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Chapter 22 — 3
Causes of CO
2
Toxicity
!
Rebreathing equipment.
Some types of military and technical diving equipment conserve gas and reduce exhaust
bubble formation by allowing the diver to rebreathe his exhaled gas (exhaust bubbles can be
detected by the enemy!). A canister of CO
2
absorbent (soda lime) is included in the circuit to
remove the CO
2
which the diver exhales (see Chapters 5 & 43).
This mechanism can fail due to exhaustion of absorbent material, extended dive duration, salt
water contamination, improper packing, excessive CO
2
production due to exertion, or
improper assembly of the equipment.
!
Diving helmet problems.
With a standard-dress helmet or with some helmets used in deep diving, the diver can partly
rebreathe his exhaled gas if the fresh gas flow in the helmet is insufficient to flush out
exhaled CO
2
.
!
Chambers and habitats.
CO
2
which is exhaled by chamber occupants must be removed by constant flushing of the
chamber with fresh air or by the recirculation of the chamber gas through a CO
2
absorbent
(scrubber). If either of these mechanisms is inadequate, the occupants can develop CO
2
toxicity by rebreathing their own exhaled CO
2
.
!
Scuba.
Since rebreathing is not possible with scuba equipment, CO
2
toxicity is not generally a
problem for scuba divers unless there is excessive resistance to breathing (regulator
resistance, increased gas density at depth) or a reduced respiratory response of the diver to
CO
2
(possibly due to voluntary control or “skip breathing”, adaptation, nitrogen narcosis, or
high oxygen levels).
Clinical Features
These depend on the rate of onset and the actual partial pressure of the inspired CO
2
.
A rapid accumulation of CO
2
may cause unconsciousness before any symptoms are
experienced.
A slower build-up causes a variety of symptoms, including :
• shortness of breath
, or air hunger.
• flushing of the face
and
sweating
(sweating is not easy to detect underwater).
• repetitive activity
, such as swimming, without awareness of this.
• light headedness
, muscular
twitching
, jerks, tremors or
convulsions
.
• impaired vision
.
•
unconsciousness
.
• a splitting or throbbing headache
, usually at the front of the head. This may be
severe and start after the CO
2
levels have been corrected. It often lasts for hours.
• death
.