Ask The Vet: Why Do Dogs Faint?

Dr. Ivana Crnec

Dogs faint for a variety of reasons, but fainting after seeing blood or after winning the lottery, as it may happen in humans, is not one of them. Rather, in dogs fainting is, more often than not, not triggered by a strong emotional response, but sign of something not right in the health department. It goes without saying therefore that if your dog faints, a vet visit is in order to determine the underlying cause.

When dogs faint, dog owners are often concerned about their canine companions Watching a dog pass out indeed can be quite a scary sight, especially considering that we are so used to seeing our dogs in perpetual movement. Veterinarian Dr. Ivana Crnec sheds some light on fainting in dogs and explains why dogs faint. 

Fainting in Dogs 

Fainting or syncope is the term used to describe the temporary loss of consciousness (usually lasting less than 1 minute) due to decreased blood flow to the brain. Simply put, fainting occurs when the dog's brain does not receive enough oxygen.

 In most cases, the decreased oxygen feed occurs as a result of abnormal blood circulation and abnormal heart function. 

Fortunately, most fainting episodes are particularly brief. The fainting episode ends and the dog spontaneously recovers once the oxygen feed in the brain increases appropriately. Another encouraging fact is that fainting in dogs is fortunately quite rare.

As one may imagine, fainting  is more of a symptom rather than a diagnosis. It therefore results from an underlying cause that requires investigation. 

Why Do Dogs Faint? 

Syncope in dogs can be mainly caused by issues located in the dog's heart, nervous system or other extraneous places. Let's take a closer look at the underlying causes. 

Heart-Related Syncope in Dogs 

There are a variety of heart conditions in dogs which can cause them to faint. Following is a list of several. 

· Bradycardia is the term used to describe an abnormally slow heart rate. Bradycardia is almost always caused by issues related with the dog's heart pacemaker or sinoatrial (SA) node such as failure to produce signals, slow firing of signals or blocked signals.

· Tachycardia is the term is used to describe an abnormally fast heart rate. When the dog's heart beats too fast, its chambers cannot effectively fill with blood. The reasons may originate in the dog's atria or ventricles.

· Low cardiac output indicates that the volume of blood ejected from the dog's heart is lower than normal. It can be caused by many culprits such as diseased heart muscle, deteriorated heart valves, congenital heart defects, parasitic infestations (heartworm disease), blood clots and heart tumors.

Nervous System-Related Syncope in Dogs 

A variety of nervous system issues can trigger dogs to pass out. Following are several nervous system disorders known to cause dogs to faint. 

· Situational syncope, as the name implies, this form of fainting can occur during common activities such as swallowing, coughing or abdominal contraction from urination or defecation.

· Vasovagal syncope is connected to the dog's vagus nerve. The vagus nerve is crucial for tension regulation inside the blood vessels. When emotionally provoked (either excited or stressed) the nervous system stimulates the heart to beat very fast which leads to temporary hypertension (high blood pressure). At such moment, the vagus nerve responds by initiating blood vessels dilatation. However, this dilatation is not accompanied by increased heart rate and increased blood flow. This results in a sudden drop in the heart rate, decreased flow of oxygenated blood to the brain and fainting.

· Carotid sinus hyperactivity is another possible cause of fainting in dogs. Located in the carotid artery, this carotid sinus regulates the dog's heart rate and blood pressure. Pulling the dog’s collar can easily stimulate the carotid sinus and lead to low blood pressure, decreased heart rate and consequently – fainting.

Other Causes of Fainting in Dogs 

There can be several other causes of syncope in dogs. For example, all medications that alter a dog's  blood pressure can potentially lead to a fainting episode.

Hypoglycemia (low blood sugar levels) and anemia (low red blood cells) can cause fainting too. 

Hypocalcemia, which is low levels of calcium in the blood and hyponatremia, that is, low levels of sodium in the blood can trigger fainting as well. 

Some dogs may also be more predisposed to fainting than others and this is an interesting subject. As previously stated, the syncope is a symptom not a diagnosis. Therefore, the prevalence of syncope in certain breeds and ages is based on the exact underlying cause. 

For example, sick sinus syndrome, is a common cause of fainting in dogs. This issue is particularly common in Pugs, Miniature Schnauzers, Cocker Spaniels and Dachshunds.

 In dogs with sick sinus syndrome the heart’s rate is irregular and varies from extremely fast to extremely slow. 

Ventricular arrhythmia is another problem that causes fainting and it is fairly common among German Shepherds and Boxers.

In general, fainting is more common in older dogs. Younger dogs are not so commonly affected, and even if affected, their signs are milder and easier to overcome or compensate for.

Fainting Versus Collapsing In Dogs 

Although commonly mistaken, fainting and collapsing are not the same. Collapsing is much more common than fainting and unlike in fainting, in collapsing there is no loss of consciousness. 

Namely, during a collapse, the dog will be weak and probably unable to get up and stand, but it will still be conscious.

 Collapse in dogs is related with many conditions such as dehydration, extremely low blood pressure, shock, heat stroke, severe anemia, internal bleeding, musculoskeletal problems (Lyme disease), neuromuscular problems (tick paralysis, botulism), endocrine problems (hypoglycemia), poisoning (with xylitol) and pregnancy.

At The Vet's Office 

The treatment for syncope in dogs is based on the underlying cause. For example, if the underlying cause is a heart disease the key to eliminating the syncope episodes is treating the heart issue.

 There are special medications that can be used to fix the heart rate, regardless of whether it is too slow or too fast. Based on the exact heart issue, some patients may require prolonged hospitalization and specialized surgeries such as implantation of a pacemaker. 

Nevertheless, before subjecting the patient to a complicated surgery, it should be determined whether he can survive the anesthesia and the surgery itself.

If the fainting is a side effect from a medication the dog is taking, the solution would be to start using an alternative medication with fewer or lesser side-effects.

The prognosis for dogs with syncope episodes depend mainly on the underlying cause. Generally speaking, the prognosis is better for dogs with non-heart-related syncope episodes. Dogs with heart-related syncope require either continuous or periodic ECG monitoring. 

To improve the prognosis, it is necessary to eliminate, or at least, minimize the potential stimuli that may trigger an episode. However, it should be well-accented that heart-related syncope is a life-threatening condition.

What Should You Do If Your Dog Faints?

Witnessing a fainting episode in your dog is a scary scenario for all dog parents. Knowing what to do and how to react is of paramount importance.

Step number 1 – call your nearest emergency hospital or 24/7 clinic and seek immediate veterinary help.

Step number 2 – put your hand over your dog's heart and check the heartbeat. Determining whether your dog’s heart rate is slow or rapid will help your vet determine the underlying cause.

Step number 3 – if possible, videotape the episode. Sometimes the physical appearance of the episode will help the veterinary specialist determine the cause. 

About the Author 

ivana

Dr. Ivana Crnec is a graduate of the University Sv. Kliment Ohridski’s Faculty of Veterinary Medicine in Bitola, Republic of Macedonia.She currently practices as a veterinarian in Bitola and is completing her postgraduate studies in the Pathology of Domestic Carnivores at the Faculty of Veterinary Medicine in Zagreb, Croatia.Ivana’s research has been published in international journals, and she regularly attends international veterinary conferences.

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