Fainting

Multiple studies have shown that 20 to 30 percent of fainting episodes have cardiac causes, 10 to 20 percent have other causes, and 30 to 50 percent remain unexplained in spite of extensive evaluation.

Several cardiac diseases may cause fainting, such as those that cause mechanical obstruction to blood flow such as narrowing of heart valves or abnormal thickening of the heart muscle, called cardiomyopathy. Blood clots from veins of the body, most often the legs, may also present with fainting due to obstruction of blood flow through the lungs and heart. Both slow heart rate (less than 40 beats per minute) and rapid heart rate (more than 140 beats per minute) may cause fainting. The rapid heart rate causes fainting by not giving the heart enough time to fill with blood before ejection into the circulation. The causes of these heart rate abnormalities include heart attacks, disease of the conduction system of the heart, and disease—most often seen in the elderly—of the sinus node of the heart, where the origin of the heart beat occurs normally.

Orthostatic hypotension is a term for blood pressure that drops more than 20 mm of mercury on position change, such as standing up from a lying or sitting position. This may cause dizziness when in mild stages but frequently causes elderly people to faint. It is either due to volume depletion or inability to constrict blood vessels under this type of stress. Volume depletion may be caused by fluid loss or blood loss or by too little fluid intake, or by medications such as diuretics, which increase excretion of fluid through the kidneys. Dilatation of the blood vessels may be caused by prolonged inactivity, such as bed rest, or by medications such as blood pressure– lowering medications and the older types of medications prescribed for depression. By only giving the lowest effective dose of medications to the elderly person, these types of adverse effects may be avoided or corrected. Disturbed autonomic function may cause dilatation of the blood vessels and cause fainting. Examples of these are central nervous system diseases—such as Multiple System Atrophy, Parkinson's disease, or Dementia with Lewy Bodies—and peripheral autonomic neuropathies caused by diseases such as diabetes mellitus or amyloidosis. Digestion of food may lead to hypotension and fainting in the elderly person due to inability to compensate for blood pooling in the gut during digestion, so-called postprandial hypotension. Finally, reflexes may cause dilatation of blood vessels, triggered by straining at stools, urination, swallowing, or coughing or by hypersensitivity of the carotid sinus, which is located in the neck.