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Ascent rate, age, maximal oxygen uptake, adiposity, and circulating venous bubbles after diving

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Decompression sickness in diving is recognized as a multifactorial phenomenon, depending on several factors, such as decompression rate and individual susceptibility. The Doppler ultrasonic detection of circulating venous bubbles after diving is considered a useful index for the safety of decompression because of the relationship between bubbles and decompression sickness risk. The aim of this study was to assess the effects of ascent rate, age, maximal oxygen uptake (VO(2 max)), and percent body fat on the production of bubbles after diving. Fifty male recreational divers performed two dives at 35 m during 25 min and then ascended in one case at 9 m/min and in the other case at 17 m/min. They performed the same decompression stops in the two cases. Twenty-eight divers were Doppler monitored at 10-min intervals, until 60 min after surfacing, and the data were analyzed by Wilcoxon signed-rank test to compare the effect of ascent rate on the kinetics of bubbles. Twenty-two divers were monitored 60 min after surfacing. The effect on bubble production 60 min after surfacing of the four variables was studied in 47 divers. The data were analyzed by multinomial log-linear model. The analysis showed that the 17 m/min ascent produced more elevated grades of bubbles than the 9 m/min ascent (P < 0.05), except at the 40-min interval, and showed relationships between grades of bubbles and ascent rate and age and interaction terms between VO(2 max) and age, as well as VO(2 max) and percent body fat. Younger, slimmer, or aerobically fitter divers produced fewer bubbles compared with older, fatter, or poorly physically fit divers. These findings and the conclusions of previous studies performed on animals and humans led us to support that ascent rate, age, aerobic fitness, and adiposity are factors of susceptibility for bubble formation after diving.
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doi: 10.1152/japplphysiol.00723.1999
93:1349-1356, 2002. First published 19 April 2002;J Appl Physiol
D. Carturan, A. Boussuges, P. Vanuxem, A. Bar-Hen, H. Burnet and B. Gardette
circulating venous bubbles after diving
Ascent rate, age, maximal oxygen uptake, adiposity, and
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Ascent rate, age, maximal oxygen uptake, adiposity,
and circulating venous bubbles after diving
D. CARTURAN,
1
A. BOUSSUGES,
2
P. VANUXEM,
3
A. BAR-HEN,
4
H. BURNET,
5
AND B. GARDETTE
6
1
Faculte´ des Sciences du Sport, Luminy, 13009 Marseille;
2
Service de Re´animation Me´dicale et
d’Hyperbarie, Hoˆpital Salvator, 13274 Marseille;
3
Laboratoire de Physiologie et de Pathologie
Respiratoire, Faculte´deMe´decine de Marseille, 13385 Marseille;
4
Universite´ Aix-Marseille III,
Faculte´ St. Je´roˆme, Institut Me´diterrane´en d’Ecologie et de Pale´oe´cologie, 13397 Marseille;
5
Centre
National de la Recherche Scientifique, Unite´ Propre de Recherche Neurobiologie et Mouvement,
13402 Marseille; and
6
Comex, Direction Scientifique, 13275 Marseille, France
Received 20 September 1999; accepted in final form 18 March 2002
Carturan, D., A. Boussuges, P. Vanuxem, A. Bar-Hen,
H. Burnet, and B. Gardette. Ascent rate, age, maximal
oxygen uptake, adiposity, and circulating venous bubbles
after diving. J Appl Physiol 93: 13491356, 2002. First pub-
lished April 19, 2002; 10.1152/japplphysiol.00723.1999.—
Decompression sickness in diving is recognized as a multi-
factorial phenomenon, depending on several factors, such as
decompression rate and individual susceptibility. The Dopp-
ler ultrasonic detection of circulating venous bubbles after
diving is considered a useful index for the safety of decom-
pression because of the relationship between bubbles and
decompression sickness risk. The aim of this study was to
assess the effects of ascent rate, age, maximal oxygen uptake
(V
˙
O
2 max
), and percent body fat on the production of bubbles
after diving. Fifty male recreational divers performed two
dives at 35 m during 25 min and then ascended in one case at
9 m/min and in the other case at 17 m/min. They performed
the same decompression stops in the two cases. Twenty-eight
divers were Doppler monitored at 10-min intervals, until 60
min after surfacing, and the data were analyzed by Wilcoxon
signed-rank test to compare the effect of ascent rate on the
kinetics of bubbles. Twenty-two divers were monitored 60
min after surfacing. The effect on bubble production 60 min
after surfacing of the four variables was studied in 47 divers.
