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AV Block

The canine and feline heart has four chambers. The top two are called the atria. The bottom two are called the ventricles. The heart’s “natural” pacemaker is called the sinoatrial (SA) node or sinus node. It’s a small mass of specialized cells in the heart’s right atrium. It produces electrical impulses that make the heart beat. For the heartheart to beat properly, the signal must travel from the SA node down a specific path to reach the ventricles. As the signal goes from the atria to the ventricles, it passes through specialized conducting tissue called the atrioventricular (AV) node.

On an electrocardiogram (ECG), a portion of the graph called the P wave shows the impulse passing through the atria. Another portion of the graph, the QRS wave, shows the impulse passing through the ventricles. As long as the impulse is transmitted normally, the heart pumps and beats at a regular pace.

What is AV Block?

Sometimes the signal from the heart’s upper to lower chambers is impaired or doesn’t transmit. This is “heart block” or “AV block.” This does not mean that the blood flow or blood vessels are blocked. Heart block is classified according to the level of impairment — first-degree heart block, second-degree heart block or third-degree (complete) heart block.

First-degree heart block, or first-degree AV block, is when the electrical impulse moves through the AV node more slowly than normal. The time it takes for the impulse to get from the atria to the ventricles (the PR interval) should be less than about 0.2 seconds. If it takes longer than this, it’s called first-degree heart block. Heartrate and rhythm are normal, and there may be nothing wrong with the heart. In this stage of heart block, the canine or feline patietn mya not show clinical signs. In fact, in very fit or active dogs and cats, it may be a normal finding due to low resting heart rate. As such, most cases of 1st degree AV block are found incidentally, requiring no treatment and offering a good prognosis.

In 2nd degree AV block, some signals from the atria don’t reach the ventricles. This causes “dropped beats.” On an ECG, the P wave isn’t followed by the QRS wave, because the ventricles weren’t activated. There are two types:

Type I second-degree heart block, or Mobitz Type I, or Wenckebach’s AV block. Electrical impulses are delayed more and more with each heartbeat until a beat is skipped. This condition is not too serious but sometimes causes mild to moderate weakness, disorientation, or excercise intolerance. This type of AV block is typically responds well to the drug atropine. Generally prognosis is fair to good with treatment.

Type II second-degree heart block, or Mobitz Type II. This is less common than Type I but generally more serious. Because electrical impulses can’t reach the ventricles, an abnormally slow heartbeat may result. Signs are similar to Type I AV block, but can be more pronounced. Some of these cases respond well to treatment with atropine, but some require a pacemaker.

3rd degree AV block (complete AV block) means that the heart’s electrical signal doesn’t pass from the upper to the lower chambers. When this occurs, an independent pacemaker in the lower chambers takes over. The ventricles can contract and pump blood, but at a slower rate than that of the atrial pacemaker.

These impulses are called functional or ventricular scope beats. They’re usually very slow and can’t generate the signals needed to maintain full functioning of the heart muscle. On the ECG, there’s no normal relationship between the P and the QRS waves. Complete heart block is most often caused in adults dogs and cats by heart disease. Complete heart block may be a medical emergency with potentially severe symptoms and a serious risk of cardiac arrest (sudden cardiac death). Treatment for third degree AV block is to implant a pacemaker as soon as possible. Prognosis with pacemaker implantation is guarded, however, as pacemaker technology in dogs and cats further advances, prognosis will continue to improve.


Roger L. Welton, DVM
Founder and Chief Editor,
President, Maybeck Animal Hospital


Article updated 9/6/2012


18 thoughts on “AV Block

  1. Jason says:

    I have a 14 year old male Siamese cat that was diagnosed with a high second degree AV block in July 2013. His heart rate was 80-84 BPM. After about 1 year or so the heart block cleared up on its own. After having him rechecked in Aug 2015, the heart block has returned and is now a low 2nd degree block. I have been to 3 different pet cardiologists and not one of them has an exact idea as to what is causing this.

    • Ken says:


      Did you ever get your cat fixed…? I have the same problem with my 18 year old kitty. She faints and collapses and I’m afraid she is really going to hurt herself if she falls…or her little heart will just eventually stop. I think she is to frail for a pacemaker but perhaps there is a medication that could help her.

