Cardiac Arrhythmias: How to Identify Atrial Fibrillation in Small Animal Patients

Caryn Reynolds, DVM, DACVIM (Cardiology)

Atrial fibrillation (AF) is one of the most common arrhythmias seen in small animal patients. It can occur in patients with significant structural heart disease, such as cardiomyopathy or chronic valvular disease, or occur in healthy patients, particularly giant breed dogs, and it can be triggered by elevated vagal tone or anesthesia. It is important to recognize this arrhythmia, as most of these patients will benefit from rhythm conversion or rate control.

Physical Exam

Auscultation of an irregular heart rhythm is often the first hint of artrial fibrillation. Because of the different time intervals between beats, the femoral pulse pressure is variable, and pulse deficits will be present. Other irregular rhythms, such as sinus rhythm with frequent atrial or ventricular premature complexes and sinus arrhythmia can sound just like AF, so an ECG is the next diagnostic step.


In this arrhythmia, there is no organized electrical activity of the atrial chambers. Chaotic electrical activity is occurring throughout the tissue, causing random wavelets of variable size and strength. Therefore, a hallmark of atrial fibrillation is that there are no P waves present on the ECG. The electrical activity might show some random “bumps” in the ECG baseline, but this is not always present and is not required for the diagnosis of AF. Some of the wavelets will be strong enough to get through the atrioventricular node, which starts ventricular depolarization and is recorded on the ECG as the QRS complex followed by a T wave. Since the conduction to the ventricle is disorganized, an irregular rhythm of the QRS complexes is the other diagnostic criteria for atrial fibrillation. Atrial fibrillation can occur at a wide range of heart rates, as seen in the examples below.

Example of normal sinus rhythm

Example of a normal sinus rhythm with consistent distance between each QRS complex.

Examples of atrial fibrillation

Example of rapid atrial fibrillation.

Example of atrial fibrillation with ventricular conduction abnormalities.

Example of slow atrial fibrillation.

Test Yourself!

Which of the following is true regarding atrial fibrillation?

A. The ECG will lack P waves B. QRS complexes will be present in an irregular rhythm C. It is the least common arrhythmia D. There is of organized electrical activity of the atrial chambers E. A & B F. B & C

Highlight here to see answer: E

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