Naval Submarine Medical Research Laboratory

■■■■■(.-.

NSMRL REPORT 1196 5 FEBRUARY 1995

HE-N2-O2:

ISOBARIC SHIFT AND SATURATION DECOMPRESSION

19950329 W

C. L. Shake

P. K. Weathersby

B. G. Coras

arid

J. W. Parker

Released by

S. F. BLACKE, CDR, MSC, USN

Acting Commanding Officer

Naval Submarine Medical Research Laboratory

Approved for public release; distribution unlimited
He-N2-02: Isobaric Shift and Saturation Decompression

C.L. Shake

P.K. Weathersby

B.G. Caras

and

J.W. Parker

Naval Submarine Medical Research Laboratory

Report 1196

Naval Medical Research and Development Command

Research Work Unit 63713N M0099.01A-5201

Approved and Released by

S. F. Blacke, CDR, MSC, USN

Acting Commanding Officer

NAVSUBMEDRSCHLAB

Accesion For

NTIS CRA&I

DTIC TAB

Unannounced

Justification

D

By.

Distribution/

Availability Codes

Dist

m

Avail and/or

Special

Approved for public release; distribution unlimited
Summary Page

Problem:

A disabled submarine (DISSUB) crew exposed to increased atmospheric pressures resulting

from flooding, rupture of air lines, or emergency breathing apparatus use may incur a decompression obligation and will require gradual decompression to avoid decompression sickness (DCS).

Decompression in a nitrogen-oxygen mixture is not always operationally feasible, and decompression from an air exposure in helium and oxygen could result in isobaric counter-diffusion

DCS. An additional risk is pulmonary oxygen toxicity, especially when survivors are exposed to

high partial pressures of oxygen from air at depth. These problems led to the concept of using a

mixture of air and helium (trimix) for decompression of a DISSUB crew.

Findings:

The trimix experiments were designed to simulate the exposure of men in a DISSUB saturated at 5 atmospheres on air, followed by rescue in a pressurized rescue vehicle, and transfer to

a surface platform for safe decompression. Trimix dives resulted in 4 cases of DCS in 62 mandives. Three DCS cases presented after surfacing: one Type II DCS (44.5 hr schedule) and two

Type I DCS (41.2 hr schedule). One case presented during decompression (Type I: 41.2 hr

schedule). Only four cases of minor skin itching without rash were noted following the isobaric

shift from air to trimix.

Applications:

Specific operationally relevant recommendations have been made and are also included in

NSMRL report 1178, Pressurized Submarine Rescue. A manual for Undersea Medical Officers.

Administrative Information

This work was completed under Naval Medical Research Development Command Research

Work Unit 63713N M0099.01A-5201, Submarine related decompression problems. The views

expressed in this report are those of the authors and do not reflect the official policy or position

of the Department of the Navy, Department of Defense, or the U.S. Government. This report

was approved for publication on 5 Feb 1995 and designated NSMRL Report 1196.

u
Abstract

A disabled submarine (DISSUB) crew exposed to increased atmospheric pressures resulting

from flooding or other problems may incur a decompression obligation. They will then require

not only gradual decompression to avoid decompression sickness (DCS), but decompression in

specific gases due to operational limitations and to lower the risk for pulmonary oxygen toxicity.

Nineteen dry chamber dives were conducted at the Naval Submarine Medical Research Laboratory (NSMRL) to explore the feasibility of an isobaric shift from air at five atmospheres

(ATA) to Helium-Nitrogen-Oxygen (50% He-40% N2-10% O2, TRIMIX) followed by decompression to the surface. These procedures parallel the potential operational scenario of returning

the crew of a pressurized, disabled submarine (DISSUB) to the surface. Saturation dives were

conducted by sequential experimental design, in which there was progressive shortening of the

hold period following isobaric shift and/or shortening of decompression time. Initial pressurization was to 111 feet ofseawater (fsw) (4.4 ATA) for 60 hours followed by compression to 132

fsw (5.0 ATA) on air for 12 hours, then the isobaric shift to TRIMIX was conducted. A hold of

24,12, 8, or 4 hours at 132 fsw (5.0 ATA) was initiated, followed by a rapid upward excursion

from 132 fsw (5.0 ATA) to 116 fsw and decompression to the surface with 57.5, 50.9, 44.5, or

41.2 hour schedules. The potential for decompression sickness (DCS) existed from both isobaric

counter diffusion and also from decompression to the surface. Four cases of DCS occurred in

62 man-dives. Three cases presented after surfacing (one Type II: 44.5 hour schedule; two

Type I: 41.2 hour schedule). One case presented during decompression (Type I: 41.2 hour schedule). Four cases of minor skin itching without rash were noted following the isobaric shift.

Overall, an isobaric shift of divers saturated on air at 5 ATA to trimix followed by decompression to the surface resulted in a small risk for DCS. Specific risk for isobaric counter-diffusion DCS for this switch appears to be negligible. Guidance to the fleet for DISSUB rescue

scenarios has been provided.

