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Experimental trials to assess the risks of decompression sickness in flying after diving

Abstract and Figures

We conducted experimental trials of flying after diving using profiles near the no-decompression exposure limits for recreational diving. The objective was to determine the dependence of DCS occurrence during or after flight on the length of the preflight surface intervals (PFSI). One to three dives were conducted during a single day with dry, resting subjects in a hyperbaric chamber at depths of 40, 60, or 100 fsw (224, 286, 408 kPa). The dives were followed by PFSI of 3 to 17 hrs and a four-hour altitude exposure at 8,000 ft (75 kPa), the maximum permitted cabin altitude for pressurized commercial aircraft. Forty DCS incidents occurred during or after flight in 802 exposures of 495 subjects. The DCS incidence decreased as PFSI increased, and repetitive dives generally required longer PFSI to achieve low incidence than did single dives (p = 0.0159). No DCS occurred in 52 trials of a 17 hr PFSI, the longest PFSI tested. The results provide empirical information for formulating guidelines for flying in commercial aircraft after recreational diving.
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UHM 2004, Vol. 31, No. 4 – Flying after diving
Experimental trials to assess the risks of
decompression sickness in flying after diving.
Divers Alert Network, Department of Anesthesiology, Duke University Medical Center, Durham, NC;
Center for Hyperbaric
Medicine and Environmental Physiology, Department of Anesthesiology, Duke University Medical Center, Durham, NC;
Navy Experimental Diving Unit, Panama City, FL;
Center for Aging, Division of Biostatistics, Department of Community and
Family Medicine, Duke University Medical Center, Durham, NC
Vann RD, Gerth WA, Denoble PJ, Pieper CF, Thalmann ED. Experimental trials to assess the risks of
decompression sickness in flying after diving. Undersea Hyperb Med 2004; 31(4):431-444 We conducted
experimental trials of flying after diving using profiles near the no-decompression exposure limits for
recreational diving. The objective was to determine the dependence of DCS occurrence during or after
flight on the length of the preflight surface intervals (PFSI). One to three dives were conducted during a
single day with dry, resting subjects in a hyperbaric chamber at depths of 40, 60, or 100 fsw (224, 286, 408
kPa). The dives were followed by PFSI of 3 to 17 hrs and a four-hour altitude exposure at 8,000 ft (75 kPa),
the maximum permitted cabin altitude for pressurized commercial aircraft. Forty DCS incidents occurred
during or after flight in 802 exposures of 495 subjects. The DCS incidence decreased as PFSI increased,
and repetitive dives generally required longer PFSI to achieve low incidence than did single dives
(p=0.0159). No DCS occurred in 52 trials of a 17 hr PFSI, the longest PFSI tested. The results provide
empirical information for formulating guidelines for flying in commercial aircraft after recreational diving.
Divers usually begin and end their dives at or near sea level (101 kPa), but they
sometimes fly after diving. Most flying after diving takes place in pressurized commercial
aircraft with cabin altitudes no greater than 8,000 ft (equivalent to a barometric pressure of 75
kPa by the 1976 U.S. Standard Atmosphere (1)), the maximum commercial aircraft cabin altitude
permitted by the Federal Aviation Administration (2). Exposure to reduced barometric pressure
after diving could increase a diver’s risk of decompression sickness (DCS) unless the diver
remained at sea level long enough to allow elimination of excess inert gas from tissue.
Flying after diving history
The first evidence that flying after diving increased DCS risk was found in a 1960 U.S.
Navy study that tested altitude exposure as a possible method for assessing the adequacy of
decompression after diving (3). The first reported operational incidents of DCS while flying after
diving occurred in 1961 when the pilot and copilot aboard an intercontinental commercial
aircraft at a cabin altitude of 8-10,000 ft (75-70 kPa) were incapacitated by DCS less than four
hours after diving not deeper than 30 fsw (193 kPa) (4). The flight engineer, who had been
diving about 12 hrs earlier, was less affected and landed the aircraft safely.
In 1967, Furry et al. provided experimental evidence indicating that DCS risk was related
to the preflight surface interval (PFSI) at sea level before flying (5). They exposed dogs to 53-88
fsw (264-372 kPa) for seven hours followed by a 1, 3, 6 or 12 hour preflight surface interval
(PFSI) at sea level pressure before decompression to a simulated altitude of 10,000 ft for 2 hours.
Signs interpreted as DCS included scratching, limping, leg lifting, respiratory difficulty, or
Copyright © 2004 Undersea and Hyperbaric Medical Society, Inc. 431
UHM 2004, Vol. 31, No. 4 – Flying after diving
change in disposition. The DCS incidence was 93% with a one-hour PFSI and gradually
decreased to zero as the PFSI was lengthened to 12 hours.
In 1969, Edel et al. conducted the first human trials to estimate how long divers should
wait before flying (6). Dives to the U.S. Navy no-decompression limits of 15 min at 120 fsw
(470 kPa) and 200 min at 40 fsw (224 kPa) (7) were followed by PFSIs of 5 min, 30 min, 1 hr, 2
hrs, or 3 hrs before an altitude exposure of 112 min at 8,000 ft and a subsequent 5 min excursion
to 16,000 ft (55 kPa). The 5 min exposure at 16,000 ft was designed to provoke latent DCS
should it be incipient as in the earlier Navy study (3). In 39 exposures, there were two DCS
incidents at 8,000 ft after a 5 min PFSI following a 200 min dive to 40 fsw, and nine incidents at
16,000 ft after PFSIs of 5 min to 2 hrs (eight incidents after 200 min at 40 fsw and one after 15
min at 120 fsw). Edel’s trials were the basis of the two-hour U.S. Navy guideline for flying after
no-decompression diving that was in effect from 1985-1999 (8).
There are few other human trials of flying after diving relevant to exposures of 8,000 ft or
less (9) although Balldin (10) and Bassett (11) conducted studies at somewhat higher altitudes.
Balldin found no DCS in 20 trials of no-decompression dives at the U.S. Navy limits of 10 min
at 130 fsw (500 kPa) or 100 min at 50 fsw (255 kPa; (12)) followed by a three-hour PFSI and a
two-hour exposure at 9,843 ft (3,000 m; 70 kPa). Balldin’s study may have underestimated the
DCS risk because his subjects were given prophylactic hyperbaric oxygen upon descent to
preclude delayed-onset DCS. Bassett investigated dives that might allow immediate ascent to
altitude with low DCS risk (11). He tested two altitude exposures: (a) 10,000 ft for four hours
followed by one hour at 16,000 ft; and (b) 8,500 ft (74 kPa) for four hours followed by one hour
at 14,250 ft (59 kPa). His dives were based on an analysis of the U.S. Navy Standard Air
Decompression Tables (12) indicating that an adjustment to the theoretical tissue ratios would
compensate for reduced pressure at altitude. Dives tested were 24 hours at 10.75 fsw (134 kPa),
34 min at 40 fsw, 20 min at 60 fsw, 14 min at 80 fsw (347 kPa), 10 min at 100 fsw, and 7 min at
130 fsw. These were followed by immediate ascent to altitude. In 167 exposures, there was one
DCS incident at 10,000 ft after a dive to 80 fsw, one at 14,250 ft and one 16,000 ft after dives to
60 fsw and four at 16,000 ft after dives to 10.75 fsw, 40 fsw, 60 fsw, and 100 fsw. He also
monitored his subjects with Doppler ultrasound for precordial signals of venous gas emboli
(VGE) and terminated the altitude exposures of nine subjects (two for 10.75 fsw, one for 40 fsw,
two for 60 fsw, three for 80 fsw, and one for 100 fsw), who had Spencer VGE scores of 3 at rest
or 4 with movement (13). Thus, Bassett also may have underestimated the DCS risks of flying
after diving.
