When to see a vet urgently
Contact an emergency vet immediately if your cat has any of the following:
- fainting or sudden collapse
- difficult breathing or clearly increased breathing effort
- pale or bluish gums and severe weakness
- sudden hind leg pain or paralysis
- attempts to urinate without passing urine, with deteriorating general condition
You can find a vet near you. Even a brief loss of consciousness needs investigation: normal behaviour afterwards does not remove the possibility of an arrhythmia.
Overview
The heart's conduction system consists of the sinus node, atrioventricular node, bundle of His and Purkinje fibres. It transmits electrical impulses that determine heart rhythm. With a block, this transmission slows or stops. The disturbance may be partial or complete, temporary or permanent.
The main types in cats are:
- Sinus block: delayed impulses at the sinus node.
- Atrioventricular, or AV, block: disrupted signalling from atria to ventricles.
- Bundle branch block: slowed impulse conduction within the ventricles.
The list describes where the disturbance occurs, but its significance depends particularly on the severity of AV block. Sinus node dysfunction is rare in cats. A slow heart rate alone does not identify the conduction disturbance, and not every slow rhythm is treated in the same way.
In first-degree AV block, impulses pass from atria to ventricles more slowly than normal. In second-degree block, some impulses fail to conduct. With high-grade second-degree block, so many ventricular beats are missed that symptoms can resemble complete block.
In third-degree, or complete, AV block, atrial impulses do not reach the ventricles. The ventricles can still contract using their own, usually slower, escape rhythm. Complete block therefore does not mean the heart has already stopped. The adequacy of that escape rhythm and the cat's condition determine urgency.
Causes
Causes of heart block in cats can include:
- cardiomyopathies, particularly hypertrophic cardiomyopathy
- ischaemic heart muscle damage
- inflammatory diseases, such as myocarditis and endocarditis
- electrolyte disturbances involving potassium, calcium or magnesium deficiency
- endocrine diseases, such as hyperthyroidism and diabetes
- poisoning and medication side effects, including antiarrhythmics and sedatives
- age-related changes in the conduction system
This list includes diseases generally associated with arrhythmias, and the link of every one specifically to feline heart block is not equally well established. Finding hyperthyroidism, for example, does not alone prove it caused the block. Changes in the heart and the whole body are assessed alongside one another.
A potassium disturbance does not mean deficiency alone. Excessive blood potassium can also cause life-threatening arrhythmias. Urinary obstruction is an important cause of hyperkalaemia, or raised potassium, in cats. Weakness with inability to urinate is an emergency and must not be treated at home with mineral supplements.
With permanent block, conduction system damage may be localised. Tissue studies in cats have found loss of conducting fibres and replacement with connective tissue, called fibrosis. This damage to the electrical connection differs from general heart muscle thickening, although changes can coexist.
Risk factors
- Older cats.
- Animals with chronic heart diseases.
- Cats receiving medicines that affect heart rhythm.
- Animals with endocrine or metabolic diseases.
Risk factor information in cats is limited, and the list does not allow an individual's risk to be calculated. Even with severe block, other diseases may affect both symptoms and treatment selection. Not every cat has the same structural heart disease on ultrasound.
Give your vet a list of every current medicine. A conduction disorder is also considered before general anaesthesia because anaesthetic-related slowing of the heart rate can be problematic. The possible role of a medicine is investigated with the treating vet.
Symptoms
Heart block can cause:
- a slow pulse, or bradycardia
- weakness, lethargy and reduced activity
- fainting or sudden loss of consciousness
- breathlessness and rapid breathing
- poor exercise tolerance
- sudden death in severe cases
Not every block causes visible symptoms. Even complete AV block is found by chance in some cats. This does not make it insignificant, but treatment decisions for a symptom-free cat may differ from those for one fainting repeatedly. The type of block must be confirmed with an ECG.
Fainting or a seizure?
During fainting, blood flow to the brain briefly decreases. A cat may fall onto its side and lose consciousness but recover quickly. Urine or faeces may also be passed. This can happen during a seizure too, so these observations alone do not distinguish causes.
Seizures more often involve heavy drooling, jaw movements or confusion during recovery. Tell your vet what your cat was doing beforehand, the estimated duration of unconsciousness and how recovery progressed. A safely recorded video can help distinguish them. Do not delay seeking help to make a recording.
Possible complications
Heart blocks that do not respond to treatment can lead to:
- chronic heart failure
- severe arrhythmias
- oxygen deprivation in organs and tissues
- sudden cardiac arrest
Hypoxia means inadequate oxygen supply to tissues. A slow heart rhythm can reduce circulation enough to cause weakness or fainting. However, consequences depend on other diseases as well as rhythm. Heart failure is not an inevitable outcome of every mild or symptom-free conduction delay.
