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UHM 2007, Vol. 34, No. 6 – Decompression deep stop times
Effect of varying deep stop times and shallow
stop times on precordial bubbles after dives
to 25 msw (82 fsw).
Submitted 10/24/06; Accepted 5/15/07
P.B. BENNETT
1
, A. MARRONI
2,3
, F.J. CRONJE
4
, R. CALI-CORLEO
2,3
, P. GERMONPRE
2,5
, M.
PIERI
2
, C. BONUCCELLI
2
, M.G. LEONARDI
2
, C. BALESTRA
2,6
1
Duke University Medical Center,
2,3
DAN Europe Foundation Research Division, Division of Baromedicine, University of
Baromedicine, University of Malta Medical School,
4
DAN Southern Africa,
5
Center for Hyperbaric Oxygen Therapy, Military
Hospital, Bruxelles,
6
Haute Ecole, Paul Henri Spaak, Occupational and Environmental Physiology Department, Bruxelles,
Belgium
Bennett PB, Marroni A, Cronje FJ, Cali-Corleo R, Germonpre P, Pieri M, Bonuccelli C, Leonardi MG,
Balestra C. Effect of varying deep stop times and shallow stop times on precordial bubbles after dives to 25
msw (82 fsw). Undersea Hyperb Med 2007; 34(6):399-406. In our previous research, a deep 5-min stop at
15 msw (50 fsw), in addition to the typical 3-5 min shallow stop, significantly reduced precordial Doppler
detectable bubbles (PDDB) and “fast” tissue compartment gas tensions during decompression from a 25 msw
(82 fsw) dive; the optimal ascent rate was 10 msw (30 fsw/min). Since publication of these results, several
recreational diving agencies have recommended empirical stop times shorter than the 5 min stops that we
used, stops of as little as 1 min (deep) and 2 min (shallow). In our present study, we clarified the optimal time
for stops by measuring PDDB with several combinations of deep and shallow stop times following single and
repetitive open-water dives to 25 msw (82 fsw) for 25 mins and 20 minutes respectively; ascent rate was 10
msw/min (33 fsw). Among 15 profiles, stop time ranged from 1 to 10 min for both the deep stops (15 msw/50
fsw) and the shallow stops (6 msw/20 fsw). Dives with 2 ½ min deep stops yielded the lowest PDDB scores
– shorter or longer deep stops were less effective in reducing PDDB. The results confirm that a deep stop of 1
min is too short – it produced the highest PDDB scores of all the dives. We also evaluated shallow stop times
of 5, 4, 3, 2 and 1 min while keeping a fixed time of 2.5 min for the deep stop; increased times up to 10 min at
the shallow stop did not further reduce PDDB. While our findings cannot be extrapolated beyond these dive
profiles without further study, we recommend a deep stop of at least 2 ½ mins at 15 msw (50 fsw) in addition
to the customary 6 msw (20 fsw) for 3-5 mins for 25 meter dives of 20 to 25 minutes to reduce PDDB.
INTRODUCTION
Recent research in divers indicates that a
deep stop, during decompression from a 25 msw
(82 fsw) dive, significantly reduces precordial
Doppler detectable bubbles (PDDB) and “fast”
tissue compartment (5 min, 10 min, 20 min) gas
tensions (1-3). This research showed that the
introduction of a deep stop at 15 msw (50 fsw),
in addition to the conventional 3-5 min shallow
stop at 3-5 msw (10-15 fsw), significantly
reduced or eliminated PDDB over a 90 minute
period after surfacing. The results suggest that
supersaturation of “fast tissue compartments”
(e.g. 5, 10 and 20 min compartments) may be
responsible for the predominantly neurological
forms of decompression illness (DCI) reported
in recreational scuba divers. This may be
perhaps due to inadequate gas elimination from
the spinal cord with its “fast” half time of 12.5
mins rather than the slower compartments used
in many decompression algorithms.
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UHM 2007, Vol. 34, No. 6 – Decompression deep stop times
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Most decompression computer
algorithms and dive tables used by recreational
divers today have their foundations in the
original ideas of Haldane or Hill (4, 5).
Haldane modeled gas uptake and elimination
on 5 exponentials of “fast” to “slow” tissue
compartments, i.e. with 5, 10, 20, 40 and 75
min half times. Later this was increased
by Buehlmann to as many as 16 or 8 tissue
compartments (6, 7). The common premise
was that as long as none of these compartments
became supersaturated beyond a certain critical
threshold, decompression sickness (DCS)
could be avoided. Haldane also introduced the
concept that it was safe to come from 6 ATA
to 3ATA as it is from 4 ATA to 2 ATA etc. or
a 2:1 ratio of absolute depth. This ratio of ½
the absolute depth was gradually modified over
past decades, and now ranges from 4 to 1 for
fast tissue compartments to less than 2 to 1 for
slow tissue compartments.