Current guidelines
The limited trials described above provide little basis for comprehensive PFSI guidelines,
and published recommendations vary widely (8). The first flying after diving guideline was
promulgated by the U.S. Navy in 1972 and recommended a 12 hr PFSI before flying after a
decompression dive (14). In 1985, the recommended PFSI after no-decompression dives was
decreased to 2 hrs (12). No delay was required for altitudes of less than 2,300 feet (93 kPa) based
on work by Boni et al(15). The U.S. Air Force required a 24 hr PFSI after any diving in 1990 (8).
In 1991, the Divers Alert Network published guidelines for recreational diving that
recommended at least 12 hours before flying and longer than 12 hours after repetitive multi-day
or decompression dives (16). In 1999, the Navy introduced new guidelines based on preliminary
reports of the work described here (Dr. E.T. Flynn, personal communication) with PFSI ranging
from 0 to 24 hours depending on the flight altitude and post-dive Repetitive Group Designator
UHM 2004, Vol. 31, No. 4 – Flying after diving
(RGD; (17)). These guidelines were computed by applying the Cross corrections (18) to the U.S.
Navy Standard Air Decompression Tables (17).
We sought to estimate the longest PFSI that might be required before flying with low
DCS risk after recreational air dives near the single and repetitive no-decompression exposure
limits. We expected the ‘safe’ PFSI necessary to achieve low DCS risk would be longest near
these limits. In addition, we sought information on how the DCS risk would change with
decreasing PFSI. We conducted the study to provide further data on which flying after diving
guidelines might be based.
Subjects and medical supervision
Experiments were conducted at 2-3 week intervals with 5-12 subjects per study at the
Center for Hyperbaric Medicine and Environmental Physiology of Duke University Medical
Center from 1993-1999. Male and female subjects were certified recreational divers or
individuals with hyperbaric or hypobaric experience. They were fully briefed and signed a
Consent document that had been approved by the Duke University Medical Center Institutional
Review Board. About 2% of the subjects were rejected for conditions including prosthetic joints,
pulmonary dysfunction, or neurological abnormality. Subjects were required to avoid diving and
strenuous activity for 48 hours before and after participation in an experiment. Subjects were
allowed to participate in multiple experiments.
The subjects’ physical characteristics are presented in Table 1. This table was computed
by treating subjects who had made repeated experiments over several years as separate
individuals to account for changing characteristics. The study was open to males and females and
no effort was made to recruit divers of a particular gender. There were 495 individual subjects
who completed 802 simulated flying after diving exposures. Sixteen subjects did not complete
the entire dive and flight profile due to ear barotrauma or DCS developing during the PFSI.
Twenty-seven percent of the subjects were female, and females made 27% of the exposures. Of
subjects completing the dives and flight, 368 participated in a single trial while 127 participated
in at least two or as many as 12 trials. Fifteen subjects participated in two trials of the same
pressure profile, and one subject participated in three trials of the same profile.
Table 1. Subject characteristics.
Age Weight Height BMI*
(n=594) (years) (kg) (cm) (kgm
Mean 36.9 83.6 175.3 25.9
S.D. 10.3 13.6 7.5 3.7
Minimum 18.0 52.2 155.0 18.0
Maximum 69.0 147.4 208.3 43.0
Mean 33.8 64.0 165.1 23.4
S.D. 9.8 11.2 7.3 3.7
Minimum 19.0 45.3 147.3 16.0
Maximum 60.0 99.8 198.1 36.0
* Body Mass Index: weight (kg)/height (m)
UHM 2004, Vol. 31, No. 4 – Flying after diving
Physicians experienced in the diagnosis and treatment of DCS provided medical
coverage for each experiment. A physician administered a baseline interview and neurological
examination before each exposure to identify pre-existing conditions that might later be confused
with DCS. The same physician interviewed the subjects immediately after each dive and again at
1 and 4 hours before releasing them for the remainder of the PFSI, usually overnight. The same
physician interviewed the subjects again immediately before flight. During the flight, a study
attendant in the altitude chamber asked each subject at 30 min intervals if there were any
symptoms to report. Subjects were encouraged to report all symptoms even if very mild or
apparently unrelated to the dives or flight. The physician, who remained outside the chamber,
interviewed subjects who reported symptoms to assess the possibility of DCS. Subjects with
suspected DCS were recompressed to ground level for further evaluation and possible therapy.
Subjects who completed the flight without symptoms were interviewed immediately after the
flight, four hours post-flight, and the next morning. A study technician familiar with the signs
and symptoms of DCS interviewed the subjects by telephone at about 48 hours post-flight.
Flying after diving exposure
For our simulated flight, we selected the FAA maximum cabin altitude of 8,000 ft (2). A
four-hour altitude exposure was chosen based on our unpublished experience that most altitude
DCS symptoms occur within four hours (19).
Preliminary trials
Prior to beginning the study, we conducted preliminary trials of a dive to 60 fsw for 55
min followed by a PFSI of three hours to assess the safety of PFSIs in the range of the two hour
U.S. Navy guideline that was in effect at the time the study began (12). When these trials
resulted in three cases of DCS (one cerebral; Case 13, Appendix B) and four additional cases
occurred with PFSIs of 6, 9, and 10 hours, the experimental design described below was
Experimental design
A sequential experimental design was adopted to minimize the number of individuals
exposed to DCS risk (20) while still finding the shortest PFSI. The diving physician diagnosed
clinical DCS based on signs and symptoms at the time of occurrence, and a decision to
recompress was made independently of scientific objectives. The decision to accept and continue
testing or reject and suspend testing of a given dive profile/PFSI combination was made in
accordance with a priori accept/reject criteria. These criteria, described below, were arbitrary
and used only to decide when to stop testing one dive profile/PFSI combination and move to
another. The criteria were not used to judge the ‘safety’ of any profile.
DCS was classified as mild, moderate, or serious in severity. Mild symptoms were
defined as limb or joint pain and/or patches of abnormal sensation unrelated to peripheral nerve
or dermatome distribution. Moderate signs or symptoms were defined as neurological findings
such as specific sensory deficit or motor weakness only obvious on detailed examination. Serious
signs or symptoms were defined as respiratory, cardiovascular, cerebral, motor weakness
obvious to observation, or disorders of gait or balance.
