace tracks the breath so closely.
Everything downstream is untouched, which is why the beats are identical and only their spacing moves.
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Why this is a sign of health
Respiratory sinus arrhythmia is a measure of vagal tone, and vagal tone is a marker of autonomic health. It is most pronounced in the young, the fit and the well rested. Prevalence falls sharply with age: one study found it in under a fifth as many people over 60 as under 31, which is attributed to stiffer arteries and blunted vagal responses. Its absence in a young person is more interesting than its presence.
Breath-holding, and why it settles the question
The variation is driven by breathing, so removing the breathing removes the variation. Ask the patient to hold their breath and a respiratory sinus arrhythmia becomes regular within a few beats. An irregularity that persists is not respiratory and deserves a closer look. Exercise does the same thing by raising sympathetic drive and overwhelming the vagal modulation.
The non-respiratory form
An identical trace, uncoupled from breathing. It carries quite different company: digitalis toxicity, and structural heart disease. The distinction is made at the bedside rather than on the strip, which is another reason the breath-holding test is worth knowing.
The ventriculophasic variety
In complete heart block, the P-P interval containing a QRS is often shorter than the one without. The mechanical beat improves sinus node perfusion, or stretches the atrium, and the node responds. It is a curiosity rather than a diagnosis, but it explains P-P variation on a strip where the atria and ventricles are otherwise unrelated.
Clinical impact
The harm this rhythm causes is entirely diagnostic. It is irregular, which draws attention, and benign, which means the attention is wasted. Recognising it is what stops a normal finding becoming a referral.
Management
Recognise it and say so plainly
No treatment, no follow-up, no investigation. Telling the patient their heart rate changes when they breathe is usually the whole consultation.
Confirm it with a held breath
A few seconds of breath-holding regularises it. Nothing else is needed to make the diagnosis.
Look further only when it is not respiratory
Irregularity that survives breath-holding, or that appears for the first time in an older patient, is worth a second look for digoxin effect and structural disease.
Do not let it distract from the rest of the ECG
The rhythm being benign says nothing about the intervals, the ST segments or the axis.
Differential
The same rhythm with less of it. Under 120 ms of R-R variation it is simply sinus rhythm, and a normal heart varies a little either way.
The irregularity that matters. It has no P waves and no pattern, whereas this has a constant P before every QRS and repeats with the breath.
Also makes a regular rhythm look uneven, but by inserting early beats with a different P shape. Here every P is identical and nothing arrives early.
Also a sinus rhythm with uneven spacing, but the gaps are discrete pauses rather than a smooth cycle, and they are far longer than any breath produces.
References
- Sinus Arrhythmia — StatPearls, NCBI Bookshelf NBK537011, 2023
- Sinus Arrhythmia — Life in the Fast Lane, ECG Library, 2024
- Sinus arrhythmia (respiratory sinus arrhythmia) — ECG Waves, 2023