Uptake of Inert Gas
Breathing air at high ambient pressure may result in two "toxic"
effects as a result of the nitrogen. The first is a dose-dependent decrement in
cerebral performance known as nitrogen narcosis, which is due to the anesthetic properties
of nitrogen. This complication often becomes apparent to an outside tender trying
to communicate with a nurse inside the chamber, for whom actions requiring fine motor
control (e.g., starting an intravenous line) and rapid decision making may be more
difficult. This toxic effect is particularly evident at 6 ATA, although most therapeutic
recompressions are limited to 3 ATA, for which the narcotic effects are usually tolerable.
Narcosis may be avoided entirely by using a helium-O2
(heliox) mixture
as the chamber atmosphere rather than nitrogen-O2
. However, such treatment
is expensive and engenders communication difficulties because the speech of people
inside the chamber becomes relatively unintelligible in a helium atmosphere ("Donald
Duck voice"). Heliox recompression is recommended by some practitioners for saturation
treatment of divers with decompression sickness during or after a helium-breathing
dive. Despite anecdotal evidence that helium-O2
might be more efficacious
than nitrogen-O2
mixtures for the treatment of nitrogen-induced decompression
sickness,[164]
[165]
this contention has yet to be supported by definitive evidence.[13]
The second major adverse effect of nitrogen is that it is taken
up by body tissues and must therefore be released during and after decompression.
Overly rapid decompression may result in the formation of bubbles within tissues
and blood and lead to decompression sickness manifested as joint pains or neurologic
symptoms referable to the spinal cord. Neither this nor nitrogen narcosis is a problem
for the patient, who breathes O2
throughout most of the therapeutic hyperbaric
exposure. However, the tender in a multiplace chamber will breathe compressed air
in the atmosphere. Therefore, decompression schedules must be designed to minimize
the risk of decompression sickness in tenders. Most hyperbaric facilities use U.S.
Navy compressed air decompression tables.[144]
Additional safety for the tender can be provided by requiring the tender to breathe
100% O2
for a period
TABLE 70-7 -- Duke modifications of the U.S. Navy air tables for decompression of hyperbaric
tenders
|
Standard air decompression |
|
Within 2 ft of the standard Navy depth divisions,
use the next greatest depth |
|
Within 2 min of the standard Navy time divisions,
use the next greatest time |
|
During decompression of the chamber after a compressed
air dive, breathe 100% O2
from an equivalent depth of 50 fsw (2.5 ATA)
to the surface or for 15 min, whichever is shorter |
|
Time for decompression |
|
68–30 fsw (3.06–1.9 ATA): 3 min |
|
30–20 fsw (1.9–1.6 ATA): 1 min |
|
20–10 fsw (1.6–1.3 ATA): 1 min |
|
10 fsw (or last stop)—"surface" (1.3–1.0
ATA): 5 min |
|
Table 6 or 6A |
|
When Table 6 or 6A is extended, the tender should
breathe 100% O2
for the last 30–90 min at 30 fsw (1.9 ATA) according
to the number of extensions at 60 fsw (2.8 ATA): 30 minutes (no extensions), 60
minutes (1 extension), 90 minutes (2 extensions). One hundred percent O2
is breathed by the tender for the entire period of decompression from 30 fsw (1.9
ATA) to "surface" (1 ATA) |
|
ATA, atmospheres absolute; fsw, feet of sea water. These procedures
do not pertain to patients breathing oxygen. |
immediately before and during decompression ( Table
70-7
). O2
breathing results in increased nitrogen elimination
because of the resultant rapid lowering of the tissue partial pressure of nitrogen.
Rare episodes of decompression sickness in hyperbaric tenders are usually of a minor
nature and generally consist of mild joint pain.