111
[Blank Page]

IV
He-N2-02: Isobaric Shift and Decompression

Introduction

Submarine safety has been of interest to

the U.S. Navy since the production and

launching of the first submarines. The risk of

having an accident which would disable a

submarine (DISSUB) is greater in the shallow

waters of the continental shelf area than in

much deeper waters. This is due to congested

submarine and ship traffic, frequent surfacing

and diving, and the fact that new submarines

undergo initial sea trials in these shallower

waters (Eckenhoff & Vann, 1985). If a submarine became disabled in shallow water,

chances for the safe rescue of DISSUB personnel would be higher than in deep water,

which might be deeper than the crush depth of

the submarine.

The Navy has technology to rescue DISSUB

crews. The Deep Submergence Rescue Vehicle

(DSRV), designed to operate deeper than U.S.

submarine hull crush depth, is capable of conducting rescues from a DISSUB at depths too

great for safe direct buoyant ascent escapes.

The DSRV was also designed to conduct rescues with DISSUB internal pressures up to 5

atmospheres absolute (ATA) (132 feet ofsea

water (few)) (5.0 ATA) (Eckenhoff & Vann,

1985).

During normal operations the internal air

pressure of a submarine is maintained at

approximately 1 ATA Several events, however, can cause DISSUB internal pressure to

rise well above 1 ATA: rupture of air lines,

flooding with ocean water which causes compression of the remaining air spaces; planned

pressurization of compartments to prevent or

slow further ocean flooding; or exhaust from

the Emergency Air Breathing (EAB) face

masks.

A DISSUB crew exposed to increased atmospheric pressure may incur a decompression

obligation. Specifically, the exposure of

DISSUB crew to increased atmospheric

pressure for some period of time results in the

uptake of inert nitrogen (N2) into the blood

stream and tissues. If an individual is exposed

for an extended period of time, the blood then

becomes "saturated" (N2 in the blood is equilibrated with the higher partial pressure of N2 in

the submarine). Once this happens, the crew

should be gradually returned to 1 ATA This decompression process allows the excess inert

gas to be eliminated while it is still in a dissolved state to avoid decompression sickness

(DCS) (Hallenbeck & Andersen, 1982).

In most cases a DISSUB crew will probably

be exposed to increased pressure for at least

48 hours while waiting for the arrival of rescue

crews (Eckenhoff & Vann, 1985). This delay

will result in saturation of the survivors with

higher pressure inert gas. Although safe decompression time is exposure dependent, numerous

hours of decompression can be anticipated

even if the DISSUB crew was exposed to

relatively low atmospheric pressures.

In a DISSUB scenario the survivors would

be transferred dry to the DSRV and then

transported onto the deck of a submarine rescue

ship (ASR-21 class) with saturation diving

chambers or to the forward compartment of a

mother submarine (MOSUB) (Figure 1).

The recovery and decompression problem is

complicated by the fact that ASRs and MOSUBs do not carry large supplies of N2. Normally, the best atmosphere for decompression

would be a mix of N2-02, but this is not feasible without a large source ofstored N2. ASRs

carry only large sources of air, oxygen (02),

and helium (He), while MOSUBs carry air

only. Shifting survivors from the air
(N2-02) atmosphere to Heliox (He-02), could

result in isobaric counter-diffusion phenomenon

which creates a risk for DCS-like symptoms

(D'Aoust & Lambertson, 1982). Briefly,

this process involves the movement of two

inert gases (He, N2) in opposing directions

through tissues without a change in pressure,

but can allow locally high total inert gas partial

pressure. This supersaturation can cause gas

phase formation and result in clinical DCS

symptoms such as bubbles in superficial tissues including skin and subcutaneous tissue

(Bove & Davis, 1990).

It is also likely that survivors breathing air

at up to 5 ATA for a prolonged period will

have increased risk of pulmonary oxygen

toxicity (Harabin, Homer, Weathersby, &

Flynn, 1987). It would, therefore, be advantageous to dilute the 02 in the breathing mixture

with an inert gas (N2 or He) as soon as possible.

These problems led to the concept of using

a mixture of air and He for decompression of

DISSUB crews. The best option for DISSUB

rescue on-board an ASR would be decompression on a trimix combination of He-N2-02.

This would: a) be operationally feasible,

b) decrease 02 partial pressure, and c) lessen

risk for DCS.

The TRIMIX experiments were designed

to simulate the exposure of men in a DISSUB

saturated at 5 ATA on air, followed by rescue

under pressure in the DSRV, and transfer to

an ASR for "safe" decompression.

The focus of this paper is to explore the

feasibility and safety of an isobaric shift from

saturation on air to TRIMIX followed by

decompression. Specifically, the TRIMDC

series used a sequential experimental design and

examined the following problems: 1) establish

the practical safety of an isobaric shift from

air at 5 ATA to He-N2-02; 2) establish a

minimum safe holding period on trimix breathing gas before decompression is initiated; and

3) establish a safe, yet expedient, trimix saturation

decompression schedule.

