The heart is one enormous muscle, contracting (beating) loyally and regularly every second of every day. The normal resting heart rate of dogs sits between 60–120 beats per minute; that’s 1–2 times every second! Small breeds generally have a higher heart rate than large breeds, and the rate can of course change with exercise and the like. Atrial fibrillation in dogs is a condition which results in an irregular heart rhythm with incredibly high heart rates – 160 beats per minute or more.

Normal heart function in dogs

To understand AF better, let’s consider how, and why, the heart beats in the first place. 

The heart is made up of 4 separate chambers: two atria, and two ventricles. Together, they beat in a rhythmic and synchronised way to pump blood through your lungs to pick up the oxygen you breathe in, and then deliver this oxygen to the rest of your body. Every time the heart relaxes, the chambers fill with blood, and every time it contracts it pumps that blood around the body. 

But what actually makes it contract? A group of specialised cells in one area of the heart called the sinoatrial node (SA node), produces electrical impulses that spread out through the rest of the heart and make each heart muscle cell contract. These contractions are usually very well organised and in perfect synchrony, resulting in a nice, steady, rhythmic heartbeat. 

What is atrial fibrillation in dogs?

In atrial fibrillation, the normal rhythmic contractions of the heart are thrown into chaos because the SA node is bombarded with electrical signals which makes the heart beat very fast and erratically. Without the usual controlled contractions, the heart is unable to pump blood efficiently around the body because the chambers don’t have time to fill with blood between each beat. 

What causes atrial fibrillation?

In dogs, AF is usually a result of an underlying heart condition, especially those that cause enlargement of the atria such as dilated cardiomyopathy (DCM) or mitral valve disease. When the heart is physically stretched over a long period of time, the cells that produce the electrical impulses in the SA node are also stretched and become faulty, sending out chaotic, fast signals. 

However, it can also occur in otherwise healthy dogs, particularly larger breeds such as Great Danes and Irish Wolfhounds. This is called primary AF. 

Atrial fibrillation (whether it is primary or due to an underlying heart problem) can lead to heart failure over time, as the heart becomes less effective at delivering oxygen-rich blood to the body.

Signs and symptoms of canine atrial fibrillation

These will vary depending on the severity of the disease, but worryingly AF can sometimes have no outward symptoms, especially in the early stages of the disease. If your dog has AF because of their underlying heart disease, there may well be signs associated with this. It may also be picked up incidentally by your vet at a routine check if they hear the characteristically fast and erratic heartbeat, or a ‘pulse deficit’ where there is not a pulse felt for every heartbeat heard. Remember though, your dog might not be showing any outward signs. In advanced or serious disease, your dog may show signs of heart failure.

Symptoms can include:

  • Exercise intolerance – getting tired easily on walks.
  • Lethargy – generally being less active and willing to play / walk.
  • Panting when at rest (panting during exercise is normal, panting at rest is not).
  • Coughing 
  • A rapid and irregular heartbeat – your vet may pick this up, or you may be able to feel your dog’s heartbeat through their chest.
  • Fainting – if not enough blood is able to be pumped to the brain, your dog may temporarily lose consciousness. 

How is atrial fibrillation diagnosed in dogs?

As discussed, you may notice some worrying symptoms in your dog which warrant a vet check, but your vet may also pick up some concerning signs during a routine consultation. 

  • A fast and irregular heartbeat (which sounds like a pair of trainers going around the tumble dryer!) is often the first indicative sign. If your vet hears this, they may want to do an ECG on your dog. 
  • An ECG – there are very specific changes on an ECG trace which can confirm whether or not your dog has AF. These traces may need to be interpreted by a cardiology specialist to get a firm diagnosis. An ECG trace gives you a snapshot of the heart contractions during that time. Atrial fibrillation can sometimes be intermittent (i.e. it doesn’t happen all of the time), so a short ECG recording may happen to show a period of a normal heart rhythm. For this reason, sometimes ‘halter monitors’ are recommended. These are ECG monitors that are attached via a harness to your dog, and record their ECG for a much longer period, from 24 hours to 7 days. This gives a much better chance of any abnormal patterns being picked up. 

Because AF is usually the result of another underlying heart condition, your vet is likely to want to check for these too, as treating the underlying disease will give your dog the best prognosis. These tests may include:

  • An echocardiogram – this is an ultrasound of the chest and will show the physical structure of the heart, the valves inside it and the blood flow through it. 
  • Chest x-rays – these will assess the size of the heart and look for any possible fluid build-up in the lungs.
  • Blood tests – these will assess your dog’s general health and check for any other underlying disease which may affect the AF.

Can AF be treated?

Treatment of AF can be tricky and your dog may benefit from being under the care of a specialist cardiologist. The aim of treatment is to control and reduce the heart rate, ideally to less than 125 beats per minute. This will allow the heart chambers to fill with blood between each beat, so each heartbeat will pump blood more effectively around the body.

There are medications which can achieve this, such as beta blockers, or drugs like diltiazem or digoxin. These drugs can be dangerous though, and your dog will need to have regular check ups and monitoring to assess their heart. 

If AF is the result of an underlying heart condition 

Conditions such as DCM or mitral valve disease are common causes, and if this is the case then treating this underlying disease can help. Medications to reduce the volume of blood and the amount the heart is stretched are typical, such as ACE inhibitors and diuretics. However, because the AF is a result of the heart becoming enlarged over a long period, sometimes by the time the diagnosis is made, permanent damage is already done. 

If your dog has primary AF 

In other words, their AF is not due to any underlying disease, then electrical cardioversion is a possible treatment option. This involves an electrical current being used to ‘shock’ the heart back into a normal rhythm whilst your dog is under anaesthetic. This can only be carried out in a specialised referral hospital and is not without risk.

Additional treatments may also be advised

For example, keeping your dog on a low salt diet and limiting their exercise to minimise stress on the heart. 

What is the long-term outlook for a dog with atrial fibrillation?

In a nutshell, atrial fibrillation is a serious condition and the prognosis for a dog with AF is guarded. It is usually the result of an underlying heart condition, so the prognosis of this underlying condition must also be considered. Dogs can be managed successfully with careful monitoring on medical treatment for some time, but these diseases are generally progressive and will get worse over time. 

If your dog has been diagnosed with AF, to give them the best prognosis they will need regular veterinary checkups and monitoring tests. This will allow your vet to assess how the disease is progressing over time, and medication adjustments may well be necessary for ongoing management.