Atrial fibrillation in dogs: ECG and rate control
6-year-old Irish Wolfhound with exercise intolerance. On auscultation, a very irregular rhythm with pulse deficits. Electrocardiogram:
| Parameter | Result | Reference |
|---|---|---|
| Heart rate | 230 bpm | 70–160 |
| Rhythm | Irregularly irregular | Regular / sinus arrhythmia |
| P waves | Absent; undulating baseline | One before each QRS |
| QRS | Narrow (0.05 s) | < 0.06 s |
What is the ECG diagnosis?
chevron_right Show answer and explanation
A. Sinus tachycardia
Sinus tachycardia is regular and each QRS is preceded by a P wave. Here there are no P waves and the rhythm is chaotic.
B. Ventricular tachycardia
Ventricular tachycardia has wide, abnormal QRS complexes and is usually fairly regular. Here the QRS complexes are narrow (supraventricular origin).
C. Atrial fibrillation
Correct. Irregularly irregular rhythm, no P waves (“f” waves on the baseline) and narrow QRS complexes. The goal is rate control with diltiazem and/or digoxin (mean HR < 125 bpm on Holter) and a search for structural heart disease, especially dilated cardiomyopathy.
D. Respiratory sinus arrhythmia
It is a normal rhythm in dogs: the rate varies with breathing, but there is a P wave before each QRS and the rate is not this high.
lightbulb Key takeaway
Giant breeds (Irish Wolfhound, Great Dane) can develop “lone” atrial fibrillation without structural disease, but it often precedes dilated cardiomyopathy. In-clinic heart rate overestimates the home rate: Holter monitoring is the best guide to treatment.
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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.