Methods and Materials

All dives were conducted in the "Genesis"

hyperbaric chamber at NSMRL in Groton,

ASR < ALTERNATE PLATFORM )

Figure 1
Connecticut. Depth was maintained within

+/- 0.5 fsw. Beckman 864 analyzers were

used to continuously monitor C02, which

was maintained at or below 0.5% surface

equivalent (SEV). Temperature and humidity

were adjusted to suit subject comfort.

Oxygen was continuously monitored by dual

Beckman 755 analyzers and maintained

within +/- 0.2% of prescribed values by two

Teledyne 323 controllers. Nitrogen and

helium concentrations were also monitored by

a mass spectrometer (Perkin Elmer 1100

Medical Gas Analyzer) and were maintained

within ± 0.5% by manual addition of the

appropriate inert gas. Diet was not limited or

monitored. The use of medications was minimized, and confined to topical preparations,

antacids, acetaminophen and rare use of

decongestants and non-steroidal anti-inflammatory drugs. Ancillary scientific studies

during some or all of the exposures included

spirometry, audiometry, doppler ultrasonic

monitoring, and breathing resistance/exercise

studies.

Subjects

All subjects for the TRIMIX series were

active duty or reserve Navy trained male

divers or submariners. Subjects' ages ranged

from 20 to 42 years (Mean = 31.5 years). At

least one Navy qualified diver was present

during each dive. None of the subjects had

hypo/hyperbaric exposures for at least 48

hours preceding their dive. Informed consent

was obtained from all subjects after thorough

briefs with the investigators. All subjects had

current physical examinations. Sixty-two

man-dives were conducted during nineteen

dry chamber dives. The design of the experiment allowed for multiple dives by a single

subject. No subject, however, was allowed to

repeat the exact same dive profile.

Dive Profiles

Trimix dives were conducted by sequential

experimental design in which there was a) progressive shortening of the hold period following the isobaric shift and b) shortening of

decompression time.

The rules for progression of the profile risk

specified that the hold time and, separately,

the decompression time would be shortened if

there were: no cases of DCS in at least 10

man-dives, no more than 1 case in 15 mandives, or no more than 2 cases in 20 man-dives.

A third case of DCS in any profile required a

return to more conservative procedures (Table

1). These rules were applied separately to

either DCS-like cases resulting from the isobaric shift and hold or DCS cases resulting

from decompression.

Table 1

Decompression Rate (fsw/Hr)

Depths (fsw) Trimix-1 Trimix-II Trimix-in

3.33

Trimix-rV

3.33

Trimix-V

116-100 3.00 3.00 3.33

100-50 2.50 2.50 3.00 3.00 3.33

50-40 2.33 2.33 2.66 2.66 3.00

40-30 2.00 2.33 2.33 2.33 2.66

30-20 1.66 2.00 2.00 2.00 2.33

20-10 1.33 1.66 2.00 2.00 2.00

10-0 1.05 1.66 2.00 2.00 2.00

Total Decom- 57.5 50.9 44.5 44.5 41.2

pression (Hrs)

Hold Time (Hrs) 24 12 8 4 4
The dive profiles consisted of: a) initial

pressurization to 111 fsw (4.4 ATA) on nitrox

(ppN2= 4 ATA, pp02= 0.4 ATA) for 60 hours

to provide a saturated N2 partial pressure

equivalent to that of 132 fsw (5.0 ATA) on air

while minimizing the potential for pulmonary

oxygen toxicity; b) pressurization to 132 fsw

(5.0 ATA) with a simultaneous shift to air

(ppN2= 4 ATA, pp02= 1.05 ATA), followed

by a 12 hour stay; c) an isobaric shift to

trimix (50% He, 40%, N2,10% 02) with subsequent holds for 24,12, 8, or 4 hours; and d)

an upward excursion to 116 fsw and decompression to the surface with 57.5, 50.9, 44.5,

or 41.2 hour schedules (Table 2) and (Figure

2). The 16 fsw upward excursion was set to

equal the oxygen partial pressure (0.50 ATA)

which is thought not to incur any risk of decompression sickness (Behnke, 1947).

Table 2

Trimix Dive Series - DCS Incidence

Hold Decompression #of DCS Itching

TRIMIX Hrs

24

Time Hrs Divers Cases Cases %DCS

I (Series) 57.5 19 0 0 0.0

II (Series) 12 50.9 11 0 0 0.0

III (Series) 08 44.5 12 1 4 8.3

IV (Series 04 44.5 04 0 0 0.0

V (Series) 04 41.2 16 3 0 18.8

TRIMIX SERIES PROFILE

132 FSWG TRIMIX

p02 = 0.53

TRIMIX (p02 28-30%)

(AIR DURING TRIMIX I SERIES)

TIME (HOURS)

Figure 2

TRIMIX I

TRIMIX H

TRIMIX BE

TRIMIX IE

TRIMIX 3E
Trimix I. The first 19 man-dives did not

use the sequential design rules. After the isobaric shift was conducted, the subjects

remained on trimix for 24 hours before

commencing decompression. Decompression

was initiated with a 16 fsw upward excursion.

