Report on Decompression Illness, Diving Fatalities and Project Dive

Section 6 - Breath-Holding IncidentsBehavioral Errors: The clearest example of behavioral errors duringbreath-hold diving is likely the presence of mind altering drugs.Alcohol ingestion preceded at least two of the fatal cases reported.The true impact of the alcohol cannot be determined, but it is possiblethat it contributed to decisions that ultimately contributed to theaccidents.A more elusive behavioral error involves the use of excessive hyperventilationprior to breath-hold. Hyperventilation can dramaticallydecrease the partial pressure of carbon dioxide in the blood. Sincecarbon dioxide is the primary trigger for inspiratory drive, the urgeto breathe is delayed until normal blood levels are restored. Thistranslates into longer breath-hold times. Hyperventilation, however,increases the body’s oxygen stores only slightly. The longerbreath-hold times will result in the oxygen partial pressure in theblood falling below normal (hypoxia). Problematically, hypoxia providesa surprisingly weak trigger for the inspiratory drive. A diverwho hyperventilates too much may fall unconscious due to hypoxiawithout ever being aware of an urge to breathe. This is known ashypoxic loss of consciousness (HLOC).A further complication of breath-hold diving to depth is that theincreased pressure with depth compresses the gas in the lung andeffectively increases the partial pressure of oxygen in the blood. Theproblem here is the reversal of the effect as the diver surfaces. Theoxygen partial pressure is reduced much faster than by consumptionalone as the ambient pressure is decreased. Unconsciousnesscan develop rapidly in the final part of the ascent, when the relativerate of pressure decrease is greatest. This phenomenon is referred toas shallow water blackout.Breath-hold divers who choose to use hyperventilation normallyexperiment to try and predict their personal safe limits. However, ifused too aggressively, even small increases in dive depth, exertionor breath-hold duration can produce a very hazardous situation.Excessive hyperventilation contributing to HLOC or shallow waterblackout may represent the underlying cause of death in many ofthe breath-hold fatalities with no other apparent explanation. This isvery difficult to document in most cases, but it is a risk that canclearly be reduced by conscious choice.The final behavioral error to be mentioned is the failure to ditch aweight belt when appropriate. There are several cases of fatality victimsbeing retrieved with weight belts still attached. It is difficult inmost to know if hypoxia impaired the ability to respond. At leastone case, though, highlights a lack of thought during a stressfulevent. One of the non-fatal cases involved an exhausted breath-holdDAN’s <strong>Report</strong> on Decompression Illness, Diving Fatalities and Project Dive Exploration: 2005 Edition 97