Sick sinus syndrome in dogs – symptoms and pacemaker treatment

Overview

The sinus node normally initiates each heartbeat and adjusts the rate to the body's needs. With dysfunction, an excessively slow rate can reduce circulation enough to cause weakness or fainting.

A sinus pause is an interruption in the sinus node's usual pacing, rather than the heart permanently stopping each time a pause occurs. Slow periods can also alternate with abnormally fast rhythms arising from the atria.

This is called bradycardia–tachycardia, or slow–fast rhythm, syndrome. An ECG finding without symptoms does not always require immediate treatment.

Treatment of a clinically significant disorder aims to restore adequate circulation and reduce collapses.

Causes

The sinus node and nervous-system control

The sinus node is located in the atrial region of the heart and sends an electrical impulse that activates the atria and then the ventricles. The sympathetic nervous system increases heart rate, while parasympathetic activity slows it at rest.

The disease may involve changes in the node itself or the surrounding tissue. Scarring of the node, called fibrosis, is one recognised underlying mechanism.

Inflammation, infections and tumours may also be associated with sinus node dysfunction. However, in many dogs the precise original cause remains unknown.

Other parts of the heart can sometimes generate an escape beat during a sinus pause. If the escape rhythm cannot maintain blood flow to the brain, loss of consciousness may follow.

Other parts of the conduction system may also be diseased, so sinus node dysfunction can occur alongside atrioventricular block.

With severe dysfunction, the rate can fall as low as 10 beats per minute during physical activity, even if the dog has no symptoms at other times.

Risk factors

Sinus node disease is identified particularly in older dogs. Recognised breed predispositions include Miniature Schnauzers and Cocker Spaniels.

West Highland White Terriers are another recognised risk group. Dachshunds, Pugs and American Cocker Spaniels also have a known predisposition to sinus node dysfunction.

The disease is diagnosed proportionally more often in females than males, particularly among Miniature Schnauzers. The precise genetic mechanism and mode of inheritance are unknown.

Other heart diseases and the dog's general health also influence symptom severity.

The breed list does not limit the disease to these dogs: dysfunction can occur in any breed. Sinus node dysfunction may be accompanied by degenerative mitral valve disease.

Typical symptoms

Symptoms may be intermittent, so your dog can look entirely normal between episodes.

• Your dog suddenly faints during ordinary activity.

• Your dog briefly becomes weak without obvious pain or injury.

• Exercise tolerance declines and your dog no longer copes with exertion as before.

• Your dog seems more tired and less active than usual.

• Alongside the slow rhythm, there may be episodes when the heartbeat feels very fast and pounding.

The risk of losing consciousness increases with a pause in heart activity lasting around 6–8 seconds or longer. A markedly slow rate at the veterinary practice may be the first finding, before collapses are seen at home.

A fainting episode can sometimes resemble a seizure, so the sequence of events is an essential part of assessment.

Complications and consequences

Long pauses reduce blood flow to the brain and can cause repeated losses of consciousness. Severe bradycardia can also cause heart failure or sudden death.

Pacemaker implantation carries risks including lead displacement and wound or device infection. After pacemaker treatment, many dogs can return to good function and ordinary daily life.

However, other concurrent diseases influence the overall benefit and long-term outlook.

Lead dislodgement was reported in 5–10% of dogs in older follow-up studies involving various slow-rhythm disorders. Dislodgement often occurs during recovery from anaesthesia or in the first days and weeks after the procedure.

Lead displacement may require another procedure to restore function.

Diagnosis at the vet

Blood pressure measurement complements the examination when assessing how a slow rhythm affects circulation and the dog's general condition. A blood count, blood biochemistry and urinalysis can help look for other diseases that alter the rhythm.

ECG and the rhythm during symptoms

The vet listens to the heart rhythm and assesses the pulse before more detailed investigations. Diagnosis requires recording the heart's electrical activity with an ECG.

When dysfunction is suspected, a resting ECG may be recorded for at least 5 minutes to detect intermittent pauses. A Holter device records the rhythm for 24 hours and can show the rhythm during an episode.

A short, normal ECG at the practice does not exclude intermittent sinus node dysfunction. The ECG is examined for sinus pauses, an abnormally slow rate and any fast atrial episodes.

A slow pulse alone does not establish the diagnosis, as a sinus rhythm may naturally be slow at rest or in a fit athletic dog. A sinus rhythm that varies with breathing may also be normal in dogs.

Reversible causes and other heart changes

Blood tests can investigate causes such as electrolyte disturbances and hypothyroidism underlying a slow rhythm. Current medicines are also checked, as beta blockers and some anaesthetic agents, for example, slow heart rate.

In an atropine response test, the vet assesses how much of the slowing relates to parasympathetic control. A partial response to atropine does not rule out sinus node disease itself.

In general, symptoms from a slow rhythm may become more pronounced when the rate falls to around 40 beats per minute or below, but symptoms determine the significance of the number. Heart ultrasound helps identify a concurrent valve disorder or other structural heart disease.