A switch of chamber atmosphere to air occurred at 46 fsw for the remaining decompression time. The decompression schedule was

from previous nitrox schedules used during

the AIRSAT series (Eckenhoff & Vann,

1985). The 57.5 hour schedule was considered a conservative starting point for the

TRIMIX series (Table 1).

Trimix II. The sequential design was initiated in this series. The next 11 man-dives

used a 12 hour hold after the isobaric shift,

the same 16 fsw upward excursion, and a

shortened decompression schedule. This

schedule was 50.9 hours and was intermediate

between the TRIMIX I schedule and a linearized USN He-02 saturation decompression

rate (NAVSEA, 1987).

Trimix III. The hold period on trimix was

shortened to 8 hours in addition to shortening

of the decompression schedule to 44.5 hours.

Sixteen man-dives were completed on this

schedule.

Trimix IV. The hold period on trimix was

shortened to 4 hours while the decompression schedule was held at 44.5 hours. Four

man-dives were conducted in this phase.

Trimix V. Sixteen man-dives were completed with a hold following the shift of 4

hours. The decompression schedule was also

shortened to 41.2 hours. The schedule is

slightly faster than the linearized Navy He-02

saturation decompression rate (NAVSEA,

1987).

Decompression Monitoring: The potential

for DCS or DCS-like symptoms existed from

both the isobaric shift and the upward excursion with subsequent decompression to the

surface. The adequacy ofthe decompression

procedures was assessed by the presence or

absence ofspecific symptoms of DCS

(doppler ultrasonic recordings were not used

for diagnosis since they were being recorded

in a double-blind fashion for other research).

A qualified USN diving medical officer

(DMO) made the diagnosis in accordance

with conventional diagnostic categories as

outlined in the US Navy Diving Manual

(NAVSEA, 1988). Evaluation ofsubjects

was based on spontaneous reporting of symptoms, regular (twice daily) interviews with the

DMO, and thorough post-dive examinations.

All divers were examined within 2 hours of

surfacing, and again at 24 and 48 hours postdive.

The data generated from this dive series

will be included in a master data base at Naval Medical Research Institute (NMRI). This

information will allow mathematical models

to be developed to perform maximum likelihood analysis to assess DCS risk (Weathersby,

Survanshi, Nishi, & Thalmann, 1992).

Results

Sixty-two man dives were conducted between 1987 and 1992. The overall incidence

of DCS for the entire TRIMIX series was

6.5% (4 of 62), which was attributed to the actual decompression based on the time of DCS

symptom occurrence. The fastest decompression schedule resulted in a DCS risk of 18.8%

(3 of 16). One Type II DCS case and three

Type I DCS cases were diagnosed. Only four

cases of minor skin itching without rash were

noted following the isobaric shift (Table 2).

The first case of DCS involved a 28 year

old subject who completed TRIMIX HI (8,

44.5). The subject reported increasing right

knee pain (dull ache below patella) to the
DMO 14 hours post surfacing (at 2225 hr).

The subject stated that the pain began earlier

(1200 hr). This knee pain was soon followed

by lower back (L4/L5 level) and elbow pain.

The back pain was mostly a dull ache, poorly

localized and not affected by range of movement. The left elbow pain was an intermittent, poorly localized dull pain which began

soon after the back pain. The pain gradually

increased in the left elbow. US Navy Treatment Table 6 (TT6) was initiated immediately

(2230 hr) for these Type I symptoms. At the

end of the first 20 minute 02 breathing period

(followed by 5 minutes air breathing), the subject's left elbow pain resolved, the right knee

pain decreased in severity, and no change was

noted in the back pain. Several hours into the

treatment (0045), the subject's back pain also

decreased in intensity to a rating of 3 out of

10 (initially 10/10) and only slight knee pain

was noticed. The subject completed five 02

breathing periods (20 minutes 02 / 5 minutes

air) (3 extensions) at 60 fsw. Travel was then

commenced to 30 fsw, however at 45 fsw the

diver complained of increasing back pain

(4/10) and a slight increase in knee pain. The

diver was then returned to 60 fsw for three

more 20 minute 02 / 5 minute breathing periods

as per the U.S.N. Diving Manual Chapter 8

(NAVSEA, 1988). Approximately 2 hours

and 40 minutes later, the diver reported complete resolution of knee pain, and only a

"slight twinge" in his back, but significant

relief. The diver was then allowed to travel to

30 fsw on 02 where TT6 was continued.

During the first 02 breathing period at 30 fsw,

the subject reported complete relief of all

symptoms. Upon reaching the surface, the

patient reported some back soreness that he

explained was different than his original

symptoms.

The second DCS case involved a 30 year

old (TRIMIX V (4, 41.2)) who reported left

knee pain (intensity 1-2/10) at 22 fsw, 2355.