In a study of severe feline AV blocks, prognosis varied considerably. Some cats were followed for a long time, and not every death was heart-related. Heart failure identified at presentation indicated a poorer cardiac prognosis. Prognosis is therefore not inferred solely from the block's name.
Diagnosis at the vet
Investigations for heart block include:
- listening to the heart, or auscultation
- pulse measurement
- electrocardiography, or ECG, to identify block type
- echocardiography, or heart ultrasound, to assess heart muscle
- blood tests to check electrolytes and hormones
- Holter ECG monitoring to detect intermittent disturbances
What do the different investigations show?
An ECG records the heart's electrical events. It compares P waves representing atrial activation with QRS complexes representing ventricular activation. Their timing relationship helps identify whether conduction is slow or some or all atrial impulses fail to reach the ventricles.
Ultrasound shows heart structure and function. It can reveal heart muscle disease but does not replace ECG classification of block. Blood tests assess electrolytes and thyroid function, for example. Chest X-rays may be needed if fluid accumulation in or around the lungs is suspected.
Why is longer rhythm monitoring needed?
A short clinic ECG may be recorded while an intermittent block is absent. A Holter device records rhythm over a longer period during ordinary activity. Electrodes attached to the chest connect to a portable recorder. This measures rhythm rather than delivering electrical treatment.
During recording, note sleeping, activity and unusual events with their times. The diary is compared with the recording to assess links between collapse and rhythm changes, for example. Instructions for attaching and removing the device are provided at the clinic. Record observations as precisely as possible even when symptoms are brief.
Treatment and monitoring at home
Treatment depends on block severity, symptoms and underlying cause. Treating a correctable electrolyte disturbance or medicine-related effect can resolve an arrhythmia. Not every conduction delay needs an immediate pacemaker. Permanent, symptomatic block is assessed with a vet experienced in heart disease.
Medicines supporting heart rate may be used in selected situations, but do not repair a damaged conduction system. A pacemaker supplies an electrical stimulus when the heart's own rhythm is insufficient. It can reduce fainting associated with severe AV block and improve functional ability.
Pacemaker implantation also carries a risk of complications. Small feline studies have often found improved symptoms, but not every study has demonstrated longer survival compared with cats managed without a pacemaker. Other diseases and the types of patients selected for treatment groups also affect results.
At home, monitor particularly for new fainting episodes, weakness, appetite changes and altered breathing. Medication and any pacemaker function are checked at agreed follow-up visits. Holter monitoring can also assess treatment effects or pacemaker programming. Clear deterioration requires contact before a planned appointment.
Prevention
Heart block cannot be prevented completely, but risk can be reduced through:
- regular heart checks in older cats
- management of chronic diseases, including cardiomyopathies and hormonal disorders
- careful use of medicines affecting cardiac conduction
- maintenance of normal weight and physical condition
Weight management or good fitness has not been shown to prevent every feline conduction system disease. Follow-up aims to identify changes and assess treatment needs. Adding potassium or other minerals without a confirmed deficiency does not prevent heart block; both low and high levels can be harmful.
This text provides general information and does not replace veterinary care. Contact a vet if your cat develops symptoms.
Frequently asked questions
Does every cat with heart block need a pacemaker?
No. The decision depends on symptoms, block type and any correctable underlying cause. A symptom-free finding may need monitoring, while severe block causing fainting may require pacemaker assessment.
Can heart block be temporary?
Yes. Even severe AV block that was transient and associated with another illness has been described in cats. Permanent conduction system fibrosis can instead cause an ongoing disturbance. Investigations and rhythm monitoring distinguish these situations.
Does a normal short ECG rule out a cardiac cause of fainting?
Not reliably. An intermittent disturbance may not appear during a short recording. Longer rhythm monitoring may be needed to link symptoms with electrical activity.
This article is for general information only and is not a guide to treatment. If you notice the first signs of illness, contact your vet.
Paw4care editorial team
Sources
- Heart Disease: Conduction Abnormalities in Dogs and Cats, Merck Veterinary Manual, accessed 2026-11-03.
- Paroxysmal high-grade second-degree and persistent third-degree atrioventricular block in cats, Journal of Veterinary Cardiology / PubMed, accessed 2026-11-03.
- Fainting, Tufts University, accessed 2026-11-03.
- Arrhythmias in Cats, VCA, accessed 2026-11-03.
- Disorders of Potassium Metabolism in Cats, Merck Veterinary Manual, accessed 2026-11-03.
- Cardiology, Cornell University, accessed 2026-11-03.
- Characteristics and outcomes of cats with and without pacemaker placement for high-grade atrioventricular block, Journal of Veterinary Cardiology / PubMed, accessed 2026-11-03.
- Histological Features of the Atrioventricular Conduction System in Cats with High-Grade Atrioventricular Block, Journal of Comparative Pathology / PubMed, accessed 2026-11-03.