A PFSI was accepted for a given dive profile if there was no clinically assessed DCS in
23 subject exposures (95% binomial upper confidence limit [BUCL] 0.122), one mild DCS
incident in 35 exposures (0.128 BUCL), two in 46 (0.131 BUCL), or three in 56 (0.132 BUCL).
UHM 2004, Vol. 31, No. 4 – Flying after diving
A PFSI was rejected if there were two mild incidents in 10 or fewer subject exposures (95%
binomial lower confidence limit [BLCL] 0.037) or three mild incidents in 26 or fewer exposures
(0.032 BLCL). PFSIs were also rejected if there were four mild incidents, two moderate
incidents, or one serious incident at any time. (The Institutional Review Board approved one
Moderate incident for the first half of the study and two Moderate incidents for the second half.)
For the first dive on a given profile, a PFSI was chosen which was expected to have a low
risk of DCS based on available data. If the first PFSI met the acceptance criteria, PFSI were
shortened on subsequent profiles until the reject criteria were met. Testing of that dive profile
was then stopped. If the first PFSI was rejected, subsequent PFSI were lengthened until one met
the accept criteria.
Definition of DCS
Since treatment decisions had to be made rapidly and in real time, we anticipated that
there might be occasional false positives or negatives in equivocal cases. Therefore, all
symptoms were reviewed post hoc and a standard definition applied in deciding if symptoms
were indeed due to DCS. We chose a U.S. Navy definition of DCS that had been used in a
retrospective review of 918 case reports (21). We modified this definition to accommodate flying
after diving as described below.
Unless another cause could be identified such as illness, injury, or pre-existing
symptoms, signs and symptoms were defined as DCS if: (a) they remained constant or showed
progression within the next 10 min at 1 ata (post-dive) or with continued altitude exposure; (b)
they improved or resolved with descent from altitude, oxygen breathing, or hyperbaric oxygen
therapy; or (c) they resolved spontaneously after persisting for at least one hour. In addition,
“marginal DCS” or “niggles” were defined as minor symptoms that persisted for less than one
hour. “Skin bends” were defined as rash or itching without marbling. Recompression therapy
was not given for niggles or skin bends, and these were not classified as DCS.
Upon completion of the study, all signs and symptoms were carefully reviewed under this
definition with special attention to cases not considered clinical DCS at the time of the trials.
DCS was attributed to diving alone if relevant signs or symptoms occurred during the PFSI. DCS
that first occurred during the flight or within 24 hours after flying was attributed to the combined
effects of diving and flying.
Flying after diving profiles
The diving exposures consisted of one or more simulated dives in a hyperbaric chamber
with seated subjects under dry, resting conditions. Dive depths were chosen to reflect shallow,
moderate, and deep recreational dives, defined as 40, 60, and 100 fsw. Dive times included
descent and were generally chosen to reflect dives at or near the no-decompression exposure
limits of the Recreational Dive Planner (22, 23). Four single dive profiles and five repetitive dive
profiles were tested (Appendix B). A one-hour surface interval between repetitive dives was
chosen to reflect common recreational diving practice. Both descent and ascent rates were 30
fsw/min, the rate used by the Navy (24) and most dive tables and computers (25). Safety stops
were not used as they are not universally applied and were not used in previous flying after
diving studies (3, 6, 10, 11).
The depths and times of the planned dive profiles were followed closely unless a subject
had difficulty equalizing his or her ears. If equalization difficulty developed, the chamber was
returned to the surface, the affected subject removed from the chamber and the dive restarted.
UHM 2004, Vol. 31, No. 4 – Flying after diving
For analytical purposes, the time of the aborted descent was ignored. Eight dives were aborted
due to ear barotrauma. The aborted dives were short and shallow with mean depths and times
(±S.D.; range) of 17.5 fsw (±8.2; 7-30) and 4.3 min (±1.6; 3-7). Actual dive profiles were
available from the logs or digital recordings. Only one experiment did not adhere closely to plan
and was conducted with only 12 min at 100 fsw rather than the planned bottom time of 15 min
(Appendix B).
The dives in each experimental trial were followed by PFSIs at the Durham, NC ground
level altitude of 398 ft (nominal barometric pressure 749 mmHg or 99.9 kPa) after which the
altitude exposure took place. The chamber pressure was reduced in four minutes to an altitude
equivalent to 8,000 ft for four hours. The subjects were seated or resting supine while at altitude
except for two deep knee-bends every 30 min during precordial Doppler monitoring. Doppler
data will be reported separately.
Statistical analysis
Descriptive measures were reported as means and standard deviations for normally
distributed data and as medians and Interquartile Ranges (IQR) for non-normal data.
The relationship of PFSI to DCS risk was evaluated by logistic regression implemented
in SAS Version 8.0 (Cary, NC). The analysis was controlled for potentially confounding
covariates such as subject characteristics, dive profile characteristics, and estimated dive profile
severity. Subject characteristics tested included age, gender, height, weight, and body mass index
(BMI; weight (kg)/height(m)
). Dive profile characteristics tested included dive depth, individual
dive time, total bottom time, and whether a dive profile was single or repetitive.
Using logistic regression, four measures of dive profile severity were tested for possible
association with DCS. These included the PFSI in combination with: (a) the Repetitive Group
Designators (RGD) according to the U.S. Navy Standard Air Decompression Tables (24); (b) the
RGDs according to the Repetitive Dive Planner (22); (c) the theoretical nitrogen tensions in an
exponential (half-time) tissue compartment either post-dive or at end-PSFI; or (d) DCS risks
estimated either post-dive or at end-PSFI by a probabilistic decompression model from exact
dive profiles (26). A p-value of 0.05 was considered significant.
The above analysis assumed identical DCS susceptibility for all subjects other than as
related to reported (age, gender, weight) or computed (BMI) subject characteristics. Because
some subjects participated in multiple experiments, we also conducted a repeated measures
analysis using the General Estimating Equation and the General Linear Model (SAS Version 8.0)
to test the hypothesis that each subject might have a unique susceptibility that was distinct and
detectable from other subjects.
DCS after diving
The overall DCS incidence during the PFSI (i.e., before flying) was 1.4%. Two of these
DCS cases had symptoms that were not reported when they first occurred, and they recurred or
became worse during flight. These were attributed to DCS from the dive alone. Two cases
resolved before flying, did not recur during flight, and were not reported until after symptom-free
flights. These were also counted as post-dive DCS while the flights were counted as symptom-
free. Nine DCS cases resolved during a single treatment (four on USN Table 6 and five on USN
UHM 2004, Vol. 31, No. 4 – Flying after diving
Table 5), and one case had minor residual symptoms that resolved over two days after a single
Appendix A summarizes the 11 DCS cases that occurred before the flight. An additional
13 signs or symptoms were classified as niggles, 13 as skin bends, and six as not DCS. Of the 13
divers with skin bends (one rash and 12 itching, all after dives to 100 fsw, and all resolving
spontaneously within one hour), one developed post-dive DCS, and none had in-flight or post-
flight DCS. Six cases with limb-pain, headache, or numbness were judged to be the result of pre-
existing injury or a concurrent condition rather than DCS.
