What Causes Fainting in a Dog? And What Should You Do?

The underlying causes of fainting in dogs range in seriousness.

By: diriye
There’s usually no warning sign before a dog faints. By: diriye

Seeing your dog faint can be an alarming experience. But don’t worry – in most cases, dogs recover within a few seconds.

A faint is technically a temporary loss of consciousness, and the immediate cause is a lack of blood supply to the brain, but perhaps the bigger question is: What caused this fall in blood to the brain in the first place?

Broadly divided into 2 groups, heart-related and not-heart-related issues cause fainting. Of these, cardiac-related causes can benefit from prescription medications, while non-cardiac causes are rarely life-threatening.

You may find it difficult to the pick up the nuances that differentiate a faint from a minor seizure, or other causes of collapse, in the anxiety of the moment. Where possible, try to remember to pick up your phone and video record the episode to show your veterinarian later.


You rarely get a warning sign when your dog is about to faint — the opposite is true for a seizure, where the dog often acts strangely before and after the event.

This video shows a dog unsteady on his feet, but he does not drop immediately:

YouTube player

During a faint, the dog will flop to the ground — his limbs will be floppy, as if asleep (again, contrast this with a fit, where the dog’s legs go rigid and paddle in the air). Most faints last for no more than 30 seconds, and the animal reverts back to his normal self pretty much immediately.

Sometimes the faint is triggered by coughing, straining to go to the toilet or pulling hard on his collar. If you notice this pattern, please tell your veterinarian — it helps diagnosis and treatment.


Heart-Related Faints

Any condition that causes the heart to pump ineffectively can cause temporary interruption of the blood supply to the brain.

However, this is an unusual presentation for heart disease, which is more commonly recognized by heavy breathing, coughing and lack of energy rather than fainting.

Most commonly, heart-related faints occur because of a flawed conduction system (like faulty wiring) giving mixed messages about when the next heartbeat is due, causing erratic disturbances in the pattern of the heartbeat.

Non-Heart-Related Faints

These are largely caused when a nerve (the vagus) becomes overactive. It’s the longest nerve in the body, passing through the chest and up the neck to the brain.

The vagus is responsible for slowing heart rate, and anything that puts pressure on the vagus and triggers it to fire can cause dramatic slowing of the heart rate.

A dog that pulls on his collar may accidentally trigger increased vasovagal tone, which drops the heart rate and causes a faint (vasovagal syncope). Likewise, straining to pass feces, or even coughing, can increase pressure in the chest and stimulate the vagal nerve, resulting in a faint.

Other causes of faint include low blood glucose, calcium or sodium levels, or a condition known as hyperviscosity syndrome (extremely rare), where the blood is too thick and sticky to circulate properly through the brain.

Although rare, some medications, especially those prescribed for circulatory problems or high blood pressure, can drop blood pressure too much and cause faints.


A detailed history, a review of medications and (where possible) a video of the faint can give your veterinarian a lot of clues about your pet’s condition.

Screening blood tests are advisable to rule out causes such as low blood glucose, but if this is normal, the next step is an ECG.

If the faints are regular, but intermittent, a 24-hour ECG monitor will be necessary to record the exact moment of the irregular heartbeat (if this is the cause).


Key to treatment is identifying the underlying cause of the faint and addressing this problem.

If the dog has a heart conduction issue, he may need a pacemaker fitted or to be put on drugs that regulate his heart rate. If the issue is straining to pass feces, then fecal softeners should help prevent this scenario.


In animals who faint for non-cardiac reasons, prevention is often a matter of avoiding certain situations.

A dog who faints when he pulls on his collar should be switched to a harness. Stress increases vagal tone, and it may be appropriate to seek behavioral training for a dog who gets excessively anxious in certain situations.


  • Clinical Medicine in the Dog and Cat. Schaer. Publisher: Manson Publishing.
  • Cardiorespiratory Medicine of the Dog and Cat.Martin & Corcoran. Publisher: Blackwell Science.


This pet health content was written by a veterinarian, Dr. Pippa Elliott, BVMS, MRCVS. It was last reviewed Dec. 17, 2018.