Heart Attack in Dogs
Overview of Canine Heart Attack
A “heart attack” is the term commonly applied to people who have suffered a myocardial infarction (MI), most commonly related to coronary artery disease. The myocardium is the muscular tissue of the heart; it receives nutrients and oxygen from the coronary arteries. The coronary arteries are small blood vessels in the myocardium which bring blood from the aorta, the main artery of the body.
If the coronary arteries become narrowed or blocked, usually with hardened plaques of fats, this can interrupt blood flow to the myocardium resulting in an infarction. The “infarct” is the area of the myocardium which is sick from the lack of blood supply. This area of muscle stops functioning well and may become the source of abnormal heart rhythms (arrhythmias). Depending on the size of the infarct, the area of the heart involved and whether the blockage is partial or complete, a myocardial infarction may be mild (chest pain) or severe resulting in heart failure or even death.
It is very unusual to identify a myocardial infarction in a dog, most likely because of the differences in the way fats are processed by the body and dietary factors. Even on a fatty diet, dogs are very resistant to coronary artery disease. The few cases of coronary artery disease that have been defined in dogs were associated with hormonal imbalances, such as hypothyroidism (underactive thyroid gland). In theory, dogs with diabetes may also be at higher risk for coronary artery blockage. Even if your pet suffers from one of these diseases, the risk of MI is miniscule!
The term “heart attack” is often misunderstood in relation to dogs and cats. The term is typically used to either define a collapsing episode (more accurately termed a syncopal spell or syncope) or to describe sudden death of an animal, neither of which are really heart attacks.
Syncope is the term used to describe fainting or collapse episodes (partial or complete) related to an abnormality of the heart. Syncope may be related to poor heart function as a result of heart disease or may be related to abnormal heart rhythms. Syncope may occur with an excessively slow heart rate (bradyarrhythmias) or with an excessively fast heart rhythm (tachyarrhythmias). Syncope may occasionally be a result of low blood pressure, usually as a result of medication administered for various medical problems.
Seizures (epileptic fits secondary to disease of the brain) may mimic a syncopal spell. Typically animals with syncope are more still (with limp or stiff limbs) during a spell and return to normal very quickly (seconds to minutes). Seizures are usually accompanied by jerky limb movements and the animal remains abnormal for minutes to hours after the episode. Any animal showing either of these syndromes should be presented to a veterinarian for evaluation.
When sudden death occurs in a pet, there are many different causes. A heart attack is actually a very unlikely cause. To try to determine the cause of sudden death, your pet should be presented as soon as possible to a veterinarian for a post-mortem (autopsy) examination. This may require referral to a veterinarian who specializes in pathology.
Since a fainting or syncopal spell is often mislabeled as a heart attack, diagnostic tests and treatment are aimed at finding the underlying cause of the fainting and not specific for a “heart attack”.
Diagnosis of Heart Attack in Dogs
Complete physical examination may lend clues as to the cause of the episode. For example, a heart murmur (abnormal heart sound) or arrhythmia may be detected in patients with heart disease, the pulses maybe be weak in patients with low blood pressure or the heart sounds might be muffled in patients with an accumulation of fluid around the heart (pericardial effusion).
Complete laboratory evaluation, including CBC, biochemical profile, urinalysis, are evaluated to screen for systemic diseases resulting in collapsing episodes.
Radiographs (X-rays) of the chest and abdomen are taken to evaluate for major diseases affecting the heart, lungs or abdominal organs.
In cases where the heart is suspected to be the problem on the basis of the physical examination and initial evaluations, a cardiac evaluation including an echocardiogram (ultrasound of the heart) and an electrocardiogram (ECG). An echocardiogram will define any disease affecting the heart valves, heart muscle and the pericardium (the sac covering the heart). Any tumors on or around the heart can usually be identified on the ultrasound. An electrocardiogram (ECG) records the electrical activity of the heart and will help identify any abnormalities in the heart rhythm which could cause collapse.
Ambulatory ECG recordings may be necessary if the general cardiovascular evaluation (chest X-ray, echocardiogram ECG) fails to identify the source of collapse. A backpack type ECG can be sent home with the dog (ambulatory ECG). Either a holter monitor can be done (records all the heartbeats for a 24-hour period) or event monitoring may be done. An event monitor is very small and is attached and running at all times. This device will only store information when the “event” button is pressed, which is done when a collapse spell is witnessed. If one records an event, this is the best way to identify whether the cause of the collapse is an abnormal heart rhythm.