An unforeseen consequence of these
modifications to prevent DCS is that an
important feature of Haldane’s 1906 proposal
- staged decompression at ½ the absolute depth
- has now become lost in the most common
forms of recreational diving. Instead a linear
ascent at 10 m (30 fsw)/min is common with a
“safety” stop at 3-5 msw (10-15 fsw). However,
a comparison between the Haldane proposal
for decompression (4) and linear ascent of Hill
(5) in the early last century was in favor of the
Haldane method rather than linear ascent.
In our recent research (3), the
introduction
of a deep stop at ½ the absolute
depth appears to significantly decrease PDDB.
Spencer 3 and 4 bubble grades are, in many
cases, reduced to zero. In this paper (3)
the optimal method for reducing post-dive
bubble production and tissue compartment
supersaturation during ascent is the combination
of an ascent rate of 10 msw (30 fsw/min) with
a 5 min deep stop at 15 msw (50 fsw) and a 5
minute shallow stop at 6 msw (20 fsw).
Given the finding that the deep stop
prevents the formation of gas bubbles during the
initial part of the ascent, the subsequent shallow
stop could possibly be shorter. Presumably,
a shorter deep stop also may be sufficient to
eliminate PDDB. It was therefore proposed that
both stops may be shortened and still reduce
PDDB. Several recreational diving agencies
have empirically recommended stopping for as
little as 1 min deep and 2 mins at 6 msw (20
fsw). This is considerably shorter than the 5
mins stops in the recent research protocol (3)
and may not be sufficient. The objective of the
present research is to vary the times for deep
and shallow stops to determine the optimal stop
times as evidenced by the least occurrence of
PDDB.
METHODS
209 Open Water dives were made to 25
msw (82 fsw) for 25 min by 14 volunteer Italian
recreational scuba divers (Sub Novara Laghi)
in the same manner as previously reported
(3). A total of 15 different dive protocols
were followed with varying times at the deep
and shallow stops. Some of the dives were to
25 msw (82 fsw) for 20 min following a first
dive to 25 msw (82 fsw) for a 3.5 hr surface
interval (indicated by asterisks in the tables).
The divers wore their own computers to record
the depth and times of each dive. They also
wore “blacked out” UWATEC dive computers
(sampling time 20 seconds) so recorded data
could not be seen by them. These were used
to permit analysis of the predicted gas tissue
compartment saturations for the various profiles
and to confirm accurate depth and time profiles
for the dives.
An Oxford Instruments 3.5 MHz
Doppler probe with a digital recorder was used
to make precordial Doppler bubble recordings.
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UHM 2007, Vol. 34, No. 6 – Decompression deep stop times
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Measurements were made by specially trained
members of the diver teams. Recordings,
as previously (3), were made with the diver
standing, at rest for thirty seconds and again
for thirty seconds after performing two knee
bends. At 15 min intervals a total of six 1- min
recordings were made over a total period of 90
mins post dive. Later the recordings were all
analyzed for the presence of bubble signals by
a single experienced and blinded researcher.
The presence of PDDB was graded
according to three scales as previously
described (3): a simplified Doppler Bubble
Grading System (DBGS); the Spencer scale
(SS); and our modification of the Spencer Scale
(the Expanded Spencer Scale or ESS):
• LBG – Low Bubble Grade: occasional
bubble signals, Doppler bubble Grades
(DBG) lower than 2 in the Spencer
Scale
• HBG – High Bubble Grade: frequent to
continuous bubble signals, DBG 2 and
higher in the Spencer Scale
• HBG + - Very High Bubble Grade:
bubble signals reaching grade 3 in the
SS and 2.5 in the Expanded Spencer
Scale (see below)
Expanded Spencer Scale
The original Spencer Scale was adapted
by introducing “half grades” to allow a
more incremental grading:
• Grade 0 = No bubble signals
• Grade 0.5 = 1 to 2 sporadic bubble
signals over the 1 min recording
• Grade
1 = up to 5 bubble signals
over the 1 min recording
• Grade
1.5 = up to 15 bubble signals
over the 1 min recording, with
bubble showers
• Grade
2 = up to 30 bubble signals
over the 1 min recording
• Grade
2.5 = more than 30 bubble
signals over the 1 min recording,
with bubble showers
• Grade
3 = virtually continuous
bubble signals over the 1 min
recording
• Grade
3.5 = continuous bubble
signals over the 1 min recording,
with numerous bubble showers
• Grade
4 = continuous bubble
signals over the 1 min recording,
with continuous bubble showers
To determine a relative index of
decompression stress, a “Bubble Score Index
– BSI” was calculated for each “Dive plus
Repetitive Dive” experimental profile
♣
.
