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Description of Driving Event:

During a swimming skills test at an underwater training facility, an experienced dive instructor lost consciousness during a free dive (with mask and fins). The dive instructor was positioning a weight at the bottom of the pool when he lost consciousness. He was observed by his co-workers and the trainee, who was sent to retrieve him to the surface. Once on the surface, the instructor¹s condition was assessed and cardio-pulmonary resuscitation (CPR) was started. CPR was ceased after five or six compressions when the instructors breathing and heartbeat began. The most likely cause of the incident was thought to be shallow water blackout, or more correctly, breath-hold blackout. Although this is an uncommon event, shallow water blackout potentially can cause serious injury or death in as little as six feet of water. One of the greatest concerns of breath holding during a free dive is unconsciousness during ascent. When the diver descends, the air in his lungs is compressed, raising the oxygen partial pressure. This partial pressure increase of oxygen (PPO2) satisfies the bodies oxygen demand during descent and while on the bottom. During ascent, the remaining partial pressure of oxygen is reduced rapidly as the hydrostatic pressure on the body lessens. If the partial pressure of oxygen drops below 11 percent (83.6 mmHg) unconsciousness may result. This danger is increased when hyperventilation has eliminated normal body warning signs of CO2 accumulation.