What is Mobitz type I second-degree AV block?

Mobitz type I is a delay in the heart's electrical conduction. It has characteristic ECG changes, and clinical signs can range from none to weakness or fainting. A vet needs to confirm it with an ECG and other tests.

If my dog or cat faints, is it definitely this condition?

Fainting has many possible causes, so it is not always due to AV block. Your vet will use an ECG, blood tests, and imaging to look for the cause, and prompt assessment is recommended.

What tests are usually needed for diagnosis?

Common tests include an electrocardiogram (ECG) to confirm the heart rhythm, chest X-rays to assess the heart and lungs, and blood biochemistry to rule out metabolic or electrolyte abnormalities.

If my vet recommends monitoring, how often are rechecks usually scheduled?

Recheck frequency depends on the condition and test results, and may range from every few weeks to every few months. Follow-up may include ECGs, blood tests, and records of clinical signs. Follow your vet's recommended schedule.

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

Second-Degree AV Block (Mobitz Type I) in Dogs and Cats: Weakness, Fainting, ECG Checks & Follow-Up Care [2025]

Second-degree AV block type I is usually benign, is commonly seen in young healthy dogs, and often does not require treatment

Take 1 minute to check the symptoms first

Last updated: 2026/01/05

Pethealthmap Editorial Team

Sources: ACVIM, Taiwan Veterinary Internal Medicine Association

When to See a Vet

Seek immediate emergency care

•Brief or sustained fainting or loss of consciousness
•Sudden extreme weakness with inability to stand
•Rapid breathing with obvious severe respiratory distress

Schedule a clinic visit soon (within 24–72 hours)

•Noticeably reduced activity level but still eating
•Occasional brief loss of consciousness or marked lethargy
•Slowed heart rate or clearly abnormal breathing rhythm

01What symptoms suggest something is wrong?

Mobitz type I second-degree AV block in dogs and cats often presents with heart-rhythm-related signs. Severity varies, ranging from no symptoms to repeated weakness or fainting.

Reduced activity or obvious sleepiness that differs from their usual behaviour
Brief or repeated fainting episodes or moments of loss of consciousness
Reduced appetite or sudden refusal to eat that lasts for more than a day
Vomiting or diarrhoea that persists without improvement
You notice a slower heartbeat or a change in breathing rhythm
Arrange a veterinary assessment if any of these abnormalities appear

Seek emergency care immediately if you notice the following symptoms:

⚠️ Brief or persistent collapse or loss of consciousness
⚠️ Sudden extreme weakness and inability to stand unaided
⚠️ Rapid breathing with obvious difficulty breathing or severe panting

02How can you assess changes at home? Key observations and emergency warning signs

Key observations and records from home can help your vet judge severity and understand the pattern of episodes.

Record when the signs first appeared and the timing of each episode
Track appetite and the actual amount eaten each day
Record the number of vomiting or diarrhoea episodes and details of stool or urine changes
Take short videos showing fainting or weakness episodes
If you can measure them, record resting heart rate and breathing rate
If you notice clear changes, arrange a cardiology or general veterinary assessment promptly

03How will the vet make a diagnosis? Common tests

At the clinic, assessment usually prioritises heart function and overall health. Common tests help confirm the rhythm abnormality and look for underlying causes.

Complete physical examination

heart and lung auscultation, pulse check, and blood pressure assessment

Electrocardiogram (ECG)

identifies the heart rhythm and type of conduction abnormality

Chest X-rays

assess heart size and whether there are any related lung changes

Blood tests and biochemistry

check for metabolic or electrolyte abnormalities and infection

Urinalysis

helps assess overall metabolism and kidney function

Echocardiography or Holter monitoring may be arranged if needed

If results need further assessment, referral to a veterinary cardiologist may be recommended

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

Treatment and follow-up focus on stabilising heart rhythm, identifying triggers, and regular monitoring. Many cases need follow-up ECGs and blood tests.

Give medicines and follow procedures as advised by the vet, with regular rechecks for monitoring
Adjust diet or electrolyte management based on test results and veterinary advice
Continue home monitoring and share your records with the vet
Schedule regular follow-up ECGs, chest X-rays, or blood tests
If the condition worsens or becomes unstable, the vet will discuss advanced intervention options
Confirm the long-term monitoring plan and recheck frequency with a veterinary cardiologist

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

List when the signs occurred, how often they happen, and possible triggers

Prepare a brief record of food intake, vomiting, and toileting over the past week

Bring a list of current medicines, supplements, and doses

Record key videos, especially during an episode, for the vet to review in person or remotely

Bring previous ECG, chest X-ray, or ultrasound reports if available

Book ahead with a clinic or specialist service that has ECG equipment

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.