Doppler readings from the participants were
classified and recorded according to both
SS and ESS systems. The six recordings for
each diver were then added and divided by the
number of participating volunteer divers for
each profile to generate a mean score. Only 6
out of the 1,254 Doppler recordings could not
be interpreted adequately and these were not
included in the analysis.
While some authors have reported
concerns with a score based on medians (8, 9),
we concur with several others, who support this
method (10, 11). The Fisher’s exact test was
then applied to the HBG and LBG occurrences
to test the difference between proportions and
the method of small p-values has been applied
to calculate the two-sided p-value in analyzing
the data (12).
* The BSI is a unique Doppler bubble scoring system
that has been used extensively in research previously published
by the authors. It is a surrogate for continuous monitoring,
which is impractical for field studies and offers the equivalent
of ‘area under the Doppler bubble-time curve’. As such, it
cannot be compared with peak Doppler grades using any other
scoring system. The authors have found utility and consistency
when using this method to reflect collective decompression
stress.
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UHM 2007, Vol. 34, No. 6 – Decompression deep stop times
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Tissue Compartment Gas
Saturations
As previously (3) the 5 tissue-
compartment gas saturation was downloaded
from the blacked out depth-time recorders
worn by the divers and analyzed using a
modified Buehlmann algorithm (9) to predict
the saturation peaks for each of the 8 tissue
compartments during the ascent. The changes
in supersaturation were computed as fractions
of their respective M values from commencing
the ascent until reaching the surface.
RESUL
TS
Dive Profiles
The dive profiles were downloaded and
compared mathematically to the experimental
profile for consistency for time, depth and
ascent rate. The results confirmed that the
divers observed the prescribed dive profiles
with an accuracy of no less than 99.7% for all
parameters (SD = 0.0058; p = 0.12 Wilcoxon
Signed Rank Test). This shows no significant
difference from the experimental profile.
Doppler Bubbles Scores
Table 1 shows the BSI scores for the
15 profiles with varying deep and shallow stop
times ranging from 0 to 10 mins. There are three
groups of profiles: Profiles 1 to 5 had deep stop
times from 0 to 2 minutes. Profiles 6 to 10 are
all 2.5 min with shallow stop times gradually
decreasing from 5 to 1 minute. Profiles 11 to 15
have deep stops ranging from 3 to 10 minutes.
The greatest reduction in BSI was associated
with dive profiles with deep stops at 15 msw
(50 fsw) greater than 2.5 mins (Profiles 6 to
15). Shorter times, as in protocols 1 through
5 had higher bubble scores. Longer times, as
in profiles 11 to 15, gave no further advantage.
After the 2.5 min deep stop, the shallow stop at 6
msw (20 fsw) showed no significant difference
as the time was shortened between 5 min down
to only 1 min (see Profiles 6 to 10).
Without a deep stop at 15 msw (50
fsw) shallow stops as long as 10 minutes did
not reduce the BSI as effectively as any profile
for which a deep stop of more than 2 minutes
was performed. Without a deep stop, a 10 min
shallow stop was slightly better than only a
Profile
No.
Dives Depth
(m)
15 m Deep
Stop (mins)
6 m Shallow
Stop (mins)
T
Time
BSI
1 24 25 0 0 2.5 7.98
2 26 25 0 5 7.5 6.23
3 21 25 0 10 12.5 5.48
4 16 25 1 3 6.5 8.04
5 18 25 2 3 7.5 3.98
6 24 25 2.5 5 10 2.23
7 7 25 2.5 4 9 2.71
8 6 25 2.5 3 8 3.58
9 6 25 2.5 2 7 2.58
10 7 25 2.5 1 6 3.36
11 8 25 3 2 7.5 4.94
12 8 25 3 1 6.5 5.63
13 25 25 5 5 12.5 2.14
14 4 25 5 2.5 10 5.5
15 9 25 10 0 12.5 2.89
Table 1. Bubble Score Index for various deep (15 msw/50 fsw) and
shallow (6 msw/20 fsw) stops on ascent from 25 msw (82 fsw) at 10
msw/min (33 fsw/min).
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UHM 2007, Vol. 34, No. 6 – Decompression deep stop times
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5 min shallow stop. However, the deep stop
appears essential to ensure the lowest bubble
production (See Figure 1).
In Figure 2 the BSI for Doppler bubbles
is compared at various times. The highest BSI
is with the 1 min deep and 3 min shallow stops
(1/3 in the Figure). With no deep stop but a 10
min shallow stop the BSI is less (0/10 in the
Figure).
