Arrhythmias in dogs: symptoms and treatment

Overview

The normal electrical impulse begins in the sinus node, the heart's natural pacemaker. It first travels through the atria to the atrioventricular node. A short delay there lets the atria fill the ventricles before ventricular contraction. Ventricular contraction then pumps blood onwards.

An arrhythmia may arise from structural heart disease, conduction problems or disease elsewhere. Respiratory sinus arrhythmia is normal in dogs. Heart rate increases during inhalation and slows during exhalation. Unequal intervals between beats therefore do not always mean heart disease. A slow rate is always assessed in context, rather than using one universal numerical threshold.

Causes

Heart disease or another body disorder

Heart muscle diseases such as dilated cardiomyopathy can predispose dogs to arrhythmias. Valve disease can also underlie them. Myocarditis can cause ventricular arrhythmias or conduction blocks.

Electrolyte disturbances, injuries and systemic infections can alter rhythm. Splenic disease or gastric dilatation and volvulus can also cause arrhythmias. Chocolate poisoning and certain medicines are other possibilities. Anaesthetic agents can also affect cardiac conduction. Scarring or fat accumulation in heart muscle can create electrically unstable ventricular areas. Insufficient oxygen in heart muscle can also predispose to ventricular premature beats. Excessive blood potassium can prevent normal atrial electrical activation. Hypothermia and hypothyroidism can instead slow sinus rate.

Fast rhythms, slow rhythms and premature beats

In atrial fibrillation, atrial electrical activity is disorganised and ventricular beats are often fast and irregular. A ventricular premature beat originates from an electrically unstable ventricular area rather than the usual impulse pathway. Consecutive rapid ventricular beats may form ventricular tachycardia.

In sick sinus syndrome, the natural pacemaker may generate impulses too infrequently or pause for long periods. In atrioventricular block, electrical messages do not travel normally from atria to ventricles. In a mild block, conduction only slows and visible symptoms may be absent. In complete block, subsidiary ventricular pacemakers may maintain a slow escape rhythm. Stress, pain and fever can raise normal sinus rate without a separate abnormal rhythm focus.

Risk factors

Arrhythmias can affect every age and breed. Boxers and Dobermanns are particularly affected by ventricular arrhythmias. Boxer arrhythmogenic cardiomyopathy often involves premature beats originating in the right ventricle.

Atrial fibrillation occurs particularly in large breeds such as Great Danes, Dobermanns and Irish Wolfhounds. Smaller dogs can also develop it with advanced heart disease. Sick sinus syndrome occurs particularly in older Miniature Schnauzers and Cocker Spaniels.

A particular inherited ventricular arrhythmia in German Shepherd Dogs occurs at 3–24 months of age. Associated sudden deaths are concentrated at 5–9 months. This specific juvenile arrhythmia generally disappears after 24 months.

Typical symptoms

A mild arrhythmia may cause no signs and be found only when the heart is listened to. A symptomatic disturbance may cause changes in condition or performance at home.

• Your dog tires on walks or plays less energetically.
• Your dog seems weak or unusually tired.
• Your dog suddenly faints or collapses.
• Breathing becomes faster or takes more effort.
• Your dog may cough.
• Appetite decreases or weight falls.
• The abdomen may swell with fluid.

Some signs result from concurrent heart disease rather than rhythm alone. An isolated ventricular premature beat usually causes no visible symptoms. However, a dangerous arrhythmia can cause sudden death without warning.

Complications and consequences

Circulation and cardiac strain

An excessively fast, slow or irregular rhythm can reduce cardiac output. Reduced brain blood flow may cause fainting. Ventricular tachycardia can deteriorate into ventricular fibrillation, stopping effective pumping.

Persistent rapid atrial fibrillation can worsen cardiac function and heart failure. Severe conduction block can also cause failure and a risk of sudden death.

Outlook depends on rhythm and underlying disease

In a follow-up of 60 dogs with atrial fibrillation, median survival was 608 days when mean daily rate was no more than 125 beats per minute. With a daily mean over 125, median survival was 33 days. Structural disease was identified in 56 of 60 dogs. Fifty of 60 had heart failure. Sick sinus syndrome generally responds well to pacing. An arrhythmia caused by disease elsewhere may resolve when that cause is treated successfully.

Diagnosis at the vet

ECG identifies the arrhythmia type

Your vet listens to the heart while assessing arterial pulse regularity. An electrocardiogram, or ECG, records cardiac electrical activity. It distinguishes rhythms such as atrial fibrillation, ventricular premature beats and atrioventricular block.

A Holter monitor records rhythm during normal daily life for at least 24 hours. It helps find intermittent disturbances missed by a short ECG at the practice. Holter monitoring is external and does not require electrodes inside the heart. After clipping, electrodes attach to cleaned skin on both sides of the chest and around the sternum. A soft dressing protects the device, with a vest over it if needed. Monitoring also assesses whether atrial fibrillation medication slows the rate sufficiently throughout the day.

In consecutive daily recordings from 25 medicated atrial fibrillation patients, mean rate varied clinically significantly in only 2 dogs (8%). One 24-hour recording is often sufficient to assess mean rate in this setting. Variation in ventricular premature beats can nevertheless require longer monitoring. Day-to-day variation in premature beat counts was 52% in the same 25 dogs.