The pain had started the night before after the

subject slipped and banged his knee. The subject denied any change in initial pain severity

during decompression so he did not report it

at that time. It was not until later (22 fsw,

2355 hours) that he reported it because the

pain had become more persistent. No DCS

treatment was initiated at this time because

the DMO was not convinced the symptoms

were DCS related. At 6 fsw (0730) the subject

reported no change in the pain (intensity still

1-2/10). He found that the intensity of the

pain was lessened with massage and movement. Approximately 5 hours (1600) postsurfacing, the subject reported a gradual

increase in pain on the medial aspect of the

left knee. The pain was steady with occasional

episodes of throbbing and severe pain lasting

15-30 seconds (intensity 6/10). Treatment

Table 6 was then initiated, and after the first

20 minute 02 breathing period (followed by

5 minutes air) (1700) , the pain decreased to

an intensity of about 2/10. Following the

second 02 treatment, the patient was able to

stand, bear full weight, and reported feeling

remarkably better. TT6 was continued, and

the subject continued to improve. At 1750 hr,

the subject had mild residual medial pain and

mild tenderness in his left knee. At 30 fsw

(1820 hr) and again at the surface, the diver

reported only minor residual soreness. Fortyeight hours post-surfacing, the diver still had

very minor soreness which eventually resolved.

The third case of DCS (TRIMIX V

(4, 41.2)) was the only case to be diagnosed

and treated before surfacing. A 31 year old

reported (day 5 of dive) left knee pain at 88

fsw (0130 hr) which woke him from sleep (intensity=2/10). The chamber was pressed 5

fsw deeper where the subject reported some

relief of pain (1/10). The chamber was then

pressed another 5 fsw and the subject was

treated with two periods of 20 minute 02

breathing (P02 = 2.8 ATA) with 5 minutes of

air breathing between 02 treatments. Follow-
ing this, the subject reported that the pain was

entirely resolved. Assessment by the DMO was

Type I DCS. The subject also reported fleeting "ache" symptoms in the same knee several

times during the last day of the dive (various

depths). These aches were mild and did eventually resolve; however, following the first

two episodes of achiness, the subject was

given three 20 minute periods of pure 02

(23 fsw) each followed by 5 minutes air

breathing. The subject again experienced two

more episodes of fleeting aches before decompression was complete, a single episode just

as the chamber surfaced, and one more while

walking to his barracks (2 hours post-surface).

No 02 treatment was given for these symptoms. Thirty-six hours after surfacing, the

subject reported a residual "stiffness" in his

left knee which had gradually diminished

during the last 24 hours and was absent at the

time of the interview. The final diagnosis was

Type I DCS, full resolution after treatment

during saturation decompression with episodic

knee pain (spontaneously resolved).

The last case of DCS (TRIMIX V (4, 41.2))

was not treated until 3 hours post surfacing.

The 33 year old subject had reported right

knee pain (both sides of patellar tendon, not

within joint) at 5 fsw following twisting his

knee earlier at 19 fsw (2300). During the remainder of decompression and upon surfacing

the subject stated that the pain was present

only during movement with no weakness or

numbness. Approximately three hours post

surfacing the subject noted pain at rest as well,

but had no weakness or numbness. Decompression sickness was considered possible so the

DMO initiated recompression (1300 h). No

relief was noted after the third session of 20

minutes 02 / 5 minutes air breathing at 60 fsw

(1410 hr). Although it was thought that DCS

was unlikely, TT6 was continued. Later, at

30 fsw (1600) the subject reported a significant decrease in discomfort when at rest and

during movement. Treatment on Table 6 was

completed and the subject reported feeling

much improved with less discomfort at rest

and during movement. Fourteen hours post

TT6 (0900), the subject felt "100% better"

than he did immediately post treatment.

Twenty-four hours post dive, the subject admitted the pain had actually worsened during

the experimental decompression but he failed

to report. The case was documented as DCS

Type I.

The four cases ofskin symptoms resulting

from isobaric counter diffusion (TRIMIX III)

consisted ofitching, blotching and redness (no

rash). The cases were quite similar in severity

(minor), time of onset, and duration. Two

cases occurred 2.5 hours after the shift and

lasted until 6.5 hours after the shift. The other

two cases both started approximately 75 minutes

post shift. One case lasted about 40 minutes;

the other about 2 hours. All four cases resolved spontaneously without treatment.

Discussion

The TRIMIX test series was designed specifically to simulate the extended exposure of

men in a DISSUB to air at a pressure of 5

ATA followed by rescue under pressure in the

DSRV.

Three DCS cases were diagnosed and

treated post-dive while one occurred during

decompression. All involved either the right

(1 case) or left ( 3 cases) knees. The site of

the symptoms was consistent with air and

NITROX saturation dive results reported by

Eckenhoff and Vann (1). Knee pain has also

been noted in the majority of DCS symptoms

in heliox saturation decompressions (Hanson,

Vorosmarti, & Banard, 1987; Summitt &

Berghage, 1971; Summitt and Kulig, 1970).

