Syncope is the medical term for a fainting spell. The most common type of syncope is known as vasovagal syncope or a vasovagal response. These fainting episodes can be experienced by healthy individuals, tend to be recurrent, and often take place during periods of emotional stress.
Susceptible individuals may be especially prone to these fainting episodes if any of the following conditions are present: mild blood loss; anemia; fever; hypoglycemia; or injury. At times the person who experiences syncope has undergone prolonged bed rest. This is the mechanism that is at work when people faint at the sight of needles or blood. The person who experienced a fainting spell only when donating blood, however, is not likely to have a problem during a dive.
The vagus nerve is the cranial nerve that controls the heart rate by slowing it. This results in a decrease in cardiac output and blood pressure. The brain, being the organ most sensitive to a decrease in oxygen, reacts to the situation with a fainting spell. The faint causes the person to assume the supine position (lying down) and restores normal blood flow to the brain.
Carotid sinus syncope is a related phenomenon: this mechanism also works through cranial nerves (the glossopharyngeal and vagus nerves). Within the carotid arteries (large arteries in the neck) chemical and pressure receptors provide feedback on blood flow and blood pressure. When appropriate, the feedback results in a decrease in the heart rate and blood pressure. Some people have exquisitely sensitive carotid sinuses and may faint when pressure is applied directly to that area of the neck. Medical students are taught very early on not to check both carotid pulses at the same time, especially in the elderly, who may have atherosclerosis in these arteries.
For the rare person who faints with slight pressure to the carotid arteries, various terms are used to describe the phenomenon (e.g., hairbrush syncope, shaving syncope). These individuals may faint when simply turning their head to the side or wearing a tight collar. For those individuals who are more sensitive to pressure on the carotid, DAN advises caution before deciding whether to dive.
Any condition in which a person may suddenly and unpredictably lose consciousness should be considered disqualifying for safe diving. And for the rare individuals with such sensitive carotid bodies that they experience a syncopal episode with the slightest pressure to the carotid artery, they should be thoroughly evaluated by a physician and likely discouraged from diving.