  2. Marta says:

    I have the same issue in my 16 year old kitty. Hyperthyroidism caused damage to the heart and after we saw a cardiologist a pacemaker was advised. His results showed 1st-2nd degree AV block. It was explained that he may pass suddenly and to be prepared. We are currently deciding if we want to chance a passing naturally that may be swift and painless or may be worse. We don’t want him gone but he is 80 years old in cat years the pacemaker is a no way at this age. We don’t want him passing in any pain. So we are currently monitoring quality of life which for the moment is spent in the closet looking sad (he has a bed in there he likes to rest in daily.) While he is doing some normal routine things I think he is sad and tired and we remain torn. But in our case we want him departing with only love and painfree and the vets cannot promise it will be kind naturally. That is the painful struggle we are having. We don’t want to end his life we thought we had more time. But at the same time he is counting on us to make the right choices. It’s very difficult. I read this condition could be genetic as well, he had diabetes in the past and that resolved. The vets think the thyroid did this and unfortunately it went too far. We are treatin the thyroid but I see he is eating less. I think he wants peace. He tried to bite me today in anger as I patted him. He doesn’t understand what is going on. He is painfree now as we debate what is the best choice. I had hoped we had more time but the more I aatch him and see the decline in joy the more I feel he is guiding me. :(

    • Rose says:

      Hi I just read your post. My 15 year old kitty has hyperthyroidism and it too damaged his heart. I just had a pacemaker inserted and he seems to be ok so far. He almost died from his heart not functioning right and was collapsing and I coulnt bear to watch him suffer. I am asking people to help me pay some of the fees but I think it is worth getting the pacemaker cause he will die eventually and suffer with this . It is so sad to see but I would do anything to save my kitties! Good luck Rose a loving cat woman

  3. Jen says:

    Hi all. So brought my kitty in to the cardiologist today. She’s 13 & 1/2. She started having a few disorientation spells a couple weeks ago. After vet check up and cardiologist exam/ekg they said in most respects she seems a very young and fit cat. But her heart rate is dropping at times because of the signal block and eventually she might have failure. Do I put her through getting a pacemaker without getting other opinions? I see up at the top atropine is mentioned but cardiologist today said meds can’t treat it so now I’m torn. Maybe a 2nd opinion is in order.

  4. Danielle Derosia says:

    I have a almost 3 year old Sphynx, He turns 3 in October. and this past Sunday (mothers day) he what the Cardiologist has determined fainted. He fell over and we were there but no one saw it happen but heard him hit the floor in front of the litter box. And immediately looked him over and he was disoriented, pupils fully dilated weak, and limp. He ‘recovered’ quickly by the time I had him at the 24 hour vet, less then 20 mins since the episode, with him able to stand heart rate steady breathing fine, but I noticed weak. (to shorten the whole tale..) an ECG was done and his heart rate was 125 and showed 3rd degree heart block, and the next day I brought him to a Cardiologist for a ecocardiogram, and his heart rate was still only 130 and showing an arrhythmia, but the scan came back Clear! No wall thickening no other heart abnormalities (stage 2 murmur always known about, and had since a born, but that wasn’t visible so not any problem to his heart now known), and no underlying cause. So the Cardiologist considers his fainting episode was his heart stopping for 4-6 seconds. (scary..) So he is considered symptomatic already, and we just have the diagnosed now also. So no history of how long he has had this, and now had an episode (I know its degenerative but for his age it is like an idiopathic broken quickly thing) and we are unable to know if he will have another episode in the next week or month or longer before it happens again, and will his heart restart and how long will recovery take next time? So Tuesday he is feeling better and more of himself ( and ALWAYS been an active cat.. Sphynx don’t know how to NOT be active, just the night before his fainting episode he was tearing around the house for over an hour playing chase.) So got his heart rate checked and at the office it was over 190 (he was excited and shivery because the offices are cold..) a good reactive rate.. So it took about 30 plus hours for his heart to recover from the episode and be back at its more regular pace. But we already determined we wanted to be able to give him the best chance and take away the chance of sudden death from at least one known episode of heart stopping.

    But I can not seem to find any information on a 2 year old cat with this form of AV block WITHOUT other heart conditions or problems (no thickening or disease) is he a special and only case of his young age but healthy heart, just with a slacker back up node that takes a brake and WILL wear out eventually. So Wednesday he will go in for surgery to put in a pacemaker so even if his heart is behaving for the time being we can preemptive catch it before it starts to decline. Does anyone think this is jumping the gun? (his cardiologist said it is degenerative, the node is working again now but in time it will fail and it wont cause a problem kinda siting quietly if it needs because he will eventually NEED it to keep his rate up if he wants a long good quality life.

    Just looking for anyone who might have similar happenings or thoughts.

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