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205 Posts
Hello,
You may recall that one (of many) debates DL have had with others (including myself) has been over the issue of an appropriate end point for CO2 scrubber breakthrough testing. To summarise, DL have advocated a breakthrough threshold of 2% CO2 whilst others, including myself, have advocated 0.5%. My own recommendation of 0.5% has been based on extensive revision of the relevant diving physiology literature, the synthesis of which can be found here:
Doolette DJ, Mitchell SJ. Hyperbaric Conditions. Comprehensive Physiology 2011;1:163-201.
DL have argued their position largely on the basis of one paper containing no original data which was only ever designed as a "concept" document to provoke discussion. That can be found here:
Knafelc ME. Physiological basis for CO2 limits within semiclosed and closed circuit underwater breathing apparatus, NEDU TR4-00, Navy Experimental Diving Unit, Aug 2000.
The reason for posting today is that a relevant experiment has just been published from the NEDU group in the USA. It can be found here:
Shykoff BE, Warkander DE. Exercise and CO2 retention with inhaled CO2 and breathing resistance. Undersea and Hyperbaric Medicine 2012;39(4):815-828.
It is a complex study, but fundamentally demonstrates that working divers are prone to dangerous CO2 retention if the inspired CO2 is 2%. I quote from the conclusions:
"Other authors have suggested that divers may be able to compensate for 2% inspired CO2, at least in a low resistance circuit. With resistance like that of a rebreather UBA, particularly during oxygen breathing with heavy exercise, we disagree. In this study at heavy exercise, even without added resistance some air breathing subjects retained CO2, as did many of those who breathed oxygen. With 2% inspired CO2 and and resistance like that of the MK 16 UBA at 50 feet of seawater, ventilation was reduced and CO2 retention severe in some subjects. Further, subject recognition of hypercapnia was poor. For diver safety with the Mk 16 and other UBAs with both inspiratory and expiratory resistance, inspired CO2 must be maintained as close to 0% as possible".
My position has always been that DL have argued their beliefs from a position of poor understanding of both physiology and the relevant literature, and completely inappropriate reliance on a single discussion document. This latest publication from another authority on this subject, which presents arguably the most directly relevant original data published to date, strongly corroborates that view. Scrubber duration limits published by DL should be interpreted with particular attention to the CO2 breakthrough end point of the testing.
Simon M
You may recall that one (of many) debates DL have had with others (including myself) has been over the issue of an appropriate end point for CO2 scrubber breakthrough testing. To summarise, DL have advocated a breakthrough threshold of 2% CO2 whilst others, including myself, have advocated 0.5%. My own recommendation of 0.5% has been based on extensive revision of the relevant diving physiology literature, the synthesis of which can be found here:
Doolette DJ, Mitchell SJ. Hyperbaric Conditions. Comprehensive Physiology 2011;1:163-201.
DL have argued their position largely on the basis of one paper containing no original data which was only ever designed as a "concept" document to provoke discussion. That can be found here:
Knafelc ME. Physiological basis for CO2 limits within semiclosed and closed circuit underwater breathing apparatus, NEDU TR4-00, Navy Experimental Diving Unit, Aug 2000.
The reason for posting today is that a relevant experiment has just been published from the NEDU group in the USA. It can be found here:
Shykoff BE, Warkander DE. Exercise and CO2 retention with inhaled CO2 and breathing resistance. Undersea and Hyperbaric Medicine 2012;39(4):815-828.
It is a complex study, but fundamentally demonstrates that working divers are prone to dangerous CO2 retention if the inspired CO2 is 2%. I quote from the conclusions:
"Other authors have suggested that divers may be able to compensate for 2% inspired CO2, at least in a low resistance circuit. With resistance like that of a rebreather UBA, particularly during oxygen breathing with heavy exercise, we disagree. In this study at heavy exercise, even without added resistance some air breathing subjects retained CO2, as did many of those who breathed oxygen. With 2% inspired CO2 and and resistance like that of the MK 16 UBA at 50 feet of seawater, ventilation was reduced and CO2 retention severe in some subjects. Further, subject recognition of hypercapnia was poor. For diver safety with the Mk 16 and other UBAs with both inspiratory and expiratory resistance, inspired CO2 must be maintained as close to 0% as possible".
My position has always been that DL have argued their beliefs from a position of poor understanding of both physiology and the relevant literature, and completely inappropriate reliance on a single discussion document. This latest publication from another authority on this subject, which presents arguably the most directly relevant original data published to date, strongly corroborates that view. Scrubber duration limits published by DL should be interpreted with particular attention to the CO2 breakthrough end point of the testing.
Simon M
