Sinus arrhythmia
A physiologic, cyclic variation in sinus rate driven by respiration and vagal tone - the rate speeds up on inspiration and slows on expiration. Every beat is still sinus in origin.
Key ECG features
- ✓R-R lengthens and shortens in a repeating cycle
- ✓Upright, uniform P before every QRS
- ✓PR interval constant
- •Varies in step with respiration
- ±Wandering pacemaker: P height rises and falls with the cycle
✓ required · • typical · ± may be present
ECG characteristics
A regularly irregular rhythm: the R-R interval lengthens and shortens in a smooth, repeating cycle rather than randomly, with an upright, uniform P wave before every QRS and a constant PR interval. A wandering pacemaker, where P-wave height rises and falls with the same cycle, often accompanies it.
This illustration is schematic. Dashed line is a normal rhythm. The grid is not drawn to scale.
Common causes
High vagal tone. It is a normal variant, expected in dogs, and is accentuated at rest and by respiratory disease.
Why it matters
It should not be mistaken for a pathologic arrhythmia and needs no treatment. Its presence in a resting cat is less typical and can warrant a closer look, and its disappearance in a dog can itself be a clue to raised sympathetic tone.
Differentiate from
- Atrial fibrillationBoth are irregular. Sinus arrhythmia varies cyclically with breathing and keeps a P wave before every QRS; atrial fibrillation has neither pattern nor P waves.
- Sinus pause & arrestA pause interrupts the rhythm abruptly; the slowing in sinus arrhythmia is gradual and repeats with the breathing cycle.
- Normal sinus rhythmEvery beat is still sinus in origin. This is a normal variant of sinus rhythm in dogs rather than a different rhythm.
- Second-degree AV block (Mobitz I)Wenckebach is also regularly irregular, but it drops a QRS. Sinus arrhythmia never does.