Why You Feel Faint

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AS AN EAGER journalist many years ago, I arranged to interview the medical examiner of New York City and thought it a great privilege to get inside the morgue and see his operation first-hand. I had never felt squeamish dissecting frogs or cleaning up dog poop. But just a few minutes into my interview, as the ME was opening and closing those long stainless steel drawers and discussing the pasty corpses inside, my vision began to blur and sweat rolled down my face. I felt nauseous and hurried from the room.

Such physical responses can precede loss of consciousness in what’s called “vagal nerve fainting” or “vasovagal syncope” — syncope meaning loss of consciousness — the main causes of fainting (besides that caused by seizures, vertigo, hypoglycemia and serious illnesses). The vagal nerve, which runs from the brain stem towards the chest and abdomen, communicates between the brain and organs including the heart and stomach, with its main function to direct blood toward the digestive system after eating. It also directs the hiccup reflex. The vagus nerve is responsible for involuntary activities that regulate the body’s organs: activating the vagus nerve via slow, deep breathing is known as the “relaxation response.” (See MyLittleBird story “Just Breathe.”)

But overstimulation of the vagal nerve can decrease the heart rate and/or the heart’s output, both of which make it difficult for sufficient blood and oxygen to reach the brain. The result can be loss of consciousness and fainting. This overstimulation occurs with an array of triggers, among them heightened emotions, including anxiety, fear, pain, hunger or squeamishness, for example, from seeing blood or those pasty corpses. (Alternatively, anxiety and panic can also cause hyperventilation, breathing in too much oxygen and breathing out too much carbon dioxide too quickly, which can also cause fainting.) Fainting due to vagal nerve overstimulation can also occur with vomiting or straining to have a bowel movement.

An overactive vagal nerve can result from heat exposure, when extra blood goes to peripheral vessels to cool the skin, leaving less blood going to the heart and brain — as well as from anything that causes blood to pool in the legs, such as after running and other strenuous exercise or prolonged standing. As for the idea that locking your knees for too long can make you faint, medical experts say that’s a myth because prolonged standing would be the main villain – though, in people for whom knee-locking increases blood pooling, it becomes a related cause.

The other main cause of fainting is orthostatic or postural hypotension from standing up too quickly, for those people whose bodies have difficulty adequately regulating blood pressure. Standing up causes a downward shift of about two cups (500 mL) of blood to the abdomen and legs, which results in less blood returning in veins to the heart and a consequent lowering of blood pressure. These effects of standing are normally countered by the body, but when the response doesn’t happen fast enough, the amount of blood going to the brain and thus blood pressure in the brain fall below a critical threshold. Symptoms accompanying this loss of consciousness can include: lightheadedness, tremulousness, “visual graying” and “coat-hanger” neck (caused by reduced oxygen to the shoulder and neck).

Countermeasures include standing slowly and contracting the calf muscles when standing to spur movement of blood away from the legs. Fainting that occurs on getting out of bed can be countered by tilting your bed, if it’s fancy enough, to keep your head elevated. The rest of us should add pillows.

Fainting is only harmful when it causes a fall that results in injuries. Falling into a supine position restores effective blood flow to the brain, although if the head remains elevated, loss of consciousness can be prolonged. For anyone prone to fainting, drinking plenty of water can help maintain a high blood volume — and appears to bestow an array of benefits for everyone.

–Mary Carpenter