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CO2 Retention in Scuba Diving
CO2
retention with it's attendant dangers
of death from convulsions and hypoxia (low oxygen level) is primarily
of
concern to the scuba diver due to "skip breathing". Other sources of
CO2
retention are breath-hold diving, breathing in a sealed environment,
faulty
regulator, exercise at extreme depth and using contaminated air.
Symptoms
include rapid respiration in
4-6%, rapid pulse rate, shortness of breath in 7-10% and convulsions
and
unconsciousness in 11-20%.
The CO2
level in the blood is unchanged
by the ambient pressure (i.e., the depth) per se, since the partial
pressure
of carbon dioxide in a scuba diver's blood is a function only of
metabolism
and the rate and depth of breathing - the same factors that
determine
blood CO2 concentration on land.
All of
the CO2 that's developed during
breathing underwater is exhaled in the bubbles from scuba apparatus and
does not increase with depth as do other gases, such as nitrogen,
oxygen,
CO and hydrocarbons. Abnormal carbon dioxide accumulation in the blood
can occur from too high a level of metabolism (such as from exercise at
depth) and/or inadequate breathing (usually not breathing deep enough
or
skip breathing). The medical term for high carbon dioxide in the blood
is hypercapnia; when the level is high enough it can cause "CO2
toxicity,"
which can lead to shortness of breath, headache, confusion and drowning
(depending on severity).
Elevated
CO2 levels play a significant
role in oxygen toxicity and in nitrogen narcosis.
The
acceptable CO2 level for diving
operations is 1.5% surface equivalent (10.5 mmHg); the acceptable level
for hyperbaric chamber operations is one that allows a vent schedule of
4scfm/person displacement.
With the
increased usage of closed circuit
scuba diving, mainly by the military-but recently by more and more
civilian
divers, there is the possibility of hypercarbia (high CO2 levels),
among
other medical considerations.
Signs
and symptoms that need to be observed
are hyperventilation, shortness of breath and tachycardia (rapid heart
beat), headache and excessive sweating, mental impairment and finally ,
unconsciousness.
This
hypercarbia comes about due to
malfunction of the CO2 absorbent canisters and can be avoided by
decreasing
the exercise rate, watch out for the operating limits of the canister,
checking for leaks at the start of the dive and not reusing the
absorbent.
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Ernest Campbell, MD, FACS All Rights Reserved. |
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