Heart Impulse Block in Dogs

Your dog’s heart is the most important organ in the body, pumping blood to the entire body including the brain and all of the vital organs your dog needs to survive. There are four sections (chambers) of your dog’s heart; the top two are named the atria and the bottom two are the ventricles. The heart has its own pacemaker that is named the sinus node, which provides the electrical currents (impulses) that keep your dog’s heart beating. It is these currents that travel to the ventricles, passing through the atrioventricular node on its way.

When the current from the atria does not travel to the ventricles, as it should, this is called an AV block. The blood still flows through the blood vessels, but the electrical impulse needed to keep the heart beating normally is either delayed or completely blocked.

There are three degrees of AV block, which are first, second, and third degree, depending on the amount of blockage. Second-degree can be either Mobitz type I or Mobitz type II.

Heart impulse block, or atrioventricular block (AV block), is described as an interrupted impulse transfer from the atrioventricular node to the ventricles in the heart. The atrioventricular node is the communication hub in between the upper and lower heart chambers. AV block can be a partial or a complete blockage and there are three degrees of blockage with two types of second-degree AV block (Mobitz type I and Mobitz type II). First and second-degree AV block are partial blocks and third-degree blocks are complete blockages.

The symptoms of AV block depend on the degree of the blockage. First-degree AV block is only partially blocked and may not show any symptoms at all. Second-degree AV block may not show symptoms either, depending on the cause and type. A third-degree AV block is a complete blockage, which has several serious symptoms. Some of the symptoms include:

 First-Degree Blockage

No symptoms in most dogs unless it is caused by a heart medication called digoxin. These symptoms would be:

• Appetite loss
• Vomiting
• Runny stools

Second-Degree Blockage (Mobitz type I and II)

• Exhaustion
• Fainting
• Weakness
• Collapse
• Loss of appetite
• Runny stools
• Vomiting
• Inability to play or exercise

Third-Degree Blockage

• Breathing difficulties
• Dry cough
• Not wanting to play or exercise
• Refusing to eat
• Weakness
• Fatigue
• Fainting
• Slowed heartbeat
• No energy

 Types

• First-degree AV block occurs when the electrical current from the atria to the ventricles is slowed down, causing a slow heart rate but no missed beats. It is usually mild and temporary with no symptoms.
• Second-degree AV block (Mobitz type I) has a normal rhythm with some increasing delays of the electrical current which causes missed heart beats at times. This is sometimes temporary and does not progress to a more serious condition.
• Second-degree AV block (Mobitz type II) is a normal heart rhythm with constant delays of electrical current, which causes occasional missed heartbeats but is more dangerous than type I because it often progresses to third-degree AV block.
• Third-degree AV block is a complete blockage of the electrical current, making the heartbeat slower than normal, causing possible heart attack and death.

Many things depending on the degree and type can cause AV block, although sometimes there is no discernible cause.

First-Degree AV Block

• Vigorous activity
• Prolonged exercise
• Vitamin deficiency
• Certain medications (beta blockers, digitalis, calcium channel blockers)

Second-Degree AV Block (Mobitz type I)

• Congenital (Pugs)
• Certain medications (beta blockers, digitalis, calcium channel blockers)
• Tumor
• Heart inflammation
• Electrolyte imbalance

Second-Degree AV Block (Mobitz type II)

• Heart defects
• Heart attack
• Coronary artery disease
• Myocarditis

Third-Degree AV Block

• Drug toxicity
• Electrolyte imbalance
• Lyme disease
• Endocarditis
• Sick sinus syndrome
• Heart tumor

While your veterinarian is doing a physical examination he will need all of your dog’s medical background, any recent illnesses or injuries, changes in diet or exercise, etc. Some tests will need to be done to determine the cause and type of the AV block.

• Complete blood count (CBC)
• Blood chemical panel
• Urinalysis
• Electrolyte panel
• EKG
• Digital radiographs (x-rays) of the chest
• Ultrasound of the heart

The treatment your veterinarian gives your dog depends on what degree the AV block is and what is causing it.

However, if your dog has first-degree AV block there is usually not any need for treatment unless your veterinarian finds a heart problem that needs to be treated. The veterinarian will have to run more tests to decide what kind of treatment is required.

If your dog has Mobitz type I second-degree AV block, the veterinarian will most likely prescribe atropine, or he may suggest surgery to insert a pacemaker.

With Mobitz type II second-degree AV block it is essential that your dog get surgery to implant a pacemaker.

Third-degree AV block is a complete blockage of the electrical impulses of your dog’s heart and will need a pacemaker implanted as soon as possible unless it is found to be caused by a myocardial infarction (heart attack). If that is the case, the veterinarian will administer atropine and hospitalize your dog to see if it helps.

 Pacemaker Insertion

Pacemaker implantation is a safe and common surgery that only involves two small incisions, has very few risks, and a success rate of more than 90%. The risk for your dog is higher if you do not get the surgery because the heart will not pump well enough to keep the oxygen flowing in your dog’s blood. This can cause a serious lack of energy, collapse, and even death. After surgery, you will have to keep a bandage on the incision for about two weeks until the stitches can be removed. Limited activity will probably be required for about 30 days and cannot have any rough play for at least eight weeks. Your dog will need to be brought back to the veterinarian after eight weeks, six months, and one year to check the placement of the pacemaker lead.

If your dog has an AV block that is treatable with medication, you will be able to go home as soon as he is stable. If a pacemaker has been inserted, your dog will have to stay in the hospital overnight and will be able to leave the next day when he is stable. You will have to come back in two weeks for a check-up in either case and will have to follow up with visits several times in the first year. After the first year, you will be able to go back to annual visits.