The data were analyzed by multinomial log-linear model.
The analysis showed that the 17 m/min ascent produced
more elevated grades of bubbles than the 9 m/min ascent
(P 0.05), except at the 40-min interval, and showed rela-
tionships between grades of bubbles and ascent rate and age
and interaction terms between V
˙
O
2 max
and age, as well as
V
˙
O
2 max
and percent body fat. Younger, slimmer, or aerobi-
cally fitter divers produced fewer bubbles compared with
older, fatter, or poorly physically fit divers. These findings
and the conclusions of previous studies performed on animals
and humans led us to support that ascent rate, age, aerobic
fitness, and adiposity are factors of susceptibility for bubble
formation after diving.
susceptibility; bubble formation; scuba diving
THE MOST LIKELY FACTOR FOR initiating decompression
sickness (DCS) in scuba diving is believed to be the
formation of inert gas bubbles as a result of supersat-
uration of the dissolved gas in the tissues and blood.
Even if it is known that most decompressions pro-
duce gas bubbles in organisms, only intravascular bub-
bles with a 40- to 50-m diameter can be detected by
means of ultrasonic Doppler (12, 19). They are detected
in the venous circulation, most often in the precordial
region, and their sound signals are graded according to
a scale of assessment ranging from 0 to 4 (21, 40). For
more than 30 years, it has been evident that many
postdive decompressions produce Doppler-detectable
bubbles in humans (45). Their presence is not sufficient
to induce DCS, but it is known that DCS risk is linked
to the bubble grades (11, 14, 31, 32, 38, 40). They may
be indicative of bubbles elsewhere in the body, which
may be a cause of DCS (33). All of the studies have
shown that the incidence of DCS is very low for grade
0 (no bubbles) or grade 1 and that the DCS risk in-
creases when grade 2 or higher is observed, with DCS
being almost always accompanied by bubbles (38).
Bove et al. (5) have shown that circulatory alter-
ations due to air embolism (increase in right ventricu-
lar pressure, stasis in the azygos vein, and reduction in
nitrogen elimination) contributed to increase the risk
of spinal cord DCS in dogs. Moreover, retrospective
studies (4, 46) have emphasized an increased risk of
cerebral DCS in divers who had an intracardiac shunt
(patent foramen ovale). One may assume that this risk
is enhanced in case of numerous vascular bubbles.
Because of the association between bubbles and DCS
risk, it could be considered that the factors favoring the
formation of bubbles are also DCS risk factors and that
it is possible to use Doppler bubble detection as an
index of safety for diving and decompression profiles,
without any DCS symptom.
In animals, a fast decompression rate was demon-
strated to be a determining factor for DCS (34, 36, 39).
Address for reprint requests and other correspondence: D. Carturan,
La Prade, 26770 Le Pe`gue, France (E-mail: carturan@wanadoo.fr).
The costs of publication of this article were defrayed in part by the
payment of page charges. The article must therefore be hereby
marked ‘‘advertisement’’ in accordance with 18 U.S.C. Section 1734
solely to indicate this fact.
J Appl Physiol 93: 13491356, 2002.
First published April 19, 2002; 10.1152/japplphysiol.00723.1999.
8750-7587/02 $5.00 Copyright
©
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It has been reported in animals and men that a slow
ascent induced less venous gas bubbles than a fast
ascent (26, 39). Thus there are some arguments to
hypothesize that, in men, a fast ascent may be respon-
sible for an increased bubble production and then for
an increased DCS hazard.