In a DISSUB situation the most senior

survivior is given responsibility for the decision

to either await rescue or attempt an escape.
This individual must take into consideration

the survival potential in the DISSUB and the

time that can be spent in a pressurized environment without incurring an unacceptable

decompression obligation and/or risk of DCS.

Escape may be desirable yet impossible if

conditions are not correct (e.g. severe surface

envirnoment).

In the case where escape is not feasible, the

senior survivor must also be responsible for

evaluating the risk of possible DCS versus the

risk associated with escape. He then makes a

decision based on all available information.

Several tables are provided in the manual titled,

"Pressurized Submarine Rescue, A manual for

Undersea Medical Officers" (Harvey et al.

1992), which outline alternative schedules for

saturation decompression from various depths.

The more conservative lengthy schedules are

associated with a 1% or less predicted incidence of DCS. Less conservative schedules

provide an alternative means to decompress

survivors when time and resources are limited

or not available. These schedules have faster

overall decompression times, at the cost of

higher expected risk for DCS; in some cases

risk may be almost as high as 80%. Currently, the incidence rates over 30% reflect

more a numerical ranking ofschedules than a

reliable estimate of actual predicted DCS

rates. In these higher risk ranges, some very

serious and possibly fatal DCS cases may occur. The tables in the Pressurized Submarine

Rescue Manual allow a senior survivor to

make informed decisions regarding the risks

and benefits associated with the selection of

riskier decompression schedules. These tables

(which include the Trimix decompression)

have not all been directly tested. They are statistically derived from 1,992 well documented

saturation man dives and represent a highly informed, yet unconfirmed, estimate of the risks

associated with their use.

It is likely that during a real DISSUB rescue

scenario, a Commander and/or rescue team

may choose to accept a DCS risk much greater

than the 18.8% resulting from the fastest

trimix decompression schedule. However, it

should be emphasized that it would be unethical

to expose volunteer subjects to any greater

risk than is absolutely necessary, even in a

controlled research setting. Consequently,

attempts to directly test a decompression

schedule to be used for rescue purposes may

be less than ideal, resulting in relatively conservative schedules. Many more trimix decompressions with faster schedules would be

needed to directly determine a faster schedule

for DISSUB rescue with a higher acceptable

DCS risk.

Decompression testing yielding a substantial degree of confidence in the results is also

not practical for another reason. The number

ofreplicate exposures required on a single procedure is very large. For example, to be 95%

confident that the underlying incidence of

DCS is below 10% DCS, 35 test dives with

no DCS or only one case of DCS in 54 replicated exposures are required.

The authors realize that the obvious shortcoming of this research is the small number of

replicate tests conducted. However, resource

economics and ethical concerns prohibit direct,

statistically useful testing of more than a few

candidate decompression schedules.

A more powerful and practical approach is

to combine many, i.e. thousands, of well

described test dives into probabilistic models

fitted by maximum likelihood (Weathersby,

Homer, & Flynn, 1984). That approach produced controlled risk decompression tables

for air diving (Weathersby et al., 1985), and

has been invoked in the submarine rescue

scenario (Harvey et al., 1992). It is not, however, yet ready for the trimix problem.
Probabilistic models can never be better

than the data on which they are developed and

calibrated. The bulk of published studies

have considered N2 as the only inert gas.

Only a single pilot study (Tikuisis, Weathersby,

& Nishi, 1991) has examined probabilistic

models allowing for both helium and nitrogen

inspiration. Work is now in progress to extend

the compilation of primary decompression

data (Weathersby, Survanshi, Nishi, &

Thalmann, 1992) to include helium-oxygen

and TRIMIX dives. In addition to the present

data, about 2000 more dives are being reviewed.

Ofthose, this series has the only well-described

TRIMIX saturation dives available.

Conclusions:

The conclusions drawn from this study are:

1. Overall, an isobaric shift of divers

saturated on air at 5 ATA to TRIMIX

(50% He, 40% N2, 10% O2) followed by

decompression to the surface is feasible.

Specific isobaric counter diffusion DCS

risk for this switch appears to be negligible;

2. A 4-hour hold period following the shift

from 5 ATA air to TRIMIX is believed to

be adequate. A 4-hour hold did not present

greater DCS risk than longer hold periods

because we believe the temporal proximity

of the DCS cases reported for all dives

precludes any relationship to hold time;

3. Trimix decompression time of 44.5

hours or greater creates only a slight risk

of DCS (1 DCS case of 46 dives, 95%

C.L. on rate 1%-10%); and

The information and experience gained

from these TRIMIX dives has permitted

NSMRL to provide guidance to the fleet for

DISSUB rescue scenarios.

Recommendations:

The results from these dives have been

incorporated in operationally relevant recommendations as follows (Harvey et al., 1992):

1) For decompression of submarine rescue

survivors a mixture of air and He with an

O2 partial pressure of 0.5 ATA and a

N2/He ratio of 4/5 has been found effective.

