Atrial Fibrillation in Dogs

Written by Shula Berg BVSc CertAVP(GSAS) GPAdvCert(SASTS) MRCVS
Clinically reviewed by Elizabeth McLennan-Green BVM&S CertAVP(SAM) MRCVS

Table of Contents

- Overview
- Symptoms
- Diagnosis
- Treatment
- Outlook

What is Atrial Fibrilation?

An arrhythmia is an abnormal heart rhythm, meaning the heart is beating too quickly, too slowly, or in an irregular pattern. Atrial fibrillation (AF) is the most common arrhythmia seen in both humans and dogs.

The heart contains four chambers; two atria at the top, and two ventricles at the bottom. In atrial fibrillation, the top chambers of the heart generate electrical signals too quickly to fully contract, so instead are quivering (also known as fibrillating) at an extremely fast rate. If every one of these impulses was transmitted to the ventricles, the heart would be beating at over 500 beats per minute – this is not physically possible! The electrical connection between the atria and ventricles (called the AV node) will not transmit all of these impulses. Despite this, atrial fibrillation still causes a very fast heart rate, with the ventricular beat often exceeding 200 beats per minute. There is no coordination of these ventricular beats, so the heart rate becomes irregular as the body tries to cope. This heart rate is often too fast for the heart to completely empty and refill with every beat, causing less blood to be pumped around the body every minute. 

Atrial fibrillation most often occurs secondary to enlargement of the heart. This can be caused by a disease of the heart muscle itself, or a problem with one or more valves inside the heart. Rarely, AF can occur without structural changes to the heart. This is known as “Lone AF”, and is seen more commonly in giant breed dogs such as Great Danes. 

What are the symptoms of Atrial Fibrilation?

In mild cases of AF, where the heart rate is only moderately elevated, you may see no symptoms at all. As the heart rate increases, the efficiency of the heart reduces, meaning less oxygenated blood is ejected with each beat. This causes dogs to become more lethargic, and they may struggle to exercise or get tired quickly. Some dogs may collapse, especially after exertion. Affected dogs may show other symptoms depending on what type of heart disease is present, if any.   

What tests are used to diagnose Atrial Fibrilation? 

Atrial fibrillation causes a fast, irregular heartbeat which can be heard with a stethoscope during examination. The speed and sound of the heart can be very suspicious of AF, but an arrhythmia cannot be diagnosed on clinical exam alone. 

A definitive diagnosis of AF can only be made using an electrocardiogram (ECG). This records a trace of the hearts activity and allows different arrhythmias to be identified. It can be obtained quickly, with the patient awake, and carries no risks. For some patients, the heart rate can vary throughout the day. It may be recommended to take a continuous ECG recording, lasting anywhere from 24 hours to 7 days. This is known as a Holter monitor, and is usually performed under the guidance of a vet with a particular interest or training in cardiology. ECG pads and a digital recording device are fitted to your dog and secured to a vest. Your dog can then wear this at home and carry out their normal routine (avoiding getting wet). They then return to the clinic at a later date to have the monitor removed, before the data is retrieved and sent for analysis. 

Due to the likelihood of underlying structural heart disease, an ultrasound scan of the heart, known as echocardiography (commonly called an “echo”), is recommended for all dogs diagnosed with AF. An echo allows us to see the chambers of the heart, assess how they are filling and contracting, and even directly observe the function of the heart valves. 

How is Atrial Fibrilation treated? 

There are two approaches to treating atrial fibrillation; rhythm control (in which we try to return the heart rhythm to normal), or rate control (in which we manage the heart rate). Ultimately, the fibrillation itself isn’t a problem, as long as the heart is beating at a rate that doesn’t adversely affect output. 

Rhythm control 

Uncommonly, electrical cardioversion can be used to try and return the heartbeat to normal. This uses defibrillating pads to shock the heart during a short anaesthetic. It is most suited to patients with lone AF and no structural heart disease, and is only performed by specialist cardiologists. 

Rate control 

The majority of dogs with AF will be prescribed medication to manage the heart rate and keep it at an acceptable level (generally considered to be less than 125 beats per minute on average). The most commonly used medications are called Diltiazem and Digoxin. Digoxin can cause side effects such as vomiting and diarrhoea, so it is usually started at a low dose and gradually increased. Blood tests may be recommended to monitor the level of Digoxin in the blood and guide dose changes. The first blood test is usually performed a week after starting medication, and should be taken 6-8 hours after the morning dose of Digoxin. 

The majority of cases will respond to treatment, but will need to continue taking oral medication for the rest of their life. Repeat Holter monitors are required to assess the response to treatment; if the average daily heart rate remains too high, medication can be slowly increased or other medication may be used in addition or instead. 

What is the outlook for dogs with Atrial Fibrilation?

The prognosis with atrial fibrillation in isolation is reasonable. Pets require regular checks and lifelong medication, however, the condition is usually very manageable. Unfortunately, the majority of cases develop AF due to underlying heart disease. This is likely to carry a more guarded prognosis. However, your vet, in conjunction with a veterinary cardiologist, will be able to advise you further depending on the type and severity of heart disease identified. 


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Page last reviewed: 16th January 2024
Next review due: 16th January 2026