In the different decompression tables, ascent rate up
to decompression stops has been assessed arbitrarily
and then modied by empiricism and intuition. The 18
m/min ascent rate of the US Navy (USN) air tables was
for a long time considered as a standard from which the
ascent rate of the other decompression tables was de-
termined (23). Now, the ascent rate of the USN tables
has been reduced since 1993 to 10 m/min. In the case of
the French tables, the recommended ascent rate varies
from 9 m/min (1992 French Labour Ministry) to 17
m/min (1990 French Navy tables). The decompression
tables used in scuba diving are based on the Haldane
concepts (6) and generally do not take into account the
individual susceptibility to bubble formation or DCS;
they are elaborated for all of the divers without dis-
tinction of age or physical tness. Often, recreational
divers use military tables that are made for young and
trained men (1, 29).
The incidence of individual factors such as age and
adiposity on DCS occurrence and bubble formation is
recognized (810, 16, 22, 41). In the present practice of
diving, it is known that physical tness is a determin-
ing biological factor for DCS hazard, even if few studies
have shown its statistical incidence. Gardette (14) has
shown that divers who were bubble producers had
lower scores on the Rufer index (37) than no bubble
producers. Rattner et al. (35) reported that rats aero-
bically trained had a signicantly reduced risk of DCS
after hyperbaric exposure. Broome et al. (7) subjected
pigs to an aerobic training and found a signicant
reduction of the rate of DCS in the conditioned pigs,
compared with the control sample. When various do-
mestic, nonprimate animals are compared with hu-
mans, the pig seems the most physiologically similar,
and several authors have considered it to be quite a
good model for decompression in humans (17, 27, 42).
We have tried to conrm, using bubble Doppler de-
tection, the ndings of previous studies carried out on
animals and men concerning the factors most currently
mentioned in the literature as favoring bubble produc-
tion and/or DCS risk. For ethical reasons, we chose to
limit our study to factors whose assessment is not
invasive, i.e., ascent rate, age, aerobic tness, and
adiposity.
METHODS
We have carried out a Doppler monitoring of postdive
decompression in a sample of 50 male sport divers who were
medically t to dive (mean age: 37 9.6 yr, mean weight:
80 10.7 kg, mean height: 177 5.7 cm) and who gave
informed consent in accordance with the French law about
biomedical research. They performed two dives in open water
at 35 m for 25 min. The dives were performed on a at,
regular bottom; the descent time (30 s) was included in the
dive time. One of the two dives was followed by an ascent at
9 m/min (1992 French Labour Ministry tables); the other one
was followed by an ascent at 17 m/min (1990 French Navy
tables). The ascent rate was linear and controlled by one
investigator who performed all of the dives himself. The
control was made by using a chronometer, a depth meter, and
a dive computer equipped with a bar graph of ascent (Mae-
stro Pro Beuchat). Ascent time to the rst decompression
stop was 195 s for the 9-m ascent and 102 s for the 17-m
ascent (time difference 93 s).
In the two cases, the divers performed the decompression
stops of the 1992 French Labour Ministry Tables: 3 min at
6 m and 15 min at 3 m. The prole of the dives is represented
by Fig. 1.
The investigators randomly imposed the order of the two
dives. The dives were performed with a minimal 24-h inter-
val. Only three divers performed their dives with this inter-
val; all of the others performed their two dives with an
interval between 3 and 7 days. The ascent rate of the two
dives was not determined with a preferential order. Some
divers performed rst the ascent at 9 m/min; the others
performed the ascent at 17 m/min. The temperature of the
water varied during the period of the study from 15 to 20°C
at the surface and from 12 to 16°C on the bottom. The divers
were equipped with neoprene diving suits whose thickness
was in accordance with the temperature of the water, and
none reported suffering from cold. Before, during, and after
the dives, the subjects were required to avoid excessive ex-
ercise. After surfacing, venous gas emboli were detected by
using ultrasonic Doppler. The divers did not take a warm
shower before the end of the bubble detection, in order not to
bias the results.