The trimix decompression procedure

offers the advantage of a lower partial

pressure of O2 but does not produce

isobaric counter-diffusion symptoms at

depth (132 fsw) (5.0 ATA).

2) A required, yet conservative, hold of 8

hours is recommended after switching the

survivors to trimix before beginning decompression. A hold time of 4 hours is

suggested as sufficient if time is crucial.

3) A decompression time of 44.5 hours is

recommended.

REFERENCES

Behnke, A R. (1947). A review of physiologic

and clinical data pertaining to decompression sickness (NMRI Report No. 4).

Bethesda, MD: Naval Medical Research

Institute.

Bove, A.A. & Davis, J.C. (Date). Diving

Medicine, 2nd Ed. W. B. Saudners

Compnay: Philadelphia.

D'Aoust, B.G. & Lambertson, C.J. Isobaric

gas exchange and supersaturation by

counter diffusion. (1982). In: P.B. Bennett

& D.H. Elliot (Eds.), The Physiology and

Medicine ofDiving, 3rd Edition (pp. 383-

403). Best: San Pedro.

Eckenhoff, R.G. & Vann, R.D. (1985). Air

and nitrox saturation decompression: a
report of 4 schedules and 77 subjects.

Undersea Biomedical Research, 12 (1),

41-52.

Hallenbeck, J.M. & Andersen, J.C. Pathogenesis of the decompression discords.

(1982). In: P.B. Bennett & D.H. Elliot

(Eds.), The Physiology and Medicine of

Diving, 3rd Edition (pp. 435-460). Best:

San Pedro.

Hanson, R. de G, Vorosmarti, J., Jr. & Barnard,

E. E. P. (1987). Decompression sickness

after saturation diving: In: C. W. Shilling

& M. W. Beckett, (Eds.). Underwater

Physiology VI. Proceedings ofthe sixth

symposium on under water physiology

(pp. 537-545). Bethesda: Federation of

American Societies for Experimental

Biology.

Harabin, A. L., Homer, L. D., Weathersby, P.

K., & Flynn, E. T. (1987). An analysis of

decrements in vital capacity as an index of

pulmonary oxygen toxicity. Journal of

Applied Physiology, 63, 1130-1135.

Harvey, D. A., Stetson, L. M., Burns, A. C,

Weathersby, P. K., Parker, J. W., & Mole,

D.M. (1992). Pressurized Submarine

Rescue. A Manualfor Undersea Medical

Officers (NSMRL Report 1178). Groton,

CT: Naval Submarine Medical Research

Laboratory.

NAVSEA. (1987). U.S. Navy Diving Manual

Vol. 2. (NAVSEA 0994-LP-001-9020).

Washington: Navy Department.

NAVSEA. (1988). U.S. Navy Diving Manual

Voll. (NAVSEA0927-LP-001-9010).

Washington: Navy Department.

Summitt, J. K. & Berghage, T. E. (1971).

Review and analysis of cases ofdecompression sickness occurring underpressure

(NEDU Rep 12-71). Panama City, FL:

Navy Experimental Diving Unit.

Summitt, J. K. & Kulig, J. W. (1970). Saturation dives, with excursion, for the development ofa decompression schedule for

use during SEALAB III (NEDU Rep 9-70).

Panama City, FL: Navy Experimental

Diving Unit

Tikuisis, P., Weathersby, P. K., Nishi, R. Y.

(1991). Maximum likelihood analysis of

air and He02 dives. Aviation Space and

Environmental Medicine, 62(5), 425-31.

Weathersby, P. K., Homer, L. D., and Flynn,

E. T. (1984). On the likelihood of decompression sickness. Journal ofApplied

Physiology: Respiratory, Environmental

and Exercise Physiology, 57(3), 815-825.

Weathersby, P. K., Survanshi, S., Homer, L.

D., Hart, B. L., Nishi, R. Y., Flynn, E. T.,

and Bradley, M. E. (1985). Statistically

based decompression tables I. Analysis of

standard air dives: 1950-1970 (NMRI

Report 85-16). Bethesda, MD: Naval

Medical Research Institute.

Weathersby, P. K., Survanshi, S. S., Nishi, R.

Y., and Thalmann, E. D. (1992). Statistically based decompression tables. 7.

Selection and treatment of primary air

andN2 Ö2 data (Joint Report: NSMRL

Report 1182; NMRI Report 92-85).

Groton, CT: Naval Submarine Medical

Research Laboratory.

