Fainting (also called syncope) is a sudden, momentary loss of consciousness with spontaneous recovery. This is a common sign in dogs with heart disease, but it is less commonly seen in cats. It can be difficult to distinguish between a true seizure and a fainting (syncopal) episode, although true seizures are less common in animals with significant heart disease. A very specific description of the circumstances of the collapse, the duration of the event, and the recovery time will help your veterinarian distinguish between a fainting episode and a seizure.
In animals with heart disease, a fainting episode may occur only once, or they can occur quite frequently. As fainting can be a sign of very serious heart disease, and since some causes of fainting can be associated with sudden death, it is important to perform diagnostic tests to search for the cause of collapsing episodes. When routine testing does not identify the cause for fainting, a 24-hour Holter ECG monitor or an event recorder may be used to record the activity of the heart to help determine the cause of fainting.