Bubble Detection
Bubble detection was performed with a continuous-wave
Doppler apparatus (DUG, Sodelec), equipped with a 5-MHz
probe. The subjects were placed in left lateral decubitus, and
they laid at rest 1 min before the beginning of the detections.
Unlike experimental studies in hyperbaric chambers, our
subjects had to exercise in accordance with actual conditions
of diving, i.e., to equip themselves, swim, come back to the
diving boat, take off, and tidy their equipment. They were
affected by the effects of cold and immersion: diuresis,
hemoconcentration, change in the repartition of the blood
mass, and dehydration. Thus their level of bubble release
Fig. 1. Proles of the dives.
1350 DECOMPRESSION AND BUBBLES
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corresponded to the actual exercise of all divers respecting
the safety rule: little exercise before, during, and after diving.
Consequently, we did not monitor bubbles after movement (3
deep knee bends), as it is usually performed in the protocols
of the studies conducted in hyperbaric chambers, when the
subjects are really at rest. The probe was placed in the
precordial site, with the ultrasonic wave being directed into
the pulmonary infundibulum. The signals were tape recorded
for 2 min and graded in a blind manner by two experienced
investigators, according to the Spencer Doppler code (40).
Bubble signals were classied on a scale from 0 to 4, on the
basis of the number of bubble signals per cardiac cycle and
the number of cardiac cycles containing bubbles. If any dis-
crepancy in the interpretation of the signals occurred, the
recordings were studied again to reach a consensus. Before
the dives, we performed a tape recording of their cardiac
signal to have a reference.
Twenty-eight divers were Doppler monitored on the diving
boat, at 10-min intervals, until 60 min after surfacing,
whereas 22 divers were monitored on the ground 60 4 min
(30 s) after surfacing.
Assessment of the Individual Factors
Aerobic tness was assessed by the maximal exercise test
to assess the maximal oxygen uptake (V
˙
O
2 max
). To assess
the adiposity, we used the percent body fat (PBF).
V
˙
O
2 max
. The maximal exercise test was performed in the
Laboratory of Respiratory Function and Muscular Metabo-
lism Exploration of the Sainte Marguerite Hospital (Mar-
seilles).
Protocol. Exercise was performed on a cycle ergometer (ER
900 Jaeger). Testing sessions took place in the morning, 2 or
3 h after breakfast. Ambient temperature ranged from 21 to
23°C.
After a 2-min warm-up period, the load was increased by
20 W every 2 min. The test was stopped in case of exhaustion,
excessive hypertensive reaction (diastolic pressure 10
mmHg, systolic pressure 25 mmHg), or when the subject
presented two of the three accepted criteria of maximal
exertion: 1) maximal theoretical heart rate according to the
Astrand formula, 220 beats age (in yr); 2) respiratory ratio
1.05; and 3) no increase of oxygen uptake (V
˙
O
2
), although
the minute respiratory ow kept increasing.
The following parameters were measured (BG Electro-
lytes, Instrumentation Laboratory): 1)V
˙
O
2
(STPD l/min and
ml min
1
kg body wt
1
); 2) minute expiratory volume (BTPS
l/min); 3) expired carbon dioxide (V
˙
CO
2
STPD ml/min), respi
-
ratory ratio (R V
˙
CO
2
/V
˙
O
2
); and 4) respiratory frequency.
The averages corresponding to the 1-min intervals were
displayed, and the average of the second minute of each load
level was retained for the study. Heart rate was recorded by
means of standard electrocardiogram derivations during ex-
ertion and recovery periods. Systemic arterial blood pressure
was measured at the end of each load level (Sphygmanomet-
rie).
Measure of the PBF. PBF was measured by electrical
impedance with a Bodystat 1500 apparatus. This process is
more accurate and reproducible than the classical process of
skinfold thickness (18). Before the test, the subjects had not
eaten for at least 4 h and had to urinate. They were placed in
dorsal decubitus.
