What Does a Second-Degree AV Block Mean for Your Dog?

If your vet just mentioned a second-degree AV block after looking at your dog’s ECG, here’s the honest answer: it depends on the type. A second-degree AV block means the electrical signal traveling from the heart’s upper chambers to the lower chambers is occasionally getting dropped, causing some beats to be missed. Some types are completely benign and need nothing more than monitoring. Others are more serious and may eventually require a pacemaker. The difference comes down to which type your dog has.

How AV blocks are graded: first, second, and third degree

To understand what an AV block actually is, it helps to know how the heart’s electrical system works in plain terms. Every heartbeat starts with an electrical signal in the upper chambers of the heart (called the atria). That signal travels down through a relay station called the AV node into the lower chambers (called the ventricles), which then squeeze and pump blood out to the body. When vets look at an ECG, they can see this whole process as a series of waves on a graph. The first little bump (called a P wave) shows the upper chambers firing. The bigger spike that follows (called a QRS complex) shows the lower chambers responding. In a healthy heart, every P wave is followed by a QRS, like clockwork.

An AV block happens when that relay station, the AV node, starts dropping the ball. Sometimes it slows the signal down. Sometimes it drops a beat entirely. Sometimes it stops passing signals through altogether. Vets grade AV blocks on a scale of one to three depending on how severely that process is disrupted:

• First-degree AV block: The signal gets through every time, just a little more slowly than it should. Usually harmless and often found incidentally.
• Second-degree AV block: Most signals get through, but occasionally one doesn’t. A beat is dropped here and there, but the heart keeps going.
• Third-degree AV block (complete heart block): No signals get through at all. The upper and lower chambers beat completely independently of each other. This is a serious emergency that almost always requires a pacemaker.

Second-degree sits in the middle, and that’s exactly why the type matters so much.

Mobitz Type I vs. Type II: which kind does your dog have, and why it changes everything

There are two main subtypes of second-degree AV block, and they have very different implications for your dog’s health.

Mobitz Type I (Wenckebach)

In Mobitz Type I, the electrical signal gets progressively weaker with each beat until one beat is dropped, then the whole cycle resets and starts again. If you’re looking at an ECG, you’ll see the gap between the P wave and QRS complex getting a little longer with each beat until a QRS disappears entirely, then it starts over. This repeating pattern is called Wenckebach.

This type is most commonly caused by high vagal tone, which means the part of the nervous system responsible for slowing the heart down is being a little too enthusiastic. Athletic dogs, flat-faced breeds, and dogs that are calm or sedated often show this pattern. It also comes up during anesthesia. In many cases, it’s not a disease at all. It’s just how that particular dog’s nervous system operates.

The atropine response test is the key here. Your vet gives a small dose of atropine, a drug that dials back vagal tone, and then repeats the ECG. If the block disappears, it confirms that high vagal tone was the cause, and the prognosis is generally excellent. No treatment is needed beyond periodic monitoring.

Mobitz Type II

In Mobitz Type II, beats are dropped suddenly and without warning. The gap between the P wave and QRS stays consistent right up until a QRS just doesn’t appear. There’s no pattern leading up to it. This points to a structural problem with the AV node itself, not just an overactive nervous system response.

This type doesn’t respond to atropine. It’s more likely to progress to a complete heart block over time, and dogs showing symptoms with this finding are often candidates for a pacemaker. Cocker Spaniels are among the breeds known to be more prone to the kind of AV node disease that leads to this pattern.

High-grade AV block is a related term used when two or more P waves in a row fail to trigger a QRS, meaning the heart is dropping beats frequently. This is always taken seriously, regardless of Mobitz subtype.

What causes second-degree AV block in dogs?

There are several reasons a dog’s AV node might start dropping beats, and finding the cause is an important part of deciding how to treat it:

• High vagal tone: The most common cause of the benign Mobitz Type I pattern. The parasympathetic nervous system is putting the brakes on the heart a little too hard.
• Medications: Digoxin and beta blockers can both slow conduction through the AV node. Always mention any medications your dog is taking to your vet.
• Electrolyte imbalances: High potassium levels in particular can interfere with the heart’s electrical system.
• AV node disease: Gradual degeneration of the node itself, more common in older dogs and certain breeds like the Cocker Spaniel.
• Infections or inflammation: Conditions affecting the heart muscle, like myocarditis, can disrupt the normal conduction pathway.
• Underlying heart disease: Structural heart conditions can interfere with the electrical signals moving through the heart.

