What is Heart Block?
“Heart block” refers to atrioventricular block. In heart block, the cardiac impulses are blocked periodically inside the atrioventricular (AV) node. The severity of the condition is measured in degrees. In a first-degree heart block, the signal travels more slowly through the AV node. In second-degree heart block, some electrical signals don’t get to the ventricles, and in third-degree heart block, electrical impulses do not pass from the atria to the ventricles at all.
The atrioventricular node is located in the two top of the cat’s heart. Normally, the heart’s sinoatrial node sends electrical impulses from the top of the heart to the atria, through the atrioventricular node, and to the ventricles in the bottom of the heart.
Symptoms of Heart Block in Cats
Heart block symptoms vary with types and degrees:
- May not cause symptoms
- Mild or moderate weakness
- Exercise intolerance
- Some disorientation
- Difficulty breathing
- Congestive heart failure
- Slow heartbeat
- Fainting (syncope)
- Potential cardiac arrest
Second-degree heart block is found in two types, Type I and II or Mobitz Type I and II. In this form of heart block, electrical impulses may become progressively more delayed and can result in skipping heartbeats. Symptoms are consistent with those above, but may be more severe.
Causes of Heart Block in Cats
Causes of heart block in cats vary:
- Heart tissue is scarred by unknown causes (idiopathic fibrosis)
- Infiltrative cardiomyopathy
- Hypertrophic cardiomyopathy (left ventricle heart muscle thickens or enlarges)
- Myocarditis (inflammation of heart muscle)
- Endocarditis (inflammation of the heart’s lining)
- Myocardial infarction (heart attack) that leads to death of heart tissue
- Lyme disease
- Electrolyte disorder
- Chagas’ disease (tick-borne illness)
- Electrolyte imbalance
- Vagal stimulation
- Idiopathic fibrosis
- Congenital heart defects
- Xylazine (used as anesthetic)
Diagnosis of Heart Block in Cats
Before diagnosing a heart block, the vet will do a complete physical on the cat. This will include drawing blood for a complete blood count and chemistry profile. Cats with heart blocks caused by infections will have abnormally high white blood cell counts. Those cats with electrolyte imbalances will have abnormal biochemistry profiles.
The cat will undergo an echocardiogram and an electrocardiogram. Thoracic X-rays are taken so the vet can begin to narrow down where the problem has started. The cat should be tested using Holter monitoring, which is a continuous ECG recording over 24 hours. This test is recommended when the cat has begun to collapse. A doppler ultrasound may also be useful in arriving at a diagnosis.
The cat should also be tested for feline immunodeficiency virus (FIV) and feline leukemia virus (FeLV), hyperthyroidism, especially if it is older than six years of age.
Treatment of Heart Block in Cats
Treatments may vary depending on the type and degree of heart block the cat is diagnosed with. For first degree and Mobitz Type I heart blocks caused by xylazine, digitalis, or acetylpromazine, the vet should either stop the medication or lower the dosage. Illnesses that cause heart blocks of this type should be treated so the cat’s heart begins to function normally.
Mobitz Type II, being a more serious form of the illness, can be treated with theophylline, atropine, or propantheline. Depending on the severity of the heart block, the vet may recommend the implantation of a pacemaker for the cat.
Cats with the most severe form of heart block may respond well to some of the medications used for Type II block. Corticosteroids may also help because of their anti-inflammatory properties. Cats should not take beta-blockers, digoxin, acepromazine or calcium channel blockers with this form of heart block.
The cat may also have to go on a special diet, depending on how severe any underlying disease may be.
If the vet decides that the only treatment for the cat is the insertion of a pacemaker (permanent or temporary), the cat will need to be placed in cage rest before surgery. Cage rest after surgery will also be necessary because the vet has to place a non-restrictive bandage around the cat’s torso for up to five days. This bandage helps to keep a seroma (an accumulation of serum under the skin) from developing. The bandage also reduces the likelihood of pacemaker movement.
Recovery of Heart Block in Cats
Cats with the most severe form of heart block don’t receive a good prognosis. Cat owners should simply do their best to keep the cat comfortable and happy.
Once the cat has gone home after undergoing pacemaker surgery, the owner should return to the vet regularly for ECG monitoring and chest X-rays.
The owner and vet will need to be on the lookout for potential complications. If the pacemaker is a pulse generator, it will need to be replaced when the battery is ready to die, if it malfunctions or if an exit block begins to develop. The leads of the pacemaker can become dislodged and the cat can develop an infection in the area where the pacemaker leads have been placed. If this develops, it will need to be promptly treated.
If a cat has not had a pacemaker installed, it should be closely monitored for any possible worsening of its symptoms.