Statistical Analysis
Statistical analysis was composed of two parts. In the rst
analysis, we compared the effects of the two ascents on the
kinetics of the bubble scores obtained at 10-min intervals
until 60 min after surfacing in the group of 28 divers. In the
second analysis, we analyzed the effects of the variables,
ascent rate, age, V
˙
O
2 max
, and PBF, on the bubble grades
detected 60 min after surfacing, on the whole sample of 47
divers.
Effect of ascent rate on bubble kinetics. The Doppler scores
were compared in the slow ascent vs. fast ascent at each
period of measurement, i.e., 10, 20, 30 min, etc., until 60 min,
in the group of 28 divers, using a paired test, i.e., Wilcoxon
matched-pairs signed-rank test. The P values obtained were
then adjusted and corrected according to Holms step-down
procedure (25). One may notice that, before 60 min, many
bubble scores are not affected, and, therefore, these values
are eliminated from the analysis. This effect on the sample
sizes and the conclusions of the test concern only divers
presenting differences in bubble scores for the different as-
cent rates.
Effect of ascent rate and individual variables on bubble
production. First, a multinomial log-linear model was used to
study the relationship between bubble scores and the four
variables. In addition, the potential for interaction among
these variables was also considered by the formulation of a
multinomial log-linear model. The interactions eligible for
inclusion were determined by examining the Akaike Infor-
mation Criterion (AIC). It should be emphasized that lower
values of AIC indicate a best tting model (2). Wilcoxon tests
were computed with Sigmastat (Jandel Scientic), and multi-
nomial log-linear models were done with S-Plus (Ref. 44,
section 7.3). We used the multinom function of the nnet
library. A Poisson model could be tted, but, in this analysis,
the number of individuals is xed. This condition on the
marginals of the Poisson model leads to a multinomial model.
Because the bubble score is an ordinal factor, it is also
possible to try a proportional odds model. In fact, results (in
term of AIC) are not good (and not presented here). A rea-
sonable explanation is the inadequacy of the bubble scale for
the proportional odds model (see Ref. 13, section 3, for details
on generalized linear models).
RESULTS
None of the divers had DCS.
Effect of Ascent Rate on Bubble Kinetics
The Doppler signals, graded according to the Spen-
cer scale, are shown in Table 1. The bubble grades were
signicantly increased after the 17 m/min ascent com-
pared with the 9 m/min ascent at each measurement
interval, except at the 40-min interval (Table 1). Per-
centages of increased bubble scores after fast ascent
are as follows: 10 min, 50% (14 of 28 dives); 20 min,
39.3% (11/28); 30 min, 32.14% (9/28); 40 min, 25%
(7/28); 50 min, 50% (14/28); and 60 min, 53.57% (15/28).
Effect of Ascent Rate and Individual Variables on
Bubble Production
Table 2 presents the raw data to be used in the
predictive model of bubble scores: ascent rate, age,
V
˙
O
2 max
, and PBF. Table 3
presents estimates in the
main effects model, along with their estimated stan-
dard error. AIC and residual deviance for this model
are also indicated. The results of the multinomial log-
linear model, i.e., including interactions between co-
variates, are presented in Table 4.
1351
DECOMPRESSION AND BUBBLES
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The difference of AIC criterion between the model
with and without the variable follows approximately a
2
distribution. This is a classic way to test that a
variable has a signicant effect on the model (see Ref.
44, for example). It is also possible to test a particular
coefcient by noting that asymptotically a coefcient
divided by the standard error follows a normal law.
These tests are not very interesting because coef-
cients are not independent and the level would be
strongly affected.
To give a graphic description of the data, we have
divided our sample of divers on both sides of the me-
dian of the variables, and we have realized histograms
for age (Fig. 2), V
˙
O
2 max
(Fig. 3),
and PBF (Fig. 4).
