What outward signs are common with atrial standstill?

Common signs include lethargy, reduced activity, a slow heartbeat, brief fainting, or reduced exercise tolerance. If your pet has breathing difficulty or repeated fainting, contact a vet immediately.

What tests will the vet use to assess a heart rhythm problem?

The main tests are an ECG to confirm heart rhythm and conduction, echocardiography to assess the atria and heart function, and blood, biochemistry, and urine tests to look for other causes.

What information should I record after going home?

Record when signs started, how often episodes happen, changes in alertness and activity, appetite, and toileting. Use your phone to capture any fainting or breathing episodes to show your vet.

What follow-up care is needed after treatment?

Regular rechecks are needed to monitor ECG and ultrasound changes. At home, track appetite, body weight, and activity, and follow your vet's advice on check-ups and adjustments.

How much do the tests and treatment usually cost?

Costs vary by clinic and region — ask your vet for an estimate before proceeding.

Disease Guide

Atrial Standstill in Dogs and Cats: Lethargy, ECG Diagnosis & Care [2025]

Atrial standstill is a serious heart rhythm disorder that may be linked to high potassium levels, heart muscle disease, or inherited disease and requires urgent care.

Take 1 minute to check the symptoms first

Last updated: 2026/01/05

Pethealthmap editorial team

Sources: ACVIM, AAHA, National Taiwan University Veterinary Hospital

When to See a Vet

Immediate veterinary care needed (emergency)

•Repeated fainting or brief loss of consciousness with slow recovery
•Rapid breathing or bluish discoloration of the mouth or nose that does not settle
•Sudden extreme weakness and inability to stand without help

Schedule a veterinary appointment soon (within 24-72 hours)

•Persistent low energy but still able to move around briefly
•Noticeably reduced exercise tolerance and tiring easily
•Brief dizziness or momentary confusion or altered awareness

01What symptoms may indicate a problem?

In dogs and cats, atrial standstill often causes a marked decline in energy and activity. Some cases are emergencies and need immediate treatment.

Lethargy, a clear drop in activity, or unusual sleepiness
A noticeably slow or irregular heartbeat, or an abnormal pulse when you feel it
Brief fainting, unsteady standing, or a dazed appearance
Reduced exercise tolerance, tiring easily on walks or reluctance to move
If you notice any of these changes, arrange a cardiology assessment as soon as possible

Seek emergency care immediately if you notice the following symptoms:

⚠️ Repeated collapse or brief loss of consciousness with slow recovery
⚠️ Rapid breathing or blue-tinged lips or nose that does not settle
⚠️ Sudden extreme weakness and inability to stand without help

02How can I assess changes at home? What to monitor and when it is an emergency

Home observation and detailed records help your vet assess the condition. Timing and frequency are especially useful for diagnosis.

Record when changes in alertness and activity began and how often they occur
Write down when the first episode happened and how long each episode lasts
Track appetite, water intake, urination, and bowel movements for any changes
Use your phone to record videos of fainting, breathing difficulty, or abnormal movements
Weigh your pet daily and record any weight changes; contact your vet if anything is abnormal
If symptoms suddenly worsen, arrange veterinary care or contact an emergency clinic immediately

03How will the vet diagnose it? Common tests

Your vet will focus on cardiac testing while also ruling out metabolic or whole-body causes of the rhythm problem.

Physical examination

listening to heart sounds and checking pulse and blood pressure

Electrocardiogram (ECG)

checking heart rhythm, electrical conduction, and temporary pauses

Echocardiography

assessing atrial size, contraction, and structural abnormalities

Blood tests and biochemistry

screening for metabolic, electrolyte, or thyroid abnormalities

Urinalysis and other imaging, such as CT if needed, to rule out concurrent disease

If more advanced assessment is needed, testing at a hospital with cardiology equipment is recommended

04How is it treated and managed after diagnosis? Medication, diet adjustments, and care

Treatment depends on the cause and severity. Close monitoring of ECG findings and clinical signs is needed, along with careful home observation.

Short-term medication or supportive care to stabilise heart rate and clinical signs
Some cases require assessment of whether a device or interventional treatment is feasible and what the risks are
Fluids and supportive care may be needed to maintain circulation and electrolyte balance
Regular rechecks to monitor changes on ECG and echocardiography
Ongoing home monitoring of appetite, activity, body weight, and episode frequency
Adjust diet and daily care as directed by your vet, and schedule cardiology follow-up

05What should I prepare before seeing the vet? Key details that help identify the cause

Record when symptoms began, how often episodes occur, and any possible triggers

List all current prescription medicines, supplements, and dosing times

Bring video files of fainting or abnormal behaviour for your vet to review

Prepare daily notes on appetite, drinking, urination, and bowel movements

Bring any previous ECG or ultrasound reports if available

Use a secure, stable carrier when travelling to reduce stress during transport

This content is for reference only and cannot replace a professional veterinary diagnosis. If your pet shows signs of illness, please seek veterinary care promptly.

Related Symptoms (You May Also Be Looking For)

Bradycardia

Abnormally slow heart rate

Lethargy

Decreased activity

Syncope

Fainting