Syncope: Fainting in Dogs: Symptoms, Causes & Treatment

Syncope: Fainting in Dogs: Symptoms, Causes & Treatment

Author WDF Staff


Dogs and humans share a lot of similar health conditions. One of those conditions is fainting or syncope. If you notice your dog loses consciousness for no apparent reason, you might be dealing with syncope. Most of us will get very worried about our dog’s wellbeing, and the best thing we can do is take them to our vets. In the meantime, here are the most important things you should know about syncope in dogs.

What is precisely syncope in dogs?

Syncope (or fainting) is defined as a brief loss of consciousness caused by a lack of oxygen in the brain. The most common cause of decreased oxygen to the brain is a circulatory abnormality, which usually involves how the heart beats. Most dogs who experience syncope recover spontaneously once adequate oxygen levels reach the brain.

Normal blood circulation

To fully understand syncope in dogs, we need to understand how the proper blood circulation in dogs works. Like the heart of a human, a dog's heart has four chambers: two on top and two on the bottom. The right and left atria are at the top, and the right and left ventricles are at the bottom. Valves connect the right atrium to the right ventricle, the left atrium to the left ventricle, the right ventricle to the main pulmonary (lung) artery, and the left ventricle to the aorta, the body's main artery.

cocker puppy

For blood to circulate throughout the body, the heart's four chambers must work in unison. The sinoatrial (SA) node generates the electrical impulse that causes the heart to contract and propels blood between sections in the heart and out to the lungs and body. The SA node is also known as the heart's "pacemaker." The impulse causes the atria to constrict, causing blood to flow into the ventricles. The impulse then enters the ventricles, causing them to contract and pump blood into the lungs (via the right ventricle) and into the body (from the left ventricle).

What causes syncope in dogs?

Syncope can be caused by the heart, the nervous system, or other external factors. Syncope can be caused by a number of heart-related factors, including:

  • Bradycardia - An abnormally slow heart rate caused by the very slow firing of the SA node, failure of the SA node, blockage of the signal from the SA node to the ventricles, or atria standstill.
  • Tachycardia - Tachycardia is characterized by an abnormally rapid heart rate that results in ineffective filling of the heart chambers. Tachycardia can be caused by either the ventricles or the atria.
  • Low cardiac output 
    • A lower than average blood volume ejected from the heart can be caused by a variety of factors, including:
    • The heart muscle may be infected or degenerated (cardiomyopathy)
    • Over time, the heart valves may deteriorate.
    • The dog could have been born with a heart valve defect, which causes abnormal narrowing of the vessels leading from the heart.
    • Canine heartworm disease can cause clogging of the heart chambers and surrounding blood vessels, interfering with blood flow.
    • Blood clots in the heart chambers or lungs, known as thromboemboli, can obstruct blood flow.
    • Heart tumors can cause a decrease in cardiac output.

Among the nervous system-related causes of syncope are:

  • Vasovagal syncope - The vagus nerve helps to regulate blood vessel tension against which the heart beats. In times of high emotional stress or excitement, the nervous system may stimulate the heart to beat quickly for a brief period, resulting in a temporary hypertension state (high blood pressure). The vagus nerve may respond to transient hypertension by dilating blood vessels without a consistent increase in heart rate and blood flow. Feedback may cause the heart rate to slow, the flow of oxygenated blood to the brain to decrease, and the dog to faint.
  • Situational syncope - Situational syncope can occur as a result of a deep cough, swallowing, or an abdominal press associated with urination or stool passage.
  • Carotid sinus hyperactivity - The carotid sinus is located in the carotid artery that leads into the head and aids in the regulation of heart rate and blood pressure. A dog's collar pull may stimulate the carotid sinus, resulting in low blood pressure (hypotension) or bradycardia.

dachshund puppy running

Other causes of syncope include:

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

Symptoms of syncope in dogs

As you might expect, the symptoms of syncope or fainting will be fairly obvious. The dog will simply pass out, and if you're nearby, you should notice them fairly quickly. The following are the most common syncope symptoms in dogs:

  • Flaccidity and abrupt collapse
  • During the episode, there is usually no urination or defecation.
  • During the episode, the dog is unresponsive.
  • Complete and immediate recovery
  • Mucous membranes are pale

Breeds predisposed to syncope

Sick sinus syndrome” can occur in

The SA node does not function normally in sick sinus syndrome, causing the heart rate to fluctuate from very slow to very fast.

Boxers and German Shepherds may develop an irregular heart rate that originates in the heart's lower chambers outside of the SA node. This is known as a "ventricular arrhythmia."

pug puppy

Syncope is more common in older dogs than in young dogs, and the symptoms that develop are determined by the underlying cause of the fainting episodes.

How do vets diagnose it?

It is critical to provide the veterinarian with as much information about the fainting episode as possible; this will assist them in differentiating seizures from syncope and determining the underlying cause.

Because most fainting episodes occur in the presence of the dog owner rather than the veterinarian, it is critical to observe how the dog collapses. Taking a short video and showing it to the veterinarian, if possible, can be a great way to help your vet make a diagnosis.

Because fainting is not a disease but rather a syndrome or symptom, it is critical to determine what is causing the episodes in order to properly treat the patient. The diagnosis process begins with a thorough physical examination and collection of the patient's clinical history. Any medications that the patient is taking should be disclosed to the veterinarian.

The veterinarian will begin by performing a thorough physical examination to detect abnormalities, particularly in heart functions.

Extensive auscultation and physical examination will allow the detection of cardiac disease symptoms such as a murmur, arrhythmia, pulse deficit, or signs of reduced cardiac output such as pale mucous membranes. If evidence of cardiovascular disease is found, the following diagnostic tests may be required:

  • An electrocardiogram (ECG) determines the resting heart rate and rhythm.
  • Echocardiography is used to evaluate the size and function of the heart.
  • Complete hematology and serum biochemistry to determine whether there is evidence of decreased organ perfusion and/or other systemic diseases.
  • Thoracic radiographs are taken to see any signs of congestive heart failure.

How is syncope in dogs treated?

The underlying cause of the syncope determines how it is treated. For example, if a dog is taking a medication that can cause syncope as a side effect, an alternative medication should be sought.

If your dog has heart disease, you should get them the proper treatment for it. Medication can help regulate the heart rate, whether it is too fast or too slow. Dogs with severe heart disease may need to be hospitalized. Activity may need to be reduced in dogs with low cardiac output. In the case of vasovagal syncope, some medications can help to reduce the effects of vagus nerve activity.

english cocker spaniel sad

Finally, if they are surgical candidates, 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.

Do dogs with syncope have to be monitored?

Follow-up care for dogs with syncope, like treatment, is heavily influenced by the underlying cause. Periodic or continuous electrocardiogram (ECG) monitoring may be prescribed for dogs experiencing heart-related syncope. Limiting any stimuli that may trigger an episode is prudent, including limiting activity for dogs with low cardiac output.

What is the prognosis?

The majority of non-heart-related syncope is not fatal. Although heart-related syncope is often treatable, the risk of death is higher in these patients. In any case of canine syncope, it is critical to obtain as accurate a diagnosis as soon as possible to understand how to proceed and the potential outcomes.

World Dog Finder team

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