DISCUSSION
We emphasize that our study was conducted in ac-
tual diving conditions, because more bubbling has been
reported when the dives were performed in open water
rather than when they were performed in hyperbaric
chambers (15). The rst part of the statistical analysis
has shown a signicant relationship between a slow
ascent and decreased bubble production. As ambient
pressure decreases faster at 17 m/min than at 9 m/min,
the increase in the gas pressure gradient between
tissues and alveoli is greater as well; it may then reach
the critical ratio pressure for nucleation and promote
bubble formation. The ascent at 9 m/min was 93 s
Table 1. Grades of bubbles detected at 10-min intervals
Diver No.
Interval, min
10 20 30 40 50 60
Ascent rate, m/min
917917917917917917
1 010111110100
2 010222121212
3 000112221100
4 011112221101
5 022233332312
6 233333333333
7 344444444434
8 000101000000
9 223333331303
10 233333333323
11 010122111200
12 010211110100
13 101112120201
14 000112110000
15 000011121100
16 000011010100
17 223333333323
18 122233333323
19 111333332313
20 333334443434
21 010112120101
22 333333333333
23 102122231302
24 333333333333
25 233333332313
26 122323333333
27 131313131302
28 212121101000
P 0.011 0.0104 0.0196 0.0546 0.0008 0.0002
P* 0.033 0.0416 0.0392 0.0546 0.004 0.0012
P 0.0416 0.0416 0.0416 0.0546 0.004 0.0012
SSSNSSS
P values are Wilcoxons P values. *Holm adjusted P values: Wilc-
oxons P values were arranged in increasing order. The rst P value
was calculated as (Wilcoxons P) (no. of intervals), i.e., 6. The second
P value as (Wilcoxons P) (no. of intervals - 1), i.e., 5, and so on until
all intervals have been compared. Corrected P values in order to
maintain the monotonicity of ranking. If a P value was lower than
the previous one, then this P value was declared to be the same as
that for the preceding comparison. S, signicant; NS, not signicant.
Table 2. Individual factors and Doppler scores
Diver No. Age, yr
V
˙
O
2max
,
ml min
1
kg
1
PBF, %
Bubble
Score
Ascent rate,
m/min
917
1 47 53 18.1 0 1
2 54 31.5 21.3 3 3
3 47 16.4 16.2 2 3
4 30 40.6 17.5 0 0
5 26 41.2 13.4 0 1
6 32 40.8 10.8 1 1
7 54 19.7 16.1 0 0
8 45 19.8 17.2 3 3
9 48 20.2 26.3 1 2
10 19 56 6.4 1 0
11 41 21.4 24.3 1 3
12 37 23.3 21.6 3 3
13 26 8.4 0 0
14 47 31.5 19 1 3
15 26 40 18.5 0 2
16 46 26.9 20.6 3 3
17 26 44 7.4 0 0
18 20 52.4 4.3 0 0
19 26 43.7 3.9 0 0
20 23 47.1 11.5 0 0
21 46 25.5 18.1 1 2
22 31 35 14.1 0 0
23 22 43.5 20.6 0 1
24 36 60.1 12.4 1 2
25 44 17.7 3 3
26 33 21.1 4 4
27 27 56 11.6 0 0
28 37 33.9 13.2 0 3
29 46 45.6 11.8 2 3
30 30 40.7 24.9 0 0
31 37 36 11.5 0 0
32 49 52.6 15.3 0 1
33 25 52.2 14 0 0
34 30 40.7 18.9 0 0
35 32 43.9 18.3 0 0
36 48 40.3 18.7 2 3
37 30 31 20.2 2 3
38 36 45 12.8 1 3
39 42 38.3 22 3 4
40 47 46.3 12.6 0 1
41 49 32 16.6 3 3
42 33 39.5 19 0 2
43 39 36.7 18.3 3 3
44 46 37.6 18.5 3 3
45 38 41.2 20 0 1
46 43 53 17 0 2
47 43 37.2 19.6 0 0
V
˙
O
2max
, maximal O
2
uptake; PBF, percent body fat. Average val
-
ues: age 37 9.6 yr; V
˙
O
2max
38.9 10.8 ml min
1
kg
1
; PBF
16.2 5.2%.
