Respiratory sinus arrhythmia and wandering pacemaker in dogs
Healthy 3-year-old English Bulldog at a pre-anaesthetic check. The rhythm is irregular: it speeds up on inspiration and slows on expiration. Femoral pulse synchronous with every beat. ECG:
| Parameter | Result | Reference |
|---|---|---|
| Heart rate | 70–130 bpm (mean 95) | 70–160 bpm |
| Rhythm | Irregular, cyclical with breathing | Regular or irregular sinus |
| P wave (lead II) | Before every QRS; variable height (0.1–0.3 mV) | ≤ 0.4 mV |
| PR interval | 0.10 s, constant | 0.06–0.13 s |
| QRS | 0.05 s, normal morphology | ≤ 0.06 s |
What is the correct interpretation?
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A. Respiratory sinus arrhythmia with wandering pacemaker: a normal variant due to vagal tone
Correct. The rate varies with breathing, every P wave conducts with a constant PR and QRS complexes are normal. The changing P wave height reflects the pacemaker shifting within the sinus node with vagal tone. It is normal, very marked in brachycephalic breeds, and is not treated.
B. Atrial fibrillation with a slow ventricular response
In atrial fibrillation there are no P waves and the rhythm is irregularly irregular, unrelated to breathing. Here there is a P wave before every QRS.
C. Second-degree AV block, Mobitz I
Second-degree block requires P waves not followed by a QRS, and in Mobitz I the PR lengthens before the blocked P. Here every P conducts with a constant PR.
D. Frequent atrial premature complexes: Holter and atenolol
Atrial premature complexes have an early P wave of different shape and do not follow the respiratory cycle. Treating a normal variant with atenolol would be a mistake.
lightbulb Key takeaway
In dogs, marked respiratory sinus arrhythmia indicates good vagal tone and makes congestive heart failure, in which sympathetic tone predominates, unlikely. In cats in the clinic it is rare: if you see it, think of respiratory disease or high vagal tone.
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Updated: October 31, 2026 · Educational content for veterinary professionals. It does not replace current clinical guidelines or the judgement of the attending veterinarian.