Assessing cardiac structure and general health

Ultrasound assesses valves, atria, ventricles and function. It can identify conditions such as degenerative valve disease or dilated cardiomyopathy. X-rays can investigate enlargement and lung fluid. Blood and urine tests look for causes such as electrolyte disturbances, infection and hormonal disease.

Troponin may be measured if heart muscle damage is suspected. Inflammatory markers may also be assessed when investigating myocarditis. A dog needing a pacemaker is assessed for other diseases before the procedure.

Treatment

Treatment depends on clinical significance

Your vet chooses care according to arrhythmia type, symptoms and underlying disease. An isolated premature beat without symptoms does not always need medication. Normal respiratory sinus variation needs no antiarrhythmic. Electrolyte disturbances caused by another disease are corrected as part of care.

Fast arrhythmias are treated with intravenous or oral medicines if needed. Sotalol, for example, may be used for ventricular arrhythmias. However, it can reduce contractility and worsen failure in a dog with dilated cardiomyopathy. Atrial fibrillation is often treated with diltiazem or diltiazem combined with digoxin. The aim is usually to slow ventricular rate rather than permanently eliminate fibrillation.

A Holter mean of around 125 beats per minute or lower may be used as a treatment target. This is an atrial fibrillation patient's daily mean, rather than a normal-rate threshold for every healthy dog. Electrical cardioversion can restore normal rhythm, but fibrillation often recurs.

A pacemaker replaces a dangerously slow rhythm

A pacemaker generates electrical impulses when the natural rhythm is dangerously slow. Symptomatic sick sinus syndrome often needs pacing. Medicines increasing rate may provide only temporary help in this disease. In complete atrioventricular block, pacing is recommended even without symptoms because of sudden death risk.

The pacing lead is usually passed through a neck vein into the heart. Settings and function are checked with a separate programmer. A pacemaker does not remove the possible need for medication for other cardiac diseases. Older dogs can also benefit if other conditions do not prevent the procedure.

Follow-up and recovery after implantation

Activity is kept minimal for 30 days after implantation. This allows healing and reduces lead displacement risk. Device and battery function are checked after about 1 month. Checks generally continue every 6 months thereafter.

Response to antiarrhythmic medication is monitored with repeat ECG or Holter recordings. Concurrent structural disease is followed by ultrasound. Holter monitoring can also assess the effect of pacemaker programming on rhythm. Concurrent heart failure is treated separately, for example with medicines supporting contraction and removing fluid.

Prevention

Breed-appropriate cardiac tests assess risks associated with inherited heart muscle disease. Some official screening schemes can include auscultation, ECG, Holter monitoring and ultrasound. Listening alone is insufficient for every breed predisposed to cardiomyopathy.

Where cardiac testing is required by a breed health programme, a breeding dog's report must meet that programme's requirements at mating. A previous normal result does not exclude later disease. A German Shepherd Dog recovered from inherited juvenile ventricular arrhythmia can still pass susceptibility to offspring. Appropriate treatment of disease elsewhere can resolve an associated arrhythmia.

Important for dog owners

Record when fainting occurs, preceding exertion and other symptoms. Note changes in stamina, appetite and breathing. During Holter monitoring, keep an activity diary throughout the 24-hour period. Rhythm changes can then be compared with events such as rest or walks. New fainting or breathing difficulty warrants urgent assessment.

Keep the pacemaker wound clean and dry. If the device lies in the neck region, use a harness rather than a collar for walks. Mention the pacemaker before MRI or CT. An ordinary microwave oven is not dangerous to a dog with a pacemaker.

Keep ECG and Holter results for follow-up visits. Recordings assess medication effectiveness even when your dog appears symptom-free at home.

Frequently asked questions

Is an uneven heartbeat always dangerous?

Respiratory sinus variation is normal in dogs. Rate rises with inhalation and slows with exhalation. An isolated premature beat can also cause no symptoms. Other abnormal rhythms are investigated by ECG, particularly with tiredness, weakness or fainting.

Can arrhythmia occur without structural heart disease?

A cause may lie elsewhere, such as electrolyte imbalance or hormonal disease. Atrial fibrillation also sometimes occurs in a structurally and functionally normal heart. Tests may therefore include blood and urine analysis alongside cardiac assessment. Successful treatment of the cause can resolve the arrhythmia too.

Why does my dog need Holter monitoring?

A wearable device records rhythm for at least 24 hours. It can detect intermittent arrhythmias missed by a short ECG. It also assesses medication effectiveness and measures daily mean rate in atrial fibrillation.

Does every arrhythmia need medication?

Normal respiratory arrhythmia is not treated with an antiarrhythmic. An isolated premature beat without symptoms may have little significance. Treatment depends on type and cause, and a dangerously slow rhythm may need pacing rather than medicine. Severe ventricular tachycardia may require immediate medication.

Can an older dog have a pacemaker?

Many dogs needing pacemakers are older. A slow rate can look like ordinary age-related slowing. General health is assessed before a decision because serious disease in another organ system may prevent implantation. Regular checks assess device function and battery status.