DCS during or after flight
The overall DCS incidence due to flying after diving was 5.0%. The difference between
male and female incidences (6.9% and 4.3%, respectively) was not significant. A repeated
measures analysis found no effect for subjects who participated in multiple experiments. Eight
DCS cases resolved spontaneously, three resolved during descent from altitude, 27 cases
resolved during a single treatment (22 on USN Table 6 and five on USN Table 5), and two cases
received an additional treatment with oxygen at 2 ata.
Appendix B summarizes the 40 subjects who reported signs and symptoms during flight
or within 24 hrs after flight that were classified as DCS. An additional 24 incidents of signs or
symptoms were classified as niggles, two as skin bends, and 10 as not DCS. Three subjects
developed DCS twice on different flying after diving profiles. One subject had post-dive DCS
and in-flight DCS during the same pressure profile. Ten cases with limb pain, back pain,
abdominal pain, or bladder pressure were judged to be the result of pre-existing injury or a
concurrent condition rather than DCS.
When tests of two repetitive dives to 60 fsw for 55 min with a 1 hr SI followed by 60 fsw
for 30 min resulted in two post-dive DCS cases relatively early in the trials, the second dive time
was reduced from 30 to 20 min. Time constraints prevented completion of testing of the single
dive to 40 fsw for 120 min and for the two repetitive 40 fsw dives with a 13 hr PFSI.
DCS Incidence
4 6 8 10 12 14 16 18
Preflight Surface Interval (hrs)
40 fsw/60 min
60 fsw/55 min
100 fsw/20 min
40 fsw/120 min
60 fsw/55 min, 60 fsw/20 min, 60 fsw/20 min
Relationship of PFSI
to DCS
Figure 1 shows the
observed DCS incidences
during or after flight for each
flying after diving profile as
a function of PFSI. For all
single dives (except 40 fsw
for 120 min not completed
due to time constraints) the
DCS incidence decreased to
zero after a PFSI of 9-12
hours. For repetitive dives,
the DCS incidence decreased
Fig. 1. Raw DCS incidence and PFSI as a result of flying after four
single dive profiles and after five repetitive dive profiles.
UHM 2004, Vol. 31, No. 4 – Flying after diving
from above 0.1 to zero over PFSI of 15-17 hours. Except for 40 fsw for 120 min, repetitive
dives required longer PFSI for a given DCS incidence than did single dives. DCS was
significantly associated with PFSI and repetitive diving. DCS tended to decrease as PFSI
increased and repetitive dives formed a separate group from single dives (p=0.0159). DCS was
not associated with subject characteristics, individual subjects, or any dive profile characteristic
other than repetitive diving.
The only measure of the combined dive/PFSI that was significantly associated with DCS
during or after the flight was the tissue nitrogen tension at the end of the PFSI in a perfusion-
limited tissue compartment with a 300 min half-time (p=0.0209).
Flights with no dives and dives with no flights
We observed no DCS in 52 exposures at a 17 hr PFSI but did not conduct control
experiments of flights without dives. Studies of altitude exposure without previous diving have
indicated thresholds for DCS and Doppler-detected venous gas emboli of 18,000 fsw (51 kPa;
(27)) and 12,000 ft (64 kPa; (28)), respectively. Thus, our 8,000 ft simulated flight by itself
would not appear to be associated with significant DCS risk.
To address the question of whether the DCS cases we attributed to flight might have been
caused by diving alone, we used the probabilistic decompression model BVM3 to estimate the
number cases expected in the absence of flight (26). This model had been calibrated to a
database of 3,322 dive profiles of known outcome (DCS or no DCS) and gave good estimates of
DCS probability for air dives in the range conducted here. Eleven DCS cases were predicted to
occur as a result of diving alone and 11 were observed, many fewer than the 40 cases reported
during or after flight. This was consistent with the conclusion that flying was the factor
responsible for the DCS.
PFSI and DCS risk
PFSI can be brief and even omitted with little DCS risk if the dives are short or shallow
(e.g., (11)). Our objective, however, was to acquire data from which to estimate the longest
PFSIs needed to achieve low DCS risk during or after flight following dives near no-
decompression limits such as specified by the Recreational Dive Planner (22). We found that
DCS decreased significantly as PFSI increased and that repetitive dives were grouped separately
from single dives with regard to the zero-incidence PFSI (Fig. 1). Single dives generally needed
PFSI of 11-12 hrs for low DCS risk while repetitive dives needed up to 17 hours. The single dive
to 40 fsw for 120 min was a possible exception to this observation, but there were insufficient
dives to estimate where the zero-incidence PFSI might fall, except that it would probably be
longer than 13 hours.
The relationship of PFSI to DCS could not be resolved for individual dive profiles as
subject safety, embodied in the Accept/Reject criteria, limited the number of DCS incidents that
were tolerable for each dive profile. Because of this limitation, we adopted a dose-response
design to help identify, in aggregate, the PFSI range over which the transition from near zero
DCS risk to the steep part of the dose/response curve might be resolved. Figure 1 shows this was
largely achieved, and it appeared that a PFSI of 17 hrs or greater should ensure a very low DCS
risk during a subsequent flight for most no-decompression dives. As the trials were conducted in
UHM 2004, Vol. 31, No. 4 – Flying after diving
a dry chamber with resting subjects, however, the results may not apply to recreational divers
who are immersed and exercising in warm water.
Comparison of old and new data
Our findings of DCS incidences as great as 17% for PFSI of 3-13 hours (Fig. 1) contrast
with the results of Edel et al. (6) and Balldin (10) where the combined DCS incidence at PFSI of
2-3 hours for altitudes of 8,000 ft or above was 3%. Two methodological differences might
explain this discrepancy. First, their altitude exposures were only about two hours in duration,
while ours were four hours. Seven of our 40 DCS cases (18%) occurred in the third and fourth
hours of flight. Second, Balldin’s subjects received prophylactic hyperbaric oxygen after flying
while ours did not. As 17 of our 40 DCS cases (43%) occurred post-flight, these might not have
been observed if hyperbaric oxygen had been administered after the flight.
DCS severity
Of the 40 flying after diving DCS cases, the 12 cases that resolved during descent from
8,000 ft or persisted for more than 60 min before resolving spontaneously might be described as
“decompression related” rather than as clinically significant. However, these cases met the U.S.
Navy definition of DCS (21) that we chose as appropriate for a study designed to provide data on
which safety guidelines might be based. In addition, 24 other subjects had mild symptoms that
persisted for less than 60 min. Temple et al. (21) defined these as “marginal DCS” or “niggles”
(29). If niggles were counted as DCS, the relationship of PFSI to DCS would still remain
significant suggesting that niggles were flight-related, even if clinically unimportant.
