Heart Block (Mobitz Type I) in Cats

Atrioventricular Block, Second Degree–Mobitz Type I in Cats

Second-degree atrioventricular block occurs when the electrical conduction within the AV node is delayed. 

The sinoatrial node (SA Node, or SAN), also called the sinus node, is the initiator of electrical impulses within the heart, triggering the heart to beat, or contract, by firing off electrical surges. The atria, the two upper chambers of the heart that receive  and send out blood, are prompted into action by the SA node’s electrical impulse, which then activates the atrioventricular node (AV node). The AV node conducts the normal electrical impulses from the atria to the ventricles, coordinating the mechanical activity so that the atria have forced the blood down into the ventricles before the ventricles contract to send the blood out into the body through the pulmonary artery and aortic artery.

Most cats with this condition show no signs, appearing to be in perfect health. Low calcium levels and certain drugs (e.g., digoxin, bethanechol, physostigmine, pilocarpine) may predispose some cats to second-degree AV block–Mobitz Type 1.  Second-degree AV block–Mobitz Type 1 can also be brought about by diseases unrelated to the heart. This condition is uncommon in cats, but has been documented.

Symptoms and Types

  • Most affected cats show no symptoms
  • If induced by digoxin (a heart medication) overdose, cat may have vomiting and lack of appetite
  • Symptoms of congestive heart failure associated with hypertrophic cardiomyopathy
  • Weakness



  • May occur in normal, healthy cats
  • Diseases that are not directly related to the heart
  • Hyperthyroidism
  • Cardiomyopathy
  • Cardiac neoplasia – masses of the heart
  • Certain drugs can affect functioning of the AV node



You will need to provide a thorough history of your cat’s health leading up to the onset of symptoms. Your veterinarian will perform a full physical exam, with a chemical blood profile and a complete blood count. A thorough history from you will allow your veterinarian to rule out masses, gastrointestinal disorders, high pressure in the eye and upper airway disease. X-rays may help detect some of these disorders as well. An atropine response test, which increases the firing action of the sinoatrial node and conduction of the AV node will indicate if the disease is originating from the heart. 

An electrocardiogram (ECG, or EKG) recording can be used to examine the electrical currents in the heart muscles, and may reveal any abnormalities in cardiac electrical conduction (which underlies the heart’s ability to contract/beat).



Pertaining to the lungs

sinoatrial node

A lump of tissue inside the right atrium; it helps to regulate the beat of the heart


A cavity within a bone; may also indicate a flow or channel


The prediction of a disease’s outcome in advance


The digestive tract containing the stomach and intestine

atrioventricular node

A mass of tissue that can be found inside the right atrium; transmits electrical impulses to certain areas of the heart.


A record of the activity of the myocardium


A large blood vessel that transports blood out of the heart.


A wave that is transmitted through nerves and nervous tissue

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