CONDITION
Atrial Fibrillation
Atrial fibrillation is a disturbance in the heart's rhythm in which the upper chambers (the atria) quiver rapidly and irregularly rather than contracting in a coordinated way. This irregular activity affects how efficiently blood moves through the heart, and it often leads to a fast, chaotic heartbeat that can be heard through a stethoscope or felt as an irregular pulse. Owners sometimes notice signs such as reduced energy, reluctance to exercise, faster breathing at rest, or episodes of weakness or collapse. In other cases, the irregular rhythm is discovered during a routine examination when no outward signs have been observed. Atrial fibrillation is more common in larger breed dogs, and it can occur alongside other heart conditions or on its own. This page explores the signals that may be present, the mechanisms that lead to the rhythm disturbance, the investigations used to confirm and characterise it, and the range of approaches that exist for managing it over time.
Why this matters now
Atrial fibrillation tends to appear in middle-aged to older dogs, often between six and ten years of age, though the range is wide. It is seen more frequently in larger and giant breed dogs, particularly Irish Wolfhounds, Great Danes, Newfoundlands, and Dobermanns. In some cases it develops alongside other heart conditions such as dilated cardiomyopathy, while in others it occurs in isolation without any detectable structural heart disease.
Once established, atrial fibrillation typically persists rather than resolving spontaneously. The heart rate may remain elevated and irregular for long periods, and the efficiency of the heart's pumping action can decline gradually over months to years. Some dogs remain relatively stable for extended periods, while others show progressive signs of reduced exercise capacity or fluid accumulation as the heart's ability to compensate diminishes.
Signals & patterns
Early signals
Reduced stamina during walks
A dog may tire more quickly than usual on familiar routes, slowing down or stopping to rest earlier than expected. This often reflects the heart's reduced ability to meet the body's oxygen demands during exertion.
Reluctance to engage in play
Activities that were previously enjoyed, such as chasing or rough play, may be avoided or abandoned sooner. The dog may appear less enthusiastic or may settle down more quickly than in the past.
Faster breathing at rest
Respiratory rate may increase even when the dog is lying quietly, sometimes becoming noticeable during sleep. This can reflect early changes in how efficiently blood is circulating and oxygen is being delivered.
Occasional coughing
A soft cough may be heard, particularly after lying down or during mild excitement. This can occur when changes in blood flow or pressure begin to affect the airways or lung tissue.
Later signals
Episodes of weakness or wobbling
Some dogs experience brief periods of unsteadiness, where they may sway, stumble, or sit down suddenly. These episodes can occur when the irregular rhythm leads to temporary drops in blood flow to the brain or muscles.
Distended or tense abdomen
The belly may appear larger or feel firmer than usual, reflecting fluid accumulation as the heart's pumping efficiency declines and pressure changes occur in the circulation. This is sometimes accompanied by increased effort to breathe.
Collapse or loss of consciousness
In some cases, a dog may collapse briefly, losing muscle tone and awareness for seconds to minutes before recovering. This tends to occur when the heart's chaotic rhythm severely disrupts blood flow for a short period.
Click to read about the biological mechanisms
How this is usually investigated
Investigation typically begins with a physical examination in which the irregular rhythm is detected through auscultation of the heart or palpation of the pulse. Once an arrhythmia is suspected, an electrocardiogram is used to confirm the specific pattern and characterise the heart rate. Further tests help to identify any underlying structural heart disease, assess the impact on other organs, and build a picture of the overall cardiovascular status.
Electrocardiography
Echocardiography
Thoracic radiography
Blood biochemistry and haematology
Biomarkers
Options & trade-offs
Management of atrial fibrillation typically involves a combination of measures tailored to the individual animal's signs, the presence of underlying heart disease, and the practical realities of home life. Approaches may include medication to control heart rate, treatment for any concurrent heart failure, and adjustments to activity and monitoring routines. Different combinations suit different households, and what works well for one dog may be less practical or less effective for another.
Rate control medication
Drugs such as digoxin, diltiazem, or beta-blockers can slow the ventricular response to the irregular atrial signals, reducing the sustained rapid heart rate and allowing the ventricles to fill more completely between beats. The aim is to bring the heart rate into a more moderate range rather than to restore normal rhythm. Medication is usually continued long-term, with periodic monitoring of heart rate and, in some cases, blood levels of the drug.
Trade-offs: Rate control does not eliminate the irregular rhythm or prevent progression of underlying heart disease. Some medications require careful dose adjustment and monitoring for side effects, particularly in animals with concurrent kidney or liver conditions, and not all dogs tolerate every drug equally well.
Management of heart failure
When atrial fibrillation occurs alongside or leads to signs of heart failure—such as fluid accumulation in the lungs or abdomen—treatment may include diuretics to reduce fluid overload, medications to support heart muscle contraction, and drugs that dilate blood vessels to ease the workload on the heart. The combination and dosing are adjusted according to the animal's response and the severity of signs.
Trade-offs: Managing heart failure involves multiple medications with different dosing schedules, which can be complex for owners and may require adjustments over time. Response varies, and some animals continue to experience episodes of fluid build-up despite treatment, necessitating periodic reassessment.
