Fainting

Classification and Causes

There are three main types of fainting: neurogenic, orthostatic, and cardiogenic. All three are associated with a temporary significant reduction in blood flow to the brain. Systolic blood pressure is decreased, typically to less than 60 mm Hg.

Neurogenic faints are of several different types. The most common of these is vasovagal fainting. Vasovagal fainting occurs mainly in young patients and people in good health. It is not suggestive of any underlying disease. There is also evidence of a familial predilection. The reason for the faint is a block of the normal sympathetic nervous system reaction to bodily stress. For instance, physical and emotional stresses normally cause an increase in sympathetic nervous system activity. This is reflected by an increase in heart rate, blood pressure, and the volume of blood pumped by the heart each minute (cardiac output). A variety of situations can trigger a vasovagal faint. These include bodily pain or injury (especially involving the internal organs), exercise, strong emotions, and situations that cause dilation of peripheral blood vessels (such as a hot, crowded room, particularly when a person is fatigued or has ingested alcohol).

Carotid sinus hypersensitivity is another type of neurogenic fainting. The carotid sinus is a dilated portion of the carotid artery, which is one of the major arteries supplying blood to the head. The sinus has nerve endings and senses changes in blood pressure. It sends feedback to the vasomotor center in the medulla—this is an area of the brainstem that controls blood pressure and heart rate. For patients with carotid sinus hypersensitivity, compression of the sinus by everyday activities such as turning one's head to the side, shaving, wearing a seatbelt, or tightening a necktie can cause the patient to faint. There are two abnormal responses that take place simultaneously: a vagal response and a vasodepressor response. The vagal response is a reflex action by the body which slows the heart rate (bradycardia) and causes a pause in the heartbeat (asystole) [Page 654]for greater than three seconds. The vasodepressor response is a decrease in the systolic blood pressure by more than 50 mm Hg without causing a significant change in heart rate. Carotid sinus hypersensitivity is most common in elderly men and patients with ischemic heart disease, hypertension, and head or neck tumors. Fainting typically occurs suddenly and while the patient is standing. The patient may have small convulsive movements during the episode. The faint rarely lasts longer than 30 seconds.

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