Syncope (Fainting) in Dogs

By Malcolm Weir, DVM, MSc, MPH; Robin Downing, DVM, DAAPM, DACVSMR, CVPP, CRPP

What is syncope?

Syncope (or fainting) is defined as a temporary loss of consciousness that occurs when the brain does not receive enough oxygen. The most common reason for decreased oxygen to the brain is an abnormality in the circulation, generally involving how the heart is beating. Most dogs who experience syncope spontaneously recover once appropriate levels of oxygen reach the brain.

How does normal blood circulation happen?

A dog’s heart has four chambers, just like humans- two on the top and two on the bottom. The top two chambers are the right and left atria and the bottom two chambers are the right and left ventricles. There are valves between the right atrium and the right ventricle, between the left atrium and the left ventricle, from the right ventricle to the main pulmonary (lung) artery, and from the left ventricle to the aorta, which is the main artery of the body.

"For blood to circulate around the entire body, the four chambers of the heart must work together in a coordinated way."

For blood to circulate around the entire body, the four chambers of the heart must work together in a coordinated way. The electrical impulse that causes the heart to contract and propel blood between chambers in the heart and out to the lungs and body originates in the sinoatrial (SA) node. The SA node is also called the “pacemaker” of the heart. The impulse causes the atria to contract, pushing blood into the ventricles. The impulse then travels into the ventricles, causing them to contract and pump the blood into the lungs (from the right ventricle) and out into the body (from the left ventricle).

Are some dogs more likely to develop syncope than others?

A condition called “sick sinus syndrome” may occur in Cocker spaniels, miniature schnauzers, pugs, and dachshunds. In sick sinus syndrome, the SA node does not function normally, causing the heart rate to vary from very slow to very fast. Boxers and German shepherds may develop an irregular heart rate that originates outside of the SA node, in the lower chambers of the heart. This condition is called a “ventricular arrhythmia”.

Syncope is more common in older dogs than young, and the signs that develop depend on the underlying cause of the fainting spells.

What are the specific causes of syncope in dogs?

The cause for syncope may originate in the heart itself, in the nervous system, or from other extraneous causes. Heart-related causes of syncope include:

Bradycardia- Bradycardia is an abnormally slow heart rate that may be caused by very slow firing of the SA node, a failure of the SA node, a block of the signal from the SA node through to the ventricles, or standstill of the atria.

Tachycardia- Tachycardia is an abnormally rapid heart rate which leads to ineffective filling of the heart chambers. Tachycardia may originate in either the ventricles or the atria.

Low cardiac output- A lower than normal blood volume being ejected from the heart may be caused by many different factors:

  • The heart muscle may be diseased/degenerated (cardiomyopathy)
  • The heart valves may deteriorate over time
  • The dog may have been born with a heart valve defect that causes abnormal narrowing of the vessels leading out of the heart
  • Canine heartworm disease can clog the heart chambers and the surrounding blood vessels, interfering with the passage of blood
  • Blood clots in the heart chambers or the lungs, called thromboemboli can interfere with blood flow
  • Tumors in the heart can cause decreased cardiac output

Nervous system-related causes of syncope include:

Vasovagal syncope- The vagus nerve helps to regulate the tension in the blood vessels against which the heart beats. In a moment of heightened emotional stress or excitement, the nervous system may stimulate the heart to beat very fast for a short time, leading to a temporary state of hypertension (high blood pressure). The vagus nerve may respond to that transient hypertension by causing dilation of blood vessels, but without any accompanying consistent increase in heart rate and blood flow. Feedback may then prompt the heart rate to drop, flow of oxygenated blood to the brain decreases, and the dog faints.

Situational syncope- Situational syncope may occur with deep coughing, swallowing, or an abdominal press associated with urination or passage of stool.

Carotid sinus hyperactivity- The carotid sinus is located in the carotid artery leading into the head, and it helps to regulate heart rate and blood pressure. A pull on a dog’s collar may stimulate the carotid sinus causing low blood pressure (hypotension) or bradycardia.

Other causes of syncope:

  • Many medications that affect blood pressure
  • Low blood sugar (hypoglycemia)
  • Low levels of calcium (hypocalcemia) or sodium (hyponatremia) in the blood

What treatments are available for dogs with syncope?

Treatment of syncope is determined by the underlying cause of the fainting spells. For instance, if a dog is taking a medication that can cause syncope as a side effect, then an alternative medication should be sought. If heart disease is present, then it should be treated. There are medications that can help to regulate the heart rate, whether it is too fast or too slow. Dogs with severe heart disease may require hospitalization. For dogs with decreased cardiac output, activity may need to be reduced. There are medications that can help to blunt the effects of vagus nerve activity in the case of vasovagal syncope. Finally, some dogs with sick sinus syndrome may benefit from the implantation of a pacemaker to take over the action of the SA node, or to overcome a blockage of the signal from the SA node to the ventricles, if they are candidates for a surgical intervention.

What kind of follow-up care is required for dogs with syncope?

As with treatment, follow-up care for dogs with syncope depends in large part upon the underlying cause. For dogs with heart-related syncope, periodic or continuous monitoring of an electrocardiogram (ECG) may be prescribed. It is wise to minimize any stimuli that precipitate episodes, including minimizing activity for dogs with decreased cardiac output.

"Most non-heart-related syncope is not life-threatening"

Most non-heart-related syncope is not life-threatening. Heart-related syncope can often be treated, but the risk of death is higher in these patients. In any case of canine syncope, it is important to get as accurate a diagnosis as possible, in order to understand how best to proceed as well as to understand the potential outcomes.

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