Only nine recreational dive profiles were tested, so we cannot state with confidence that
17 hours would be a low risk PFSI for all possible no-decompression dive profiles. Because only
one altitude was investigated (8,000 ft), our results represent an upper bound for commercial,
pressurized flight or for or unpressurized flight or mountain travel at lower altitudes where low
DCS risk might be achieved at shorter PFSI. For example, Emmerman found that the average
maximum cabin altitude of 123 commercial airline flights was 5,500 ft (81 kPa; (30)). At cabin
altitudes greater than 8,000 ft, longer PFSI would likely be needed. As the dives were conducted
during a single day with dry, resting subjects, longer PFSI might be required after multi-day
diving by immersed, exercising divers or after dives requiring decompression.
We thank the Professional Association of Diving Instructors (PADI) and the Divers Alert
Network (DAN) for providing support. The study would have been impossible without the
efforts of many people including the 518 volunteers who knew that some of them would develop
decompression sickness but participated for the benefit of all divers. We also thank the more than
35 physicians and staff of DAN and the Center for Hyperbaric Medicine and Environmental
Physiology at Duke Medical Center, especially Neal Pollock Ph.D. and Jake Freiberger, M.D.,
M.P.H. for insightful reviews and suggestions during manuscript preparation.
UHM 2004, Vol. 31, No. 4 – Flying after diving
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Appendix A. DCS after diving.
Dive Profiles
(fsw) (min)
(fsw) (min)
# of
DCS Signs, Symptoms, Treatment, and Resolution
100 20 109 1 Pain in hip on ascent. Improved on Table 5. Gone in 2 days. Did not fly.
2 Ache in tempromandibular joint on ascent persisted for 60 min &
resolved. Flew without problem.
3 Ache & burning in hip/leg after dive. Resolved but not reported.
Recurred in-flight with positive calf extension reflex. Resolved on Table
40 60 1 40 60 119 4 Calf pain during 1st dive. Numbness in foot in surface interval. Resolved
on 2nd dive. Flew without problems.
5 Post-dive pain in sternum. Resolved on Table 5. Did not fly.
6 Tingling & numbness post-dive in arm. Decreased pin-prick over C6-C7
dermatome. Resolved on Table 6. Did not fly.
60 55 1 60 30 36 7 Shoulder pain post-dive. Gone after Table 5. Did not fly.
8 Arm tingling post-dive. Resolved on Table 6. Did not fly.
100 15 1 60 35 111 9
eck pain on leaving chamber. Cracking & popping on neck rotation.
eck stiff & sore next AM. Ache in calf. All resolved on Table 5. Did
not fly.
10 Pain, tingling & decreased sensation in hand after 1
dive. Resolved on
Table 6. Did not fly.
60 55 1 60 20 1 60 20 100 11
R shoulder pain after bowling after 3 asymptomatic dives. Pain present
when flight began & unchanged during flight. Resolved on Table 6.
D=depth; BT=bottom time; SI=surface interval
a – This subject also reported in-flight symptoms described in Case 40 of Appendix B.
UHM 2004, Vol. 31, No. 4 – Flying after diving
Appendix B. DCS In-Flight or Post-Flight.
Dive Profiles
DCS Signs, Symptoms, Treatment, and
40 60 8 41 14.6 1 Cramp in calf began on leaving chamber &
lasted 4 hrs. Resolved spontaneously.
2 Foot pain 23.8 hrs post-flight became worse.
Resolved on Table 6.
3 Abnormal sensation in elbow 22 hrs post-
flight. Unchanged for a day. Resolved on
Table 6.
4 Knees sore & tingling 9.8 hrs post-flight.
Worse next day with pin-prick deficit on leg.
Resolved on Table 6.
5 Arm pain & paresthesias 2 hrs in-flight.
Improved on descent but returned over next 12
hrs. All but mild soreness resolved on Table 6.
6 Shoulder pain & abnormal arm sensation post-
flight. Resolved on Table 6 except for residual
tightness in shoulder.
0 NA
40 120 12 20 5 7 Tingling foot 3 hrs in-flight, no change on
descent. Progressed to thigh. Resolved on
Table 6.
13 30 10 8 Tingling on R side of body 1 hr inflight until
resolving 30 min post-flight.
9 Hand tingling while driving 12 hrs post-flight.
Arm sore & tingling with fullness & stiffness
in wrist & hand. Persisted for 12 hrs &
resolved spontaneously.
10 Tingling, warm sensation on ball of foot at 3
hr, 32 min in-flight. Resolved on descent 30
min later.
60 55 3 36 8.3 11 Moderate arm pain 2 hrs inflight. Decreased on
descent. Resolved on Table 5 .
12 Hand tingling & mild shoulder pain 1 hr in-
flight. No change on descent. Resolved on
extended Table 6.
13 Shoulder pain inflight resolved after 23 min.
Disoriented, 'spaced-out', and nauseated 1 hr,
30 min post-flight. Difficulty with serial-7s.
Resolved on Table 6.
6 6 16.7 14 Hand numb, tingling & decreased grip
strength 3 hrs post-flight. Improved on Table
6. Resolved on HBO (2 hrs at 2 ata) next day.
14.3 15
umb foot 4 hrs post-flight. Resolved on
Table 6.
10 23 8.7 16 Knee pain on drive 5 hrs post-flight. Pain 1/10
seated, 3/10 standing, and 5/10 walking.
Present at bedtime. Resolved by morning.
17 Transient arm pain 3 hrs post-flight progressed
to tingling & returned 4-5 times overnight.
Resolved by morning. No recompression.
0 NA
0 NA
UHM 2004, Vol. 31, No. 4 – Flying after diving
100 20 8 29 17.2 18 Intermittent foot cramp at 30 min infligh
persisted until resolving on descent.
19 At 24 hr post-flight call, reported mild fatigue
& dull ache in shoulder that lasted 2 hrs
starting 5.75 hrpost-flight.
20 Inflight hip pain, leg tingling, calf cramping &
knee ache. Nearly normal after treatment on
Table 5. Completely resolved next day.
21 Inflight paresthesias in hand, episode of
forearm tingling occurred. Symptoms lasted 60
min & resolved before descent. Table 5 given
22 Elbow pain & hand sensory deficit at 11 min
in-flight. Improved on descent. Resolved on
Table 6 except for subtle decreased fine touch
on palm.
9 55 5.5 23 Calf pain 1 hr inflight improved during
descent. Decreased pinprick on calf. Resolved
on Table 6.
24 Hip pain 6.5 hr post-flight. Resolved on Table
25 Burning, fullness & tingling in hand 40 min
inflight. Complete resolution on descent at
4000 ft. Table 5 given to ensure complete
0 NA
40 60 1 40 60 13 18 5.5 26 Knee pain 30 min in-flight. Improved on
descent & nearly resolved with surface O2.
Full resolution occurred on Table 5.
0 NA
15 38 7.9 27 Mild steady knee pain 1.5 hr in-flight.
Resolved 1.35 hr later before descent.
