Atrial fibrillation (AF)
A rhythm in which many electrical impulses arise throughout the atria instead of from the SA node. The atria quiver rather than contract as a unit, and the AV node lets only some of those impulses through, at unpredictable intervals.
Key ECG features
- ✓No organized P waves anywhere on the strip
- ✓Irregularly irregular R-R intervals with no pattern
- •QRS narrow, unless conduction is abnormal for another reason
- ±Coarse or fine undulating baseline (f-waves)
✓ required · • typical · ± may be present
ECG characteristics
Three hallmarks: no organized P waves anywhere on the strip, an irregularly irregular R-R interval with no discernible pattern, and often a coarse or fine undulating baseline (f-waves). The QRS complexes stay narrow unless conduction is abnormal for another reason, because the impulses still reach the ventricles by the normal route.
This illustration is schematic. Dashed line is a normal rhythm. The grid is not drawn to scale.
Common causes
In dogs, especially large and giant breeds, it usually reflects marked atrial enlargement from underlying structural heart disease. It occasionally arises in giant breeds with structurally normal hearts (lone atrial fibrillation). In cats it is much less common and nearly always accompanies marked left atrial enlargement, usually from cardiomyopathy.
Why it matters
Losing coordinated atrial contraction and gaining a rapid, irregular ventricular response both cut cardiac output. Finding it should prompt a search for the underlying structural disease rather than treatment of the rhythm alone.
Differentiate from
- Atrial standstillNeither shows P waves. Atrial fibrillation gives an irregularly irregular ventricular response; atrial standstill gives a slow, regular escape rhythm.
- Sinus arrhythmiaAlso irregular, but the variation is cyclic and repeats with breathing, and a P wave still precedes every QRS.
- Atrial premature complexesFrequent atrial ectopy can mimic the irregularity, but each early beat still has a discrete premature P wave in front of it.