1352 DECOMPRESSION AND BUBBLES
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longer than the ascent at 17 m/min; this seems to have
been sufcient to reduce bubble formation and thus
DCS risk. This is consistent with previous studies on
decompression (see Introduction). The analysis of the
bubble kinetics (Table 1) has shown that the signi-
cance of ascent rate was higher from 50 min after
surfacing. This could mean an enhanced safety for the
divers in case of repetitive dives because of a reduced
gas load and a reduced, preexisting gas phase in the
interval.
In the second part of the statistical analysis, accord-
ing to the AIC, the best t was obtained with the
interactive model rather than with the main effects
model. The best tting model includes the four covari-
ates and interaction terms between V
˙
O
2 max
and age as
well as V
˙
O
2 max
and PBF. It should be emphasized that
one implication of the main effects model is that the
effects of any variable do not depend on the values
assumed by the other variables. Thus, had one not ta
model with interaction terms, the effect of age and PBF
on V
˙
O
2 max
would have been missed. Such interactions
are not surprising.
Indeed, it is well known that V
˙
O
2 max
decreases with
increasing age, and generally adiposity is associated
with poor physical tness. Many studies have con-
cluded that aerobic tness and adiposity were among
the possible causes of individual susceptibility (see the
introduction), but, so far, no data have shown a rela-
tionship for men between V
˙
O
2 max
and bubbles.
For Mebane and McIver (28), obesity and poor phys-
ical condition generally coexist and represent a hazard
to the divers. Obesity is widely recognized as a DCS
risk factor because of the high solubility of nitrogen in
lipids. Dembert et al. (10) found signicantly higher
measures of weight and skinfold thickness in USN
divers who experienced DCS compared with those who
remained free of DCS. However, they did not nd an
association of DCS and age in studies of military
divers. Broome et al. (7) commented on these ndings,
hypothesizing that, in military diving units, both se-
nior and junior divers are required to maintain a high
level of aerobic tness. For Lam and Yau (22), DCS
susceptibility is increased by age, but this might be due
to an increase in adiposity because of age. If age and
obesity are widely mentioned in the literature as DCS
risk factors, there are few data about aerobic tness.
Curley et al. (9) suggested that the importance of
obesity as a risk factor for DCS may be overstated.
Furthermore, Broome et al. (7) hypothesized that, in
epidemiological studies in which body weight has been
associated with increased DCS risk, the underlying
association was, in fact, with poor aerobic tness, for
which being overweight or relatively obese was a sur-
rogate indicator. They have shown that aerobic exer-
cise reduces the risk of DCS in swine, regardless of age,
adiposity, and weight. Vann (43) had reported that
aerobically trained runners appeared to be at lower
risk for venous bubbling and bends than weight lifters
or sedentary subjects. Rattner et al. (35) speculated
that increased capillary density of muscle, as a result
of training, might explain the decreased DCS rate
observed in their treadmill-exercised rats. McKirnan et
al. (27) have reported a reduction of 20% in cerebral
blood ow at rest, in a sample of exercise conditioned
pigs, compared with the untrained control pigs. Thus,
Broome et al. (7) hypothesized that such changes in
cerebral blood ow were representative of proportional
changes in the central nervous system blood ow gen-
Table 3. Parameter estimates of multinomial
log-linear model: main effects model
Grade Intercept Age V
˙
O
2
PBF Ascent Rate
Coefcients
1 3.992 0.069 0.005 0.029 0.277
2 5.401 0.093 0.022 0.095 0.648
3 4.319 0.134 0.073 0.080 0.877
4 38.842 0.228 0.146 0.823 5.458
Standard errors
1 2.718 0.039 0.038 0.078 0.324
2 3.606 0.048 0.045 0.103 0.397
3 3.229 0.046 0.039 0.092 0.356
4 13.504 0.203 0.201 0.674 14.899
Residual deviance 186.00
AIC 226.00
A log-linear model is tted, with coefcients zero for the rst class.