Before a pacemaker is fitted, anaesthetic risk and other diseases affecting the outlook are also assessed.

The response to atropine does not reliably predict whether oral medication to increase heart rate will help later.

Treatment

Medication and the need for a pacemaker

Your vet prescribes treatment according to the symptoms, recorded rhythm and other diseases. If the slow rhythm results from a reversible underlying condition or a medicine, treatment targets that cause.

A mild finding without symptoms may sometimes be monitored without medication. However, planned anaesthesia or an exceptionally slow rhythm may change treatment needs even in a dog without symptoms.

Medication to increase heart rate, such as theophylline or terbutaline, may be tried in selected situations. The response may be temporary, even if there is an initial benefit.

A permanent pacemaker is often needed for a dog that repeatedly faints or has other symptoms. If fast-rhythm episodes are also present, they may need separate treatment alongside the pacemaker.

A temporary pacemaker may be fitted before general anaesthesia in a dog with a severely slow rhythm, if needed, to support circulation during the procedure.

How does a pacemaker work, and what does recovery involve?

A pacemaker monitors the heart's own rhythm and supplies an electrical signal when beats are missing. The device contains a power source and programmable electronics, while a lead carries the electrical signal to the heart muscle.

The device is usually placed beneath the skin of the neck, and the lead is passed through a vein into the heart. Implantation takes place under general anaesthesia, and real-time X-ray imaging can guide the lead into the correct position.

A rate-responsive pacemaker can increase heart rate when the dog is active. A protective neck bandage limits head movement and helps prevent scratching of the wound.

Exercise is usually restricted for around 4–5 weeks to allow tissues to heal and the lead to become securely attached. Games that stretch the neck strongly and jumping are avoided initially to prevent lead displacement.

A harness replaces a collar so that no pulling force is applied over the device and lead. Removal of stitches and the bandage may be planned for around 14 days, for example.

A pacemaker programming check may be scheduled 3–4 months after implantation. Further checks are needed, for example every 6 months depending on the practice's protocol.

Some monitoring programmes use annual programming checks. Checks assess the battery and adjust settings to suit the dog's needs.

Settings can be changed through the skin without another operation. When the battery runs down, the device can be replaced in a smaller procedure and the existing lead can often remain.

Programming can influence battery use and therefore device lifespan. Regular device checks provide advance warning of battery depletion before planned replacement.

Prevention

Not all sinus node diseases can be prevented, as inherited or unknown changes may be involved. Further breeding from an affected dog is approached cautiously because of the suspected inherited predisposition.

Listening to the heart during a routine health check may raise suspicion before obvious fainting occurs. Identifying an arrhythmia before planned anaesthesia helps account for reduced circulatory reserve in an individual anaesthetic plan.

Reviewing medicines and possible other causes of a slow rhythm forms part of reducing reversible risks.

Important for dog owners

For each episode, record the time, duration, what your dog was doing beforehand and how quickly it recovered. Note whether it looked weak before falling or whether the collapse was completely unexpected. A safely recorded video may help describe the event to the vet.

Bring all previous ECG and Holter reports and an up-to-date medication list to investigations. Mention any planned dental procedures or other anaesthetics too. Do not try to provoke an episode by making your dog run or experiment with medicines that change heart rate yourself.

After pacemaker implantation, follow the written rest and wound-care instructions. Watch for swelling, discharge and redness at the surgical site, as well as changes in general condition. Keep the device details and check-up schedule readily available. Report new weakness or fainting episodes immediately, even if a programming appointment is already booked.

Frequently asked questions

Is a slow heartbeat always a sign of disease?

A slower-than-usual sinus rhythm can be entirely normal, for example in a fit athletic dog. The significance of sinus node dysfunction is therefore assessed using ECG, symptoms and possible underlying diseases; treatment is not based solely on a pulse counted at home.

Does a normal ECG at the practice rule out disease?

The rhythm may be normal during a short recording even when serious intermittent pauses occur at home. Longer Holter or event monitoring then helps link the symptom to the actual heart rhythm and establish whether sinus node disease is causing the fainting.

Does every dog need a pacemaker?

Mild bradycardia without symptoms does not usually require immediate treatment. However, repeated loss of consciousness or an inadequate response to medication may lead to pacemaker treatment. Planned general anaesthesia and a particularly slow rate can also influence treatment decisions for a dog otherwise without symptoms.

Does a pacemaker also stop excessively fast rhythms?

A pacemaker prevents excessively slow periods by supplying missing electrical signals to the heart muscle, but does not necessarily eliminate all fast arrhythmias. If fast-rhythm episodes continue, separate medication may be used alongside the pacemaker.

Can a dog live an ordinary life with a pacemaker?

Many dogs can return to good function after recovery. The device still needs regular checks to monitor its battery and settings. Pulling on the neck is avoided by using a harness, and initial exercise restrictions continue according to the treating vet's plan.