From Divernet:
http://www.divernet.com/news/items/cold060601.htm
DATELINE: 6th June 2001
SUFFOCATION CLAIM REJECTED
A prominent British diving doctor has rejected the dramatic claims of US researchers that sport divers are risking suffocation by throat spasm when diving in colder waters.
Dr Chris Edge, who sits on the UK Sport Diving Medical Committee, has described as "bollocks" the researchers' claim that, due to the refrigeration effect on gas reduced in pressure by a regulator, a diver's airway can become blocked by a condition quoted as laryngospasm, brought on by the sub-zero breathing gas.
The US study has claimed that the risk of respiratory shock and suffocation can occur when diving in waters of 4.5°C or less - and that this could be the answer to some unexplained coldwater diving deaths.
Suddenly rendered unable to breathe, it says, a diver would appear to buddies to "panic" and head for the surface, resulting in fatal embolism injuries - even from shallow depths - because a blocked airway would prevent exhalation during ascent.
Even among those who survive such an incident, says the report, the sudden inability to breathe could well be mistaken for a regulator/ gas flow malfunction, such that the problem has gone "unnoticed and unreported in open-circuit scuba diving for many years".
The study was conducted by the Dive Lab of Panama City on behalf of US manufacturer Kirby Morgan Dive Systems of Santa Barbara, which has developed a heating system for scuba breathing gases, previously limited to the world of commercial diving.
The study was based on a series of tests on rigs immersed in water at different temperatures.
"It is not known at what temperature respiratory shock occurs in divers and it undoubtedly varies from diver to diver," said the authors of the report.
"We believe that -23.3°C on any gas including air, but especially helium mix [due to helium's greater heat transfer characteristics], is cold enough to cause a serious danger to any diver."
In the tests, such a gas temperature was recorded at the regulator first stage when a cylinder filled to 206.8 bar was immersed in water at 4.5°C, representing conditions often experienced by divers around the world.
Chris Edge described the study's claims as groundless because of the lack of clinical trials and "no indication that they have any anatomical, physiological or medical knowledge".
"These people haven't a clue what they're talking about," he told Divernet. "First of all, common sense would tell you that, if their claims are correct, people would be keeling over from just walking about in places like Alaska, Siberia or Antarctica, where air temperatures go lower than -23°C.
"They also show a serious ignorance of physiology by stating that their supposed respiratory shock is caused by laryngospasm in which, quote, 'the diver's airway is completely blocked by the epiglottis sealing the trachea closed'. Laryngospasm is in fact a movement of the vocal cords across the larynx."
Even the researchers' scuba rig test results were meaningless, said Dr Edge.
"They took the temperature of gas at the first stage, but what matters is temperature at the second stage. Flowing through the linking hose, gas starts to calibrate with the surrounding water temperature. It is affected further by water temperature when it enters the highly conductive, metal-cased second stage.
"And then there's the warming effect of your own mouth. Gas hits the back of your throat warmer than when it entered."
Dr Edge advises divers who read the research report, Open Circuit Compressed Air Scuba Gas/Air Temperatures May Be Dangerous To Divers In Cold Water by Pete Ryan, Bev Morgan & Trent Schultz of Kirby Morgan and Mike Ward of the Dive Lab, to ignore its content.
Ralph