What are the clinical signs to watch for?

Many dogs with a low-grade second-degree AV block show no symptoms at all. The finding turns up on a routine ECG, the vet recommends monitoring, and life goes on. But when symptoms do appear, they usually reflect how often beats are being dropped and how well the heart is managing to compensate.

• Weakness or lethargy, especially after mild activity
• Tiring faster than usual on walks or during play
• Fainting or collapsing (syncope), which can sometimes look like a seizure
• A noticeably slow heart rate that shows up as general sluggishness
• In more severe cases, pale gums or confusion from reduced blood flow

Fainting and exercise intolerance are the signs that shift a case from “monitor closely” to “treat now.” If your dog has fainted even once, call your vet the same day.

How do vets diagnose an AV block in dogs?

Diagnosis starts with an ECG, a quick painless test where small sticky leads are attached to your dog’s legs and chest to record the heart’s electrical activity. This is usually how the block gets spotted in the first place.

From there, your vet will likely give a small injection of a drug called atropine and repeat the ECG. Atropine temporarily dials back the nervous system’s influence on the heart. If the block disappears after the injection, it means the nervous system was causing it, which is the more benign scenario. If it doesn’t disappear, it suggests a structural problem with the AV node itself.

Your vet may also recommend a Holter monitor, which is essentially a small portable ECG your dog wears at home for 24 hours. It records the heart throughout a normal day, including during sleep and walks, so your vet can see exactly how often beats are being dropped in real life rather than just in the clinic. An echocardiogram, which is just an ultrasound of the heart, checks whether there’s any physical damage or disease in the heart itself. Bloodwork helps rule out other causes like electrolyte imbalances or infections that could be affecting the heart’s electrical system.

Treatment options for second-degree AV block in dogs

Treatment depends entirely on the type of block and whether your dog is showing symptoms.

For dogs with a low-grade, vagal Mobitz Type I block and no symptoms, monitoring is often all that’s needed. Periodic ECGs and Holter monitoring keep tabs on whether things are progressing.

If a medication like digoxin or a beta blocker is the culprit, adjusting the dose often resolves the block. If an electrolyte imbalance is responsible, correcting it can do the same.

For Mobitz Type II, high-grade AV block, or any dog who has fainted or shown significant exercise intolerance, a pacemaker is often the recommended path forward. The device is implanted under anesthesia and takes over the job of sending reliable electrical signals to the ventricles, replacing the unpredictable backup rhythm the heart has been relying on with something consistent and safe.

Does pet insurance cover AV block in dogs

Yes, cardiac conditions like AV block are eligible for coverage under a Lemonade Pet accident and illness policy, as long as the condition isn’t pre-existing. That means your dog shouldn’t be showing any signs or symptoms before your policy starts or during your waiting period.

A cardiac workup can add up fast. ECGs, Holter monitor rentals, echocardiograms, specialist referrals, and potentially pacemaker surgery can push costs into the thousands. Getting coverage in place while your dog is healthy is what makes it actually useful when you need it.

Before we go

An AV block on an ECG report is unsettling to hear, but it’s not automatically bad news. For many dogs, it’s a finding to monitor rather than a crisis to manage. Know which type your dog has, ask about the atropine response test if it hasn’t been done, and if your dog faints even once, call your vet the same day. And if you haven’t looked into coverage yet, now is a good time to do it while your dog is still healthy.

Frequently Asked Questions

A few quick words, because we <3 our lawyers: This post is general in nature, and any statement in it doesn’t alter the terms, conditions, exclusions, or limitations of the policies issued, which differ according to your state of residence. You’re encouraged to discuss your specific circumstances with your own professional advisors. The purpose of this post is merely to provide you with info and insights you can use to make such discussions more productive! Naturally, all comments by, or references to, third parties represent their own views, and Lemonade assumes no responsibility for them. Coverage may not be available in all states. Please note that statements about coverages, policy management, claims processes, Giveback, and customer support apply to policies underwritten by Lemonade Insurance Company or Metromile Insurance Company, a Lemonade company, sold by Lemonade Insurance Agency, LLC.  The statements do not apply to policies underwritten by other carriers.