Activity modification and monitoring
Adjusting the intensity and duration of exercise can help to reduce cardiovascular demand and allow the heart to function within its available capacity. This may involve shorter, gentler walks and avoiding sudden bursts of high-intensity activity. Regular observation of breathing rate at rest, energy levels, and appetite provides early indication of changes in cardiovascular status.
Trade-offs: Activity modification depends on owner observation and judgement, and the degree of restriction needed can be difficult to define precisely. Some dogs tolerate reduced activity well, while others find it frustrating, and striking a balance that maintains quality of life without overburdening the heart is often a matter of trial and adjustment.
Rhythm conversion
In selected cases, attempts may be made to restore normal sinus rhythm using drugs or electrical cardioversion, particularly in dogs without significant underlying structural heart disease and in whom atrial fibrillation has developed recently. This approach is less common and tends to be considered when the rhythm disturbance is thought to be the primary problem rather than a consequence of advanced heart disease.
Trade-offs: Rhythm conversion carries risks, including the possibility that normal rhythm will not be maintained or that complications may arise during the procedure. It is not suitable for all animals, and even when successful, recurrence of atrial fibrillation is common, particularly if structural changes in the atria have already occurred.
Anticoagulation
The irregular flow of blood through the fibrillating atria can, in theory, increase the risk of clot formation, though this is less commonly observed in dogs than in humans with atrial fibrillation. In some cases, anticoagulant or antiplatelet medication may be considered to reduce this risk, particularly if there is evidence of sluggish blood flow or a history of thromboembolic events.
Trade-offs: Anticoagulation carries a risk of bleeding complications, and the evidence for its routine use in canine atrial fibrillation is limited. Monitoring for signs of abnormal bleeding and adjusting doses can add complexity, and the balance between benefit and risk is not always clear in an individual animal.
Common misconceptions
"Atrial fibrillation can be cured with the right medication."
Atrial fibrillation in dogs is generally a persistent condition rather than one that resolves with treatment. Medication can slow the heart rate and manage associated signs such as heart failure, but it does not usually restore normal rhythm or reverse the underlying electrical and structural changes in the atria. The focus of management is on improving quality of life and slowing progression rather than elimination of the rhythm disturbance.
"A dog with atrial fibrillation will always show obvious signs of illness."
Some dogs with atrial fibrillation remain relatively asymptomatic for extended periods, particularly if the ventricular rate is not excessively high and heart failure has not yet developed. The irregular rhythm may be discovered incidentally during a routine examination, and the absence of outward signs does not indicate that the condition is absent or unimportant. Monitoring and management may still be warranted even when the dog appears well.
"Atrial fibrillation and heart attacks are the same thing."
Atrial fibrillation is a disturbance in the heart's electrical rhythm, not a blockage of blood supply to the heart muscle as occurs in a myocardial infarction (heart attack). Dogs rarely experience myocardial infarction in the way humans do, and the mechanisms and consequences of atrial fibrillation are distinct. The irregular rhythm affects pumping efficiency and can lead to heart failure over time, but it does not involve the sudden vessel occlusion associated with a heart attack.
Related conditions
Dilated Cardiomyopathy in Dogs
Dilated cardiomyopathy is one of the most common underlying heart conditions associated with atrial fibrillation in dogs. The weakened, enlarged chambers create an environment in which the atria may struggle to maintain coordinated electrical activity, and atrial fibrillation can develop as a secondary rhythm disturbance.
Hypertrophic Cardiomyopathy in Cats
Hypertrophic cardiomyopathy in cats can predispose to atrial fibrillation when the thickened heart muscle leads to enlargement of the atria. The stretched atrial tissue may then be more prone to the electrical disturbances that characterise atrial fibrillation.
Exocrine Pancreatic Insufficiency (EPI)
Pericardial effusion and atrial fibrillation can both present with signs of reduced exercise tolerance, weakness, or collapse, and distinguishing between them often requires imaging of the heart. In some cases, fluid around the heart can contribute to atrial stretch and rhythm disturbances.
Systemic Hypertension in Cats
Systemic hypertension in cats can place extra strain on the heart over time, and in some cases this contributes to changes in atrial structure that may predispose to rhythm disturbances. Both conditions are more common in older cats and may be investigated together when cardiovascular signs are present.
Hyperthyroidism in Cats
Hyperthyroidism in cats can cause changes in heart rate and rhythm, and in some cases the sustained metabolic and cardiac stress may contribute to atrial fibrillation. Managing thyroid hormone levels is often part of the broader approach when both conditions are present.
Atrial fibrillation often coexists with or leads to other cardiac conditions, and understanding the broader context of cardiovascular health can provide useful perspective. The metabolic health pillar includes related topics such as heart muscle disease and the management of heart failure, which may be relevant as the picture develops over time. For dogs already receiving treatment, periodic reassessment of response and adjustment of management is a common part of the ongoing conversation, and changes in signs at home—such as breathing rate, energy levels, or appetite—can inform those discussions.
Last reviewed: 30 June 2026 · Dr Alastair Greenway MRCVS