V
˙
O
2
,O
2
uptake; AIC, Akaike Information Criterion.
Table 4. Parameter estimates of multinomial log-linear model: interactive model
Grade Intercept Age V
˙
O
2
PBF Ascent Rate Age V
˙
O
2
V
˚
O
2
PBF
Coefcients
1 17.232 0.400 0.276 2.073 0.429 0.012 0.049
2 14.804 0.368 0.183 1.914 0.800 0.012 0.044
3 13.754 0.750 0.644 1.462 1.094 0.024 0.032
4 16.460 43.944 102.162 49.688 23.909 1.921 1.827
Standard errors
1 0.028 0.228 0.031 0.608 0.356 0.006 0.015
2 0.018 0.224 0.049 0.609 0.424 0.006 0.015
3 0.015 0.208 0.084 0.589 0.396 0.006 0.015
4 0.003 0.216 0.061 0.099 0.003 0.007 0.011
Residual deviance 157.00
AIC 213.00
A log-linear model is tted, with coefcients zero for the rst class.
1353DECOMPRESSION AND BUBBLES
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Fig. 2. Grades of bubbles according to age and ascent
rate. Left: age 37 yr; right: age 37 yr. Higher grades
of bubbles are associated with elevated age and faster
ascent.
Fig. 3. Grades of bubbles according to maxi-
mal O
2
uptake (V
˙
O
2max
) and ascent rate. Left:
V
˙
O
2max
40 ml min
1
kg
1
; right:V
˙
O
2max
40
ml min
1
kg
1
. Higher grades of bubbles are
associated with poor aerobic tness and faster
ascent.
Fig. 4. Grades of bubbles according to percent
body fat (PBF) and ascent rate. Left: PBF
17.5%; right: PBF 17.5%. Higher grades of
bubbles are associated with higher PBF and
faster ascent.
1354 DECOMPRESSION AND BUBBLES
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erally and thus also in spinal blood ow. They assumed
that reduced spinal cord blood ow would lessen the
gas uptake by spinal cord tissue and reduce the risk of
neurological DCS. Moreover, they assumed that the
rheological effects of aerobic training [a rise in plasma
volume and fall in hematocrit (3, 24)] might explain
this reduction of the DCS rate in aerobically trained
divers.
Thus we can support that age and the combination of
aerobic tness and adiposity take a part in the bubble
formation process and that the divers would be well
advised to keep physically t and slim by aerobic train-
ing. Our conclusion is consistent with Broome et al. (7),
who underlined that an individual could manipulate
his personal risk by being aerobically torunt.
Moreover, our ndings suggest that elderly, poorly t,
and fat divers could reduce their bubble production and
then their DCS risk on the one hand by improving their
physical tness and on the other hand by ascending
slowly. Obviously, the presence of bubbles does not
explain the whole occurrence of DCS, and some au-
thors have reported an adaptation to decompression
stress that seems to be a subsequent response of the
immune system (20). Nevertheless, it is widely recog-
nized that, because of the linkage between DCS and
bubbles, it is in the divers best interest to prevent as
much bubble formation as possible. As Moon et al.
wrote (30): the probabilistic models on which tables
and computers are based should reect the individual
reality of the divers, to enable them to conduct their
dives in accordance with their individual characteris-
tics.
It would be of interest to conduct further studies in
men to check the effect on bubbles of an aerobic train-
ing that would make V
˙
O
2 max
increase and PBF de
-
crease and, in this way, verify whether it is possible to
lighten the effects of age on bubble susceptibility.
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... From the early use of Doppler in the 1970s to more recent echocardiography studies, it is well-established that there exists large variability in VGE loads not only for different dive profiles but also between subjects and for the same subject undergoing the same controlled dive profile. [10][11][12] Additionally, the time course of VGE varies significantly post-dive, so that regular monitoring intervals are paramount for correct quantification. [12][13][14] As such, continuous ultrasound monitoring could provide a more accurate postdive assessment. ...
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