Page 46 - AlertDiver_Winter2013

O xygen toxicity has been known since the 19th
century, but we are still learning about its
causes and mechanisms. In diving practices
of years past, pure oxygen and gas mixtures
with oxygen content different than that of air were used
exclusively by military and commercial divers. In the
last several decades enriched air nitrox (EAN) became a
part of mainstream recreational diving, while technical
diving, which maximizes the use of oxygen to minimize
decompression risks, is becoming increasingly popular.
The accumulated experience in recreational and technical
diving now exceeds the previous experience of military
and commercial diving. Both scientific research and diving
practices advanced our knowledge of oxygen toxicity, and
from time to time we should take a step back to make sure
we are all sharing the benefits of this knowledge.
Oxygen convulsions were first described by Paul Bert in
1878. He noted convulsions in larks in air compressed to
a pressure of 20 atmospheres absolute (ATA). When he
compressed larks in pure oxygen, they convulsed at pressures
as low as 5 ATA. Bert concluded that convulsions represent
a sign of oxygen toxicity upon the central nervous system
(CNS), which was shown to be correct in later research.
In 1899 Lorrain Smith described pulmonary oxygen toxicity
in rats. Problems started when rats breathed 45 percent
oxygen at normal pressure. When breathing 73 percent
oxygen, rats developed fatal pneumonia in four days. At
higher oxygen pressure, these symptoms developed sooner.
Since that time we have learned more about the
manifestations and some of the underlying mechanisms of
oxygen toxicity. All manifestations of oxygen toxicity are
dose dependent. Symptoms of CNS oxygen toxicity, which
include seizures, may occur after short exposures to partial
pressures of oxygen greater than 1.3 ATA in exercising
divers, which equates to breathing pure oxygen at 10 feet of
seawater. Resting divers in comfortable conditions tolerate
1.6 ATA of oxygen well. Pulmonary manifestations occur
after days of exposure to partial pressures of inspired oxygen
greater than 0.45 ATA. In addition, repetitive and prolonged
use of hyperoxic gases (at least 30 hours in 10 days of an
oxygen partial pressure greater than 1.3 ATA) in modern
diving has revealed ocular manifestations (hyperoxia-
induced myopia). Cold, exercise, some drugs and increased
partial pressure of carbon dioxide increase susceptibility to
oxygen toxicity.
Who is Exposed to the Risk of Oxygen Toxicity?
Divers using EAN as well as both open-circuit and
rebreather divers who use mixed gases are exposed to the
risk of oxygen toxicity. CNS oxygen toxicity may occur in
divers using EAN if they exceed depth limits for a given
oxygen content or if they mistakenly breathe gas that
contains more oxygen than they thought it did. Cold,
strenuous exercise, some medications and other known
44 | WINTER 2013
Understanding
Oxygen Toxicity
RESEARCH, EDUCATION & MEDICINE // E X P E R T O P I N I O N S
B y P e t a r J . D e n o b l e , M . D . , D . S c .
Some divers use high-oxygen gas mixtures or even pure oxygen
for shallow-water decompression stops. Due to the risk of
seizure from central-nervous-system oxygen toxicity,
the maximum depth for use of pure oxygen should not exceed
20 feet of seawater (equivalent to 1.6 ATA of oxygen).
STEPHEN FRINK