To understand why fainting occurs, one must appreciate how the cardiovascular system works. The heart is a muscular pump that receives blood from the veins and pushes it out into the arteries, which are the blood vessels that carry the blood to all the other organs. Inside the heart are valves, fibrous door-like structures that open and close at the right time to keep the blood moving in the proper direction. On the outside of the heart is the “plumbing” system of the heart, referred to as the coronary arteries, which are the blood vessels that bring oxygen and nutrients to the heart muscle itself.
The heart also has an electrical system, which controls the beating, or the rhythm, of the heart. Specialized cells in the heart generate tiny electrical signals that control how fast the heart beats. The heart is able to speed up and slow down, depending on the body’s needs. When a person exercises, or if they are under stress, the heart beats faster to pump more blood. It can also squeeze more vigorously as needed.
Although the heart responds to your body’s needs, the heart takes its instructions from the brain. The brain determines how the heart behaves, and together they form a very efficient team. The lowest part of the brain, called the brainstem, controls all of our internal functions, like breathing, body temperature, sweating, bowel and bladder function, heart rate and blood pressure. The complex centers of the brain that regulate these inner workings are part of the “Autonomic Nervous System,” from the Latin term meaning “self-governing.” The Autonomic Nervous System receives information from nerve endings all over our body and responds by sending out instructions over other nerves. For example, the brain can contrict or relax blood vessels in response to your body’s needs and thus control where the blood goes and how high the blood pressure should be. All of this happens without any conscious effort on our part.
Under certain circumstances, a decrease in the amount of blood returning to the heart via the veins causes a drop in blood pressure. The autonomic nervous system compensates by making the heart beat faster and harder. Sometimes the brain responds inappropriately, however, resulting in a further drop in blood pressure and slowing of the heart rate. This causes a type of fainting known as neurocardiogenic syncope, which is also called “vasovagal fainting.” People often feel hot or nauseated, usually look very pale, sometimes get sweaty, and will pass out if they do not lie down. If they do lose consciousness, they often feel exhaused when they wake up, and vasovagal syncope will sometimes result in vomiting afterwards. Certain situations can trigger this common cause of syncope, including physical or emotional stress, pain, dehydration, hot or crowded surroundings, prolonged standing, and certain medications. Neurocardiogenic syncope can occur at any age. Unfortunately, older patients often do not get much of a warning beforehand, and will sometimes lose consciousness suddenly, a situation often referred to as a "drop attack."
Another common cause of fainting is caused by changes in the heart rhythm (“cardiac arrhythmias”). Sudden slowing or pausing of the heartbeat can cause abrupt loss of consciousness without warning. On the other hand, an abnormally rapid heart rhythm disturbance (“tachycardia”) can cause the heart to beat so fast that it stops pumping efficiently, resulting in a precipitous drop in blood pressure. Cardiac causes of fainting are usually more dangerous and even be life-threatening. Anyone with a history of heart disease who faints should seek medical attention immediately.
Other causes of fainting include a condition known as “orthostatic hypotension,” in which the blood pressure falls dramatically when a person stands up from a lying or sitting position, especially after a meal. This can be a medication side effect, but it can also be due to problems with the autonomic nervous system, dehydration, or severe anemia. The drop in blood pressure can occur fairly quickly (within a couple of minutes) or it may be gradual in onset.
Less common causes of syncope include heart valve problems, severe vascular disease, tumors within the heart, and heart rhythm problems associated with genetic disorders. A person with a family history of syncope or sudden death who faints should be referred to a specialist for further evaluation. Fainting that occurs during or immediately after exercise needs to be investigated immediately, as this may be a sign of a potentially fatal heart problem.