Dear Diana,
The medical term for "black-outs" is syncope (or near syncope if one dosen't actually pass out). This is a common but complex condition that has many causes. The most common cause is the common faint (neurocardiogenic or vasovagal syncope). This is the typical faint caused by strong emotional factors (i.e. the sight of blood) and is usually brief in duration. The person almost never harms themselves and the precipitating factor can usually be identified. More serious forms of syncope are due to cardiac and neurologic causes.
Syncope due to bradyarrhythmias (slow heart rate) or tachyarrhythmias (fast heart rates) are often hard to document. Holter monitors will only reveal the source if they are being worn during an event. "Event monitors" are devices that can be worn for months at a time and when an event occurs a button is pressed that saves the heart rhythm for the last 5 minutes. This can then be sent to the doctor over the telephone for a diagnosis. Other less common cardiac causes are carotid sinus irritability which is due to an abnormal structure in the neck that results in syncope when pressed upon.
Neurologic forms of syncope include autonomic nervous system diseases and seizure disorders. These are diagnosed with tilt table testing and seizures with an EEG. Tilt table testing is a simple test that is pretty much what it sounds like. The patient is placed on a table and ECG and blood pressure monitoring are attached. The table is then tilted upright so the person is in a vertical position and the heart rate and blood pressure are monitored. People with certain types of syncope are more likely to have symptoms during this test. The entire test lasts about a half hour.
Medications can sometimes be used for treatment of the "common faint". A beta-blocker is the most typical medication used. Other treatments that may be recommended include liberalizing salt intake, compression stockings and elevating the head of the bed with blocks 6 inches. Finally, slowing rising and "bouncing" on one's toes when a faint feeling comes helps return blood circulation to the head.