10
UNCLASSIFIED

SECURITY CLASSIFICATION OF THIS" PAGE"

REPORT DOCUMENTATION PAGE

1a. REPORT SECURITY CLASSIFICATION

UNCLASSIFIED

2a. SECURITY CLASSIFICATION AUTHORITY

2b. DECLASSIFICATION/DOWNGRADING SCHEDULE

4. PERFORMING ORGANIZATION REPORT NUMBER(S)

NSMRL Report 1196

6a. NAME OF PERFORMING ORGANIZATION

Naval Submarine Medical

Research Laboratory

6b. OFFICE SYMBOL

(IfApplicable)

6c. ADDRESS (City, State, Zip Code)

Box 900, Naval Submarine Base

NLON,

Groton, CT 06349-5900

8a. NAME OF FUNDING/SPONSORING

ORGANIZATION

Same as 7a

8b. OFFICE SYMBOL

(IfApplicable)

8c. ADDRESS (City, State, Zip Code)

Same as 7b

1b. RESTRICTIVE MARKINGS

Form Approved

OMB No. 074-0188

3. DISTRIBUTION/AVAILABILITY OF THE REPORT

Approved for public release;

distribution unlimited

5. MONITORING ORGANIZATION REPORT NUMBER(S)

NA

7a. NAME OF MONITORING ORGANIZATION

Naval Medical Research and Development

Command

7b. ADDRESS (City, State, Zip Code)

8901 Wisconsin Avenue

Bethesda, MD 20889-5606

PROCUREMENT INSTRUMENT IDENTIFICATION NUMBER

10. SOURCE OF FUNDING NUMBERS

PROGRAM

ELEMENT NO.

63713N

PROJECT NO.

M0099

TASK NO.

.01A

WORK UNIT

ACCESSION NO.

DN 242606

11. TITLE (Include Security Classification)

(U) He-N2-02: Isobaric shift and saturation decompression

12. PERSONAL AUTHOR(S)

13a. TYPE OF REPORT

Interim

C. L. Shake. P. K. Weathersbv. B. G. Caras, and J. W. Parker

13b. TIME COVERED

FROM TO.

14. DATE OF REPORT (Year, Monti, Day)

95 February 5

15. PAGE COUNT

10

16. SUPPLEMENTARY NOTATION

17. COSATI CODES

FIELD GROUP SUB-GROUP

18. SUBJECT TERMS (Continue on reverse if necessary and identify by block number)

Decompression; Decompression sickness;

19. ABSTRACT (Continue on reverse if necessary and identify by block number)

A disabled submarine (DISSUB) crew exposed to increased atmospheric pressures resulting from flooding

or other problems may incur a decompression obligation. They will then require not only gradual decompression to avoid decompression sickness (DCS), but decompression in specific gases due to operational

limitations and to lower the risk for pulmonary oxygen toxicity.

Nineteen dry chamber dives were conducted at the Naval Submarine Medical Research Laboratory

(NSMRL) to explore the feasibility of an isobaric shift from air at five atmospheres (ATA) to HeliumNitrogen-Oxygen (50% He-40% N2-10% O2, TRIMIX) followed by decompression to the surface. These

procedures parallel the potential operational scenario of returning the crew of a pressurized, disabled

submarine (DISSUB) to the surface. Saturation dives were conducted by sequential experimental design, in

which there was progressive shortening of the hold period following isobaric shift and/or shortening of

decompression time. Initial pressurization was to 111 feet ofseawater (fsw) (4.4 ATA) for 60 hours followed

by compression to 132 fsw (5.0 ATA) on air for 12 hours, then the isobaric shift to TRIMIX was conducted.

A hold of 24,12, 8, or 4 hours at 132 fsw (5.0 ATA) was initiated, followed by a rapid upward excursion from

132 fsw (5.0 ATA) to 116 fsw and decompression to the surface with 57.5, 50.9, 44.5, or 41.2 hour schedules.

20. DISTRIBUTION/AVAILABILITY OF ABSTRACT

X UNCLASSIFIED/UNLIMITED SAME AS REPORT DTIC USERS

22a. NAME OF RESPONSIBLE INDIVIDUAL

Susan D. Monty, Publications

21. ABSTRACT SECURITY CLASSIFICATION

Unclassified

22b. TELEPHONE (Include Area Code)

(203) 449-3967

22c. OFFICE SYMBOL

DD Form 1473, JUN 86 Previous editions are obsolete.

S/N 0102-LF-014-6603

SECURITY CLASSIFICATION OF THIS PAGE

UNCLASSIFIED
UNCLASSIFIED

SECURTTrCCASSIFlC7mölT0FTHlSTrAGE""

19. cont.

The potential for decompression sickness (DCS) existed from both isobaric counter diffusion and also from

decompression to the surface. Four cases of DCS occurred in 62 man-dives. Three cases presented after

surfacing (one Type II: 44.5 hour schedule; two Type 1:41.2 hour schedule). One case presented during

decompression (Type I: 41.2 hour schedule). Four cases of minor skin itching without rash were noted

following the isobaric shift.

Overall, an isobaric shift of divers saturated on air at 5 ATA to trimix followed by decompression to the

surface resulted in a small risk for DCS. Specific risk for isobaric counter-diffusion DCS for this switch

appears to be negligible. Guidance to the fleet for DISSUB rescue scenarios has been provided.

DD Form 1473, JUN 86 (Reverse) SECURITY CLASSIFICATION OF THIS PAGE

UNCLASSIFIED