28 Abnormal sensation in hand 7.75 hr post-
flight. Decreased in intensity 25 hrs after onset
on extended Table 6. Resolved by next day.
29 Decreased arm sensation & pin-prick in C5
distribution 5.2 hrs post-flight. Resolved on
Table 6 except for mild numbness which
resolved by next day.
2.8 30 Mild elbow pain 3.5 hrs in-flight. Improved on
descent & resolved on Table 6.
60 55 1 60 20
14 27 7.4 31 Pain & paresthesias in both forearms 2.2 hrs
in-flight. Improved on descent, improved with
surface O2 & resolved on Table 6.
32 Shoulder & elbow pain 1.5 hrs in-flight,
improved on descent. Sensory deficit in hand.
Resolved with surface O2 but given Table 5.
15 39 2.6 33 Elbow pain 3.6 hrs in-flight resolved in 30
min. Recurred post-flight. Resolved on surface
O2. Returned & increased in hot shower.
umb area on deltoid. Resolved on Table 6.
0 NA
60 55 1 60 30 14 8 12.5 34 Dull shoulder ache 20 min in-flight. Improved
on descent. Improved further on Table 6.
Resolved by 48 hr check.
0 NA
UHM 2004, Vol. 31, No. 4 – Flying after diving
100 15
1 60 35 14 18 16.7 35 Mild elbow pain 1 hr in-flight, lasted 2 hr,
resolved spontaneously before descent.
Arm pain, tingling, numbness & abnormal
temperature sensation 4 hrs post-flight.
Resolved with surface O2, returned on air.
Resolved on Table 6.
Tingling & numbness 2.3 hrs post-flight in
hand, forearm & foot. Foot resolved on Table
6. Residual numbness in hand resolved on
HBO (60' for 90 min).
15 36 2.8 38 Sharp arm pain 26 min in-flight. Progressed to
numbness. Decreased on descent. Resolved on
Table 6 except for shoulder soreness. Resolved
by next day.
0 NA
0 NA
60 55 1 60 20 1 60 20 14 10 9.1 39 Tingling, numbness, coldness & decreased
strength in arm occurred 11 hrs post-flight.
Resolved on Table 6.
15 39 2.6 40
Discomfort in R elbow 2.5 hrs in-flight
progressed in severity. After descent, tingling
and numbness in the R hand. Resolved on
Table 6.
0 NA
0 NA
D=depth; BT=bottom time; SI=surface interval, PFSI=pre flight surface interval
a – Dive profile rejected due to two post-dive DCS cases
b – 10 of these exposures were conducted with a 12 min dive time.
c – This subject also reported post-dive symptoms described in Case 11 of Appendix A.
... 7 It has been estimated that the incidence of DCS decreases as the PFSI increases and beyond 11 hours there appears to be no additional DCS risk after single no-stop dives and beyond 17 h after repetitive, no-stop dives. 8 Current guidelines suggest a minimum PFSI of 12 h after a single, no-stop dive, 18 h after multiple dives per day or multiple days of diving, whilst intervals substantially longer than 18 h are suggested after dives requiring mandatory decompression stops. [8][9][10] The steady increase in popularity of scuba diving has implied an increase in flights to and from tropical destinations and, as a consequence, the risk of DCS during the return flight may be increased. ...
... 8 Current guidelines suggest a minimum PFSI of 12 h after a single, no-stop dive, 18 h after multiple dives per day or multiple days of diving, whilst intervals substantially longer than 18 h are suggested after dives requiring mandatory decompression stops. [8][9][10] The steady increase in popularity of scuba diving has implied an increase in flights to and from tropical destinations and, as a consequence, the risk of DCS during the return flight may be increased. For this reason, we thought further research was due and well justified. ...
Inert gas accumulated after multiple recreational dives can generate tissue supersaturation and bubble formation when ambient pressure decreases. We hypothesized that this could happen even if divers respected the currently recommended 24-hour pre-flight surface interval (PFSI). We performed transthoracic echocardiography (TTE) on a group of 56 healthy scuba divers (39 male, 17 female) as follows: first echo - during the outgoing flight, no recent dives; second echo - before boarding the return flight, after a multiday diving week in the tropics and a 24-hour PFSI; third echo - during the return flight at 30, 60 and 90 minutes after take-off. TTE was also done after every dive during the week's diving. Divers were divided into three groups according to their 'bubble-proneness': non-bubblers, occasional bubblers and consistent bubblers. During the diving, 23 subjects never developed bubbles, 17 only occasionally and 16 subjects produced bubbles every day and after every dive. Bubbles on the return flight were observed in eight of the 56 divers (all from the 'bubblers' group). Two subjects who had the highest bubble scores during the diving were advised not to make the last dive (increasing their PFSI to approximately 36 hours), and did not demonstrate bubbles on the return flight. Even though a 24-hour PFSI is recommended on the basis of clinical trials showing a low risk of decompression sickness (DCS), the presence of venous gas bubbles in-flight in eight of 56 divers leads us to suspect that in real-life situations DCS risk after such a PFSI is not zero.
... Main causes for DCI have been documented to be very rapid ascent and lack of decompression stops [3]. Decompression stops can prevent decompression illness as these delay ascent to the surface and allow inert gases to be eliminated in dissolved form rather than as bubbles [4,5]. ...
Full-text available
A professional 55-year-old female experienced diver, who surfaced after the second dive, had a lucid interval before dropping Glasgow Coma Scale (GCS) to 3/15. She was admitted to intensive care unit and commenced on hyperbaric oxygen therapy. Her initial computed tomography of the head was normal but her magnetic resonance imaging of the brain at 48 hours showed extensive bilateral cortical watershed territory infarcts. She developed acute respiratory distress syndrome which resolved within a few days. Her GCS gradually improved from 3/15 to 6/15, was repatriated to United Kingdom after about 2 weeks of the insult and admitted to a tertiary care hospital where she had myoclonic seizures and was started on anti-epileptics. Then she was transferred to the Rehabilitation Medicine Ward of Leicester General Hospital, with GCS 14/15 with poor sitting balance, for her management and rehabilitation. She had weakness of right upper and lower limbs, dysarthria, neuropathic bilateral shoulder pains, pressure ulcer of left heel, bladder and bowel incontinence and cognitive issues. She improved to have significant neurological recovery within next 3 months, became ambulant independently and bladder and bowel continent. Her Barthel index (from 4 to 17), Montreal Cognitive Assessment Test, Adembrook Cognitive Examination and Berg Balance scale (from 33/56 to 44/56) improved significantly. Early diagnosis, treatment and rehabilitation can have a significant impact on the recovery of decompression illness. (Int Marit Health 2020; 71, 2: 105-108)
... The risk of DCS in flying after diving can be decreased by reduction of exposure or by elimination of inert gas before or during decompression with intermittent high-concentration oxygen breathing. [11] In future, drugs that modify bubble generation by improving endothelial function or reducing gas nuclei population could be used as possible candidates for preventing the occurrence of DCI, particularly in risky operational dives or in emergency situations such as submarine escape. [12] obsErvation Patients with DCS can dramatically improve or have complete resolution in musculoskeletal or neurological symptoms with just oxygen and rehydration. ...
