Fainting, also called syncope, is a loss of consciousness that is typically due to a lack of normal blood flow to the brain. Fainting is uncommon in dogs and not always caused by a lack of blood flow; there are other similar medical causes that result in fainting for both humans and animals. That said, no matter how scared dogs or cats may get, they never faint from fright the way a human might (e.g., from panic attacks, hyperventilating, etc.).
Why would a dog faint?
Fainting in dogs is typically attributed to one of two main problems: neurologic (e.g., brain or spinal cord) or cardiac (e.g., heart arrhythmias, etc.). 
Neurologic problems may include:
- Seizures
- Abnormal brain activity
Cardiac problems may include:
- Abnormal heart rhythms (such as sick sinus syndrome)
- A-V block
- Ventricular tachycardia
- Ventricular fibrillation
What to do if your dog faints:
- Put your hands over the heart and see if you can feel a heartbeat. Try to tell if the heart rate is very, very slow or extremely rapid. This will help your veterinarian determine if the cause of fainting is cardiac in origin.
- Try to videotape the episode quickly. Often times, your veterinarian or veterinary specialist (e.g., neurologist, cardiologist) can determine the cause by physical appearance.
- Seek immediate veterinary attention.
Once you get to your veterinarian or emergency veterinarian, the doctor will need to check the heart rate on an electrocardiogram (ECG) right away to look for the presence of abnormal arrhythmias, and perform blood work to make sure that there are no metabolic causes (e.g., liver, kidney, etc.) or blood sugar problems causing the episode.
If neurologic problems are detected, further evaluation by a neurologist is warranted. Unique tests such as an electroencephalogram (EEG) may need to be done to monitor the brain for any unusual seizure activity. Alternatively, an MRI or spinal tap may also be necessary. If cardiac abnormalities are detected, a referral to a cardiologist for an ultrasound of the heart (e.g., echocardiogram), x-rays of the heart, and a Holter monitor (to monitor the heart rhythm) may be necessary.
Collapsing is not fainting
Fainting in dogs needs to be differentiated from the more common problem of collapsing. With collapse, loss of consciousness typically doesn’t occur—in other words, your dog or cat may be weak and unable to get up, but he’s still conscious. There are numerous causes for collapse including:
- Dehydration
- Shock or severe hypotension (e.g., low blood pressure)
- Internal bleeding or severe anemia
- Heat stroke
- Neurologic problems (e.g., seizures)
- Neuromuscular problems (e.g., botulism, tick paralysis, etc.)
- Musculoskeletal problems (e.g., Lyme disease, joint problems, etc.)
- Cardiac problems (e.g., arrhythmias, etc.)
- Endocrine problems (e.g., low blood sugar)
- Poisonings (e.g., xylitol, etc.)
- Pregnancy
Thankfully, some causes of fainting can be treated, but often require more advanced therapy. For example, if an abnormally low heart rate (e.g., sick sinus syndrome, A-V block) is detected, sometimes a permanent pacemaker needs to be placed into the heart to help stimulate it. With severe neurologic signs, anti-seizure medications may also be necessary—depending on what the preliminary tests indicate.
Thankfully, fainting is relatively rare in dogs. That said, if you notice any signs, attempt to get a heart rate from your dog’s chest cavity. This will help your veterinarian determine what the underlying cause is. Also, seek immediate veterinary attention, as untreated fainting can be potentially life threatening.
If you have any questions or concerns, you should always visit or call your veterinarian -- they are your best resource to ensure the health and well-being of your pets.