Fainting

Fainting results from inadequate energy substrate delivery to the brain. The major energy substrates are oxygen and glucose. Significant lowering of blood sugar tends to result in coma rather than fainting, and a prolonged cessation of oxygen delivery results in death. Thus transient loss of delivery of oxygen to the brain due to decreased blood flow to the brain is the final common pathway in most causes of fainting. Generalized lowering of oxygen in the blood from heart or lung disease or reduced oxygen-carrying capacity of the blood from anemia are risk factors for fainting, particularly in the elderly, but rarely the sole cause.

Blood pressure lowered enough to decrease blood flow to the brain may cause fainting, and the causes of fainting can be deduced from the blood pressure formula. Blood pressure is the product of cardiac output and total peripheral resistance of the arteries. Cardiac output is the product of stroke volume and heart rate per minute. Stroke volume is the amount of blood that the heart ejects in one beat. Total peripheral resistance is a measure of how constricted the arterial blood vessels are. Thus anything that will reduce the volume of blood that is ejected, impair heart rate, or cause dilatation of the arteries may lead to a fainting spell.

Elderly people are predisposed to fainting by the presence of multiple abnormalities, both age- and disease related. These conditions add up to threaten cerebral blood flow or reduce oxygen content in the blood. A situational stress that further reduces blood pressure, such as standing up from a lying position or straining at stools or voiding, may reduce cerebral oxygen delivery below the critical threshold and result in fainting. Several homeostatic mechanisms that normally preserve blood pressure and cerebral oxygen delivery in the face of stress become impaired with age. These mechanisms include cerebral blood flow autoregulation, blood pressure sensors in the carotid artery (baroreflexes), relaxation of the heart muscle, and sodium conservation by the kidneys.