Apnoeic diving safety - experimental approaches to oxygen augmentation

Oxygen augmentation during ascent for distressed apnoeic divers has recently been both advocated and criticized. Experiments involving 111 apnoeic divers have been conducted, shedding light on diving safety when oxygen augmentation is used at the onset of the apnoeic black-out, and are summarized in this report. It is maintained that black-out sets in gradually during ascent, taking the form of progressive tunnel vision, leaving scope for rational action by a distressed diver before complete loss of consciousness. By triggering a device (e.g. a 50 - 100 ml syringe) loaded with oxygen by means of pressure or otherwise at the onset of symptoms of hypoxic black-out during a 10 m dive, unconsciousness is believed to be delayed. This is because it takes a diver 12 seconds to rise to the surface from 10 m, while the lung-brain circulation time, inferred from textbook lung-ear circulation times for oxyhaemoglobin, is approximately 5.2 seconds. Since the oxygen is injected into the trachea by force and because of simulated involuntary ventilatory activity which dynamically mixes mouth and alveolar gas during apnoeic diving stress, the injected oxygen is believed to be quickly delivered at the alveoli. Such gas injection was shown to raise the amount of oxygen available to the diver by 25 - 50%. Intrathoracic pressure increases could ensure delivery of enough oxygen to the capillaries to delay unconsciousness. Recommendations for using the syringe at present apply only to 10 m dives and do not exclude use of other safety devices currently in use by divers or envisaged for their safety.