heart and blood vessel disorders
Fainting
Causes of Fainting
A person cannot lose consciousness unless brain function is generally disturbed. This disturbance usually occurs because overall blood flow to the brain is reduced. Sometimes, however, blood flow is adequate but the blood does not contain sufficient oxygen or glucose (blood sugar), which the brain needs to function. Blood flow to the brain can be reduced in several ways. Most often, the cause is something that interferes with the normal return of blood to the heart (and thus reduces blood flow out from the heart). Less often, the cause is a disorder that interferes with blood pumping (typically a heart disorder). Although strokes reduce blood flow to the brain, they only reduce flow to part of the brain. Thus, strokes rarely cause fainting except for the few strokes that involve the part of the brain that maintains consciousness. Themost common causesof fainting are Strong emotion (such as fear, pain, or sight of blood) Coughing or straining to pass stool or urine Prolonged standing Standing up suddenly Pregnancy Use of certain drugs Idiopathic (meaning that the cause cannot be determined) These common causes nearly always cause fainting only when people are standing up. When they fall down, blood flow to the brain is increased, quickly restoring consciousness, although people may not feel completely normal for a few minutes to a few hours. Some people feel tired or exhausted for several hours. These causes tend not to be serious unless people are injured when they fall. Most of these causes involve decreased return of blood to the heart. Strong emotion (particularly that triggered by the sight of blood) or pain can activate the vagus nerve. Vagus nerve activation widens blood vessels, reducing the return of blood to the heart, and slows the heart rate. Both of these factors cause light-headedness and sometimes fainting (called vasovagal syncope or neurocardiogenic syncope). Straining during bowel movements or urination or coughing increases chest pressure. Increased chest pressure can activate the vagus nerve and also reduce the return of blood to the heart—two factors that may cause fainting. Healthy people may faint when standing still for a long time (most common in soldiers, a phenomenon called parade ground syncope), because the leg muscles have to be active to help return blood to the heart. Sitting or standing up too quickly can cause fainting, because the change in position causes blood to pool in the legs, resulting in a fall in blood pressure. Normally, the body quickly increases the heart rate and constricts blood vessels to maintain The Body's Control of Blood Pressure . If the body does not compensate in these ways, light-headedness is common and fainting may rarely occur. Certain brain and spinal cord disorders, prolonged bed rest, and certain drugs (particularly those used to treat high blood pressure) can interfere with this compensation and lead to Hormonal changes in early pregnancy sometimes lead to fainting. Low blood sugar ( Hypoglycemia ) initially causes confusion, light-headedness, shakiness, and other symptoms, but if hypoglycemia is severe or prolonged, people can lose consciousness. Because these other symptoms usually occur before fainting, people with hypoglycemia usually have some warning before they faint. Usually, the cause of hypoglycemia is use of drugs for diabetes, particularly Less common but more serious causesinclude Heart valve disorders (most commonly, the aortic valve) A heart rate that is too fast or too slow Blockage of an artery to the lungs by a blood clot (pulmonary embolism) Heart attack or other heart muscle disorders Heart valve disorders can block blood from leaving the heart. A very fast heart rate may not allow the heart enough time to refill with blood, so less blood is pumped. A very slow heart rate may not pump enough blood. Blood clots in the lungs can keep the heart from pumping enough blood. People with a heart attack rarely have fainting when the heart attack occurs (more common in older people). Other uncommon heart muscle disorders called cardiomyopathies can cause fainting, particularly during exercise, typically because of an abnormal heart rhythm. Although most strokes do not cause fainting, a stroke or transient ischemic attack (TIA) that involves certain blood vessels at the base of the brain (posterior circulation stroke) can cause fainting. Similarly, a migraine that involves these blood vessels sometimes causes fainting. 