Full-text available
Decompression illness (DCI) describes a syndrome complex caused by inert gas bubbles generated by an inappropriate rate of reduction in ambient environmental pressure or decompression. This “umbrella term” covers both traditional decompression sickness caused by in-situ bubble formation from dissolved inert gas and arterial gas embolism (AGE), in which alveolar gas or venous gas emboli (via shunts or by-passing pulmonary vessels) are introduced into arterial circulation. DCI occurs in divers, compressed air workers, aviators, and astronauts, but AGE could also arise from iatrogenic causes unrelated to decompression. A hundred years ago, serious manifestations and deaths were frequent in divers and caisson workers due to DCI, but they decreased greatly when decompression stops were introduced in diving practice. This review article is of interest to the doctors who face the dilemma of treating the rare syndrome of DCI that could present in the clinical spectrum ranging from itching and minor pain to severe neurological symptoms or other systemic pathology. The first aid lies in the administration of 100% oxygen, and definitive treatment is therapeutic recompression. With appropriate and adequate treatment, recovery is complete, but some severe cases may have lifelong residual deficits, even after extended and multiple recompressions.
... The chance of suffering DCS on any single dive is about 4 in 10,000 in warm water and 59 per 10,000 in cold water. 21 Risk factors for DCS are those that enhance the accumulation of inert gases in tissues or slow their release. These include advanced age, female sex, obesity, elevated arterial carbon dioxide tension, low water temperature, poor physical fitness, and the level of physical activity during the dive. ...
Context: As of 2015, more than 23 million scuba diver certifications have been issued across the globe. Given the popularity of scuba diving, it is incumbent on every physician to know and understand the specific medical hazards and conditions associated with scuba diving. Evidence acquisition: Sources were obtained from PubMed, MEDLINE, and EBSCO databases from 1956 onward and ranged from diverse fields including otologic reviews and wilderness medicine book chapters. Study design: Clinical review. Level of evidence: Level 5. Results: Otologic hazards can be categorized into barotrauma-related injuries or decompression sickness. Conclusion: When combined with a high index of suspicion, the physician can recognize these disorders and promptly initiate proper treatment of the potentially hazardous and irreversible conditions related to scuba diving.
... These prevention efforts include diving education and use of dive computers, oxygen-enriched-air, flying after diving guidelines, conservative diving practices, first-aid surface oxygen and banning emergency free ascent during training dives. [31][32][33][34][35][36] However, most fatalities occur due to causes other than DCS or AGE. 8,10 Behavioral interventions allow divers to actively participate in ensuring their own safety. ...
Background : Scuba diving mishaps, caused by equipment problems or human errors, increase the occurrence of injuries and fatalities while diving. Pre-dive checklists may mitigate mishaps. This study evaluated the effect of using a pre-dive checklist on the incidence of diving mishaps in recreational divers. Methods : A multi-location cluster-randomized trial with parallel groups and allocation concealment was conducted between 1 June and 17 August 2012. The participants had to be at least 18 years of age, permitted to dive by the dive operator and planning to dive on the day of participation. They were recruited at the pier and dive boats at four locations. The intervention group received a pre-dive checklist and post-dive log. The control group received a post-dive log only. The outcomes, self-reported major and minor mishaps, were prompted by a post-dive questionnaire. Mishap rates per 100 dives were compared using Poisson regression with generalized estimating equations. Intent-to-treat, per-protocol and marginal structural model analyses were conducted. Results : A total of 1043 divers (intervention = 617; control = 426) made 2041 dives, on 70 location-days (intervention = 40; control = 30) at four locations. Compared with the control group, the incidence of major mishaps decreased in the intervention group by 36%, minor mishaps by 26% and all mishaps by 32%. On average, there was one fewer mishap in every 25 intervention dives. Conclusions : In this trial, pre-dive checklist use prevented mishaps which could lead to injuries and fatalities. Pre-dive checklists can increase diving safety and their use should be promoted. Trial Registration : ID NCT01960738.
... Cross sectional surveys are used widely to measure the prevalence of diving-specific injuries within populations of divers. Prior to constructing this instrument, questionnaires used within WA (6) and overseas (22,32,38,53,96) were obtained, yet none was entirely satisfactory for use within WA in this instance. Each question within each questionnaire was considered for relevance to the present study and like variables were grouped according to type, for example, demography, injury history, etc. ...
Introduction: Divers are recommended to observe a pre-flight surface interval (PFSI) ≥ 24 hours before boarding a plane following a diving vacation. Decompression sickness (DCS) symptoms may occur during or post-flight. This study aimed to examine the adherence of PFSI ≥ 24 in vacationing divers, and if any perceived signs and symptoms of DCS during or after flight were experienced. Methods: An anonymous online survey was publicised through diving exhibitions and social media. Data included diver/diving demographics, PFSI before flight, flight details, and perceived signs and symptoms of DCS during or after flight. Results: Data from 316 divers were examined (31% female) with the age range 17-75 years (median 49). Divers recorded 4,356 dives in the week preceding the flight, range 1-36 (median 14). Overall, 251/316 (79%) respondents reported a PFSI of ≥ 24 hours. PFSIs of < 12 hours were reported by 6 respondents. Diagnosed and treated DCS developing during, and post flight was reported by 4 divers with PFSIs ≥ 24 hours and by 2 divers with PFSIs < 24 hours. Fifteen divers boarded a plane with perceived symptoms of DCS. Conclusions: These data suggest that most divers in this study observed the recommendations of a ≥ 24 hour PFSI with safe outcomes.
The Cockpit EnvironmentEffects of AccelerationPressure Oxygen Breathing and HypoxiaReferencesFurther ReadingUseful Web Sites
The only text to cover lung function assessment from first principles including methodology, reference values and interpretation New for this edition: -More illustrations to convey concepts clearly to the busy physician - Text completely re-written in a contemporary style: includes user-friendly equations and more diagrams - New material covering the latest advances in the treatment of lung function, including more on sleep-related disorders, a stronger clinical and practical bias and more on new techniques and equipment -Uses the standard Vancouver referencing system What the experts say: "I have always considered Dr Cotes' book the most authoritative book published on lung function. It is also the most comprehensive. "Dr Robert Crapo, Pulmonary Division,LDS Hospital, Salt Lake City, USA"I think I can fairly speak on behalf of staff in lung function departments the length and breadth of the country - that a sixth edition of Cotes would be gratefully received."Dr Brendan Cooper, Clinical Respiratory Scientist,Nottingham City Hospital.
Fifty subjects performed 106 simulated dives at a final ambient pressure of 0.7 at (3000 m above sea level). One hundred and forty-three subjects performed 278 actual controlled dives at altitudes 900-1700 m above sea level. From the experience of these dives, air-decompression tables for altitudes 0-3200 m above sea level were calculated. Tables up to 2000 m above sea level were tested on humans under wet conditions.
The direct decompression limits for a group of divers over a range of pressure-time air exposures was determined using ultrasonic detection of venous gas emboli (VGE). In addition to dry chamber exposures, ranging from 233 ft for 7 min to 25 ft for 720 min, we exposed six divers to open ocean dives at 165 ft for 10 min. Findings demonstrated a strong individual propensity to form VGE, correlating with susceptibility to bends. No bends developed without the prior detection of precordial VGE. The present concept of no problems after any period of time at 30 fsw was not confirmed. Isopleths of equal percentage occurrence of VGE were computed between 10 and 60%. Open ocean exposures increased the percentage of VGE and bends, when compared to dry chamber exposures. Limiting tissue half times computed from the 20% VGE isopleth suggested that saturation exposures are controlled by a greater sensitivity of the short-half-time tissues than previously appreciated, rather than by additionally extended half times.
The dose-response relationship for decompression magnitude and venous gas emboli (VGE) formation in humans was examined. Pressure exposures of 138, 150, and 164 kPa (12, 16, and 20.5 ft of seawater gauge pressure) were conducted in an underwater habitat for 48 h. The 111 human male volunteer subjects then ascended directly to the surface in less than 5 min and were monitored for VGE with a continuous-wave Doppler ultrasound device over the precordium or the subclavian veins at regular intervals for a 24-h period. No signs or symptoms consistent with decompression sickness occurred. However, a large incidence of VGE detection was noted. These data were combined with those from our previously reported experiments at higher pressures, and the data were fit to a Hill dose-response equation with nonlinear least-squares or maximum likelihood routines. Highly significant fits of precordial VGE incidences were obtained with the Hill equation (saturation depth pressure at which there is a 50% probability of detectable VGE [D(VGE)50] = 150 +/- 1.2 kPa). Subclavian monitoring increased the sensitivity of VGE detection and resulted in a leftward shift [D(VGE)50 = 135 +/- 2 kPa] of the best-fit curve. We conclude that the reduction in pressure necessary to produce bubbles in humans is much less than was previously thought; 50% of humans can be expected to generate endogenous bubbles after decompression from a steady-state pressure exposure of only 135 kPa (11 ft of seawater). This may have significant implications for decompression schedule formulation and for altitude exposures that are currently considered benign. These results also imply that endogenous bubbles arise from preexisting gas collections.
Recreational divers face a difficult choice when trying to select the appropriate surface interval between diving and flying. Differences in diving techniques and lack of readily available hyperbaric treatment make guidelines for commercial and military divers inappropriate for recreational divers. A literature review revealed that proposed surface intervals ranged from zero to 24 h, but few were human-tested. On 24 February 1989, the Undersea and Hyperbaric Medical Society (UHMS) Workshop formalized guidelines for recreational divers. Do not push the tables, i.e., do not go to the maximum exposures allowed by the tables. For no-decompression dives: (1) With less than 2 h total dive time (surface to surface) during the previous 48 h, divers should wait 12 h before flying; (2) With multiday, unlimited diving, wait 24 h before flying. Recreational divers should not make dives that require decompression stops, but if such dives should occur, delay flying for at least 24 h and, if possible, for 48 h. Divers with DCS symptoms should not fly, unless it is required to obtain hyperbaric treatment. The UHMS guideline is based on current scientific information and expert opinion, and is anticipated to be conservative, safe surface intervals for the vast majority of divers.
Before a decompression procedure is recommended for general use it is subjected to a limited number of human trial dives. Based on the trial, one attempts to reject unsafe procedures but accept those with a low incidence of decompression sickness (DCS). Binomial confidence regions are often so broad that even after 40 dives it may be impossible to distinguish between the possibility that the table being tested has a 0.6% risk of DCS and the possibility that it has a 17% risk. Our proposed alternative is to select some rule (e.g., one or more cases of DCS in 10 dives) for rejecting tables and to calculate the probabilities of accepting tables as a function of the probability of DCS. With such calculations we conclude that (a) generally one cannot reduce the risk of adopting unsafe tables without increasing the risk of rejecting safe ones unless one chooses to increase the number of test dives; (b) truncated sequential designs could reduce the number of dives required for testing by 15 to 20%; and (c) rules similar to the ones tested will always have a zone of indifference. Tables with a probability of DCS in this zone will be accepted or rejected with nearly equal frequency even if tested with hundreds of dives. The use of models describing the probability of DCS as a function of dive parameters should allow us to combine information from dives previously analyzed separately and perhaps to improve our selection of new tables to be tested.
Decompression venous gas bubbles were detected with the precordial Doppler utrasound technique in humans at simulated altitudes of 1,000-3,000 m 3 h after no-stage decompression dives to 15 or 39 m. Bubbles were detected at 3,000 m in a total of 60% of the subjects: in 90% after the 100-min shallow dives to 15 m with some bubbles present in the first minutes (mean onset 12 min), and in only 30% after the 10-min deeper dives to 39 m with later appearances of bubbles (mean onset 28 min). At both 2,000 and 1,000 m bubbles could also be detected, sometimes in the first minutes. The risk of decompression sickness must be considered high with the amount of gas bubbles found, even though only uncertain symptoms appeared in this study. Thus, a safe interval between ordinary SCUBA-diving and flying in airliners or general aviation aircraft seems to be more than 3 h.
Probabilistic models of the occurrence of decompression sickness (DCS) with instantaneous risk defined as the weighted sum of bubble volumes in each of three parallel-perfused gas exchange compartments were fit using likelihood maximization to the subset of the USN Primary Air and N2-O2 database [n = 2,383, mean P(DCS) = 5.8%] used in development of the USN LE1 probabilistic models. Bubble dynamics with one diffusible gas in each compartment were modeled using the Van Liew equations with the nucleonic bubble radius, compartmental volume, compartmental bulk N2 diffusivity, compartmental N2 solubility, and the N2 solubility in blood x compartmental blood flow as adjustable parameters. Models were also tested that included the effects of linear elastic resistance to bubble growth in one, two, or all three of the modeled compartments. Model performance about the training data and separate validation data was compared to results obtained about the same data using the LE1 probabilistic model, which was independently implemented from published descriptions. In the most successful bubble volume model, BVM(3), diffusion significantly slows bubble growth in one of the modeled compartments, whereas mechanical resistance to bubble growth substantially accelerates bubble resolution in all compartments. BVM(3) performed generally on a par with LE1, despite inclusion of 12 more adjustable parameters, and tended to provide more accurate incidence-only estimates of DCS probability than LE1, particularly for profiles in which high fractional O2 gas mixes are breathed. Values of many estimated BVM(3) parameters were outside of the physiologic range, indicating that the model emerged from optimization as a mathematical descriptor of processes beyond bubble formation and growth that also contribute to DCS outcomes. Although incomplete as a mechanistic description of DCS etiology, BVM(3) remains applicable to a wider variety of decompressions than LE1 and affords a conceptual framework for further refinements motivated by mechanistic principles.