On the other hand, if the deep stop is 10
min with no shallow stop (10/0 in the Figure)
the BSI is, as discussed above, much the same
as for only a 2.5 min deep stop. Again, with a
2.5 min deep stop, decreasing the shallow stop
had no significant effect.
Figure 3 shows the statistical significance
comparing a deep stop longer than 2 minutes
versus no deep stop (p < 0.0001). However, if
the comparison is done for deep stops overall
(even less than 2 minutes) the difference is
still at p < 0.004 Figure 3. The difference
between the proportions of occurrences for the
higher bubbles grades (HBG and VHBG) is
very significant for the deep stop longer than 2
minutes vs. no deep stop (p<0.0001), yet if the
comparison is done for the deep stops overall
(even less than 2 minutes) the difference is still
present (p<0.004) by the Fisher exact test (12).
the Fisher exact test (12).
Fig 3. The difference between the proportions of
occurrences for the higher bubbles grades (HBG and
VHBG) is very significant for the deep stop longer
than 2 minutes vs. no deep stop (p<0.0001), yet if the
comparison is done for the deep stops overall (even less
than 2 minutes) the difference is still present (p<0.004)
by the Fisher exact test (12).
Tissue Compartment Gas
Saturations
The calculated tissue compartment gas
saturations for the various stop times are shown
in Table 2 (See page 404) from data recorded
by the UWATEC computers worn by the divers
during the dives.
Fig. 1. ZBG : Zero bubble grade; LBG : Low bubble
grade; HBG : High bubble Grade VHBG : Very high
bubble grade
Fig. 2. BSI and total decompression time at various deep
and shallow stop times.
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The origin at 5 min deep 5 min shallow
stops (Profile 13) showed the lowest tissue
saturation of 29 for the 5 min tissue compartment
(14) with a BSI of 2.14. The 2.5 min deep and
5 min shallow (6) also showed a low gradient
of 35 with a BSI of 2.23. Interestingly the very
long shallow stop of 10 mins with no deep stop
had a gradient of only 30 but a BSI of 4.93. So
lengthening the shallow stop was ineffective at
reducing bubbles.
DISCUSSION
These results give further support to
the findings reported in the previous paper (3)
that a deep stop significantly reduces PDDB.
However, the correlation with lower
fast tissue compartments is not as strong due to
the aberration of Profile 3 with a low gradient
but high BSI. According to the present results,
the optimal time for a deep stop at 15 msw (50
fsw) is 2.5 mins for a 25 msw (82 fsw) profile.
Shorter times increase the BSI. Further it
would seem that after the 2.5 min deep stop,
the shallow stop time at 6 msw (20 fsw) makes
little difference to the BSI but could be less
than the 3 to 5 min currently recommended.
Results of our earlier deep stop research
and technical diving data were discussed at
the U.S. National Association of Underwater
Instructors meeting in 2003 in Florida. NAUI
has since recommended that instead of the 3
min safety stop at 20 fsw (6 msw), divers should
take a 1 min stop at half the bottom depth and a
2 min stop at 20 fsw (13). The present data do
not support this recommendation as shown by
Profile 4 whose high BSI is virtually the same
as Profile 1 with no stops. A time of 2.5 mins
appears necessary for the deep stop at 15 msw
(50 fsw) after a 25 msw (82 fsw) dive. The
data indicates that the shallow stop is not as
important as a deep stop for this profile, but for
practical purposes, the original 3-5 min shallow
stop could be retained as it also attenuates the
risk for pulmonary barotrauma.
Table 2. Tissue compartment gradients for 5, 10, 20 and 40 min tissue compartments calculated from
UWATEC computers worn by the divers as compared to decreasing BSI. The values are given as percent
saturation with 100% fully saturated.
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Our research seeks to control bubble
growth by the intervention of a deep stop to
prevent supersaturation at depth and bubbles
forming at that time but growing in size with
ascent in accordance with Boyle’s Law. If
the deep stop is effective in stopping this
bubble formation, the shallow stop, as the
present research indicates, becomes much less
important. This method of decompression may
be designated as “beating the bubble” rather
than “treating the bubble”.
Deep stops also have been commonly
used by recreational technical divers who have
empirically devised their own methods for
decompression. This has led to other methods for
determining ascent profiles based on reduction
of bubbles rather than supersaturation, such as
the Wienke ‘Reduced Gradient Bubble Model
(RGBM)’ (14). The RGBM is now utilized
in a number of dive computers. There have
been no extensive formal field trials validating
the RGBM, but a data bank has been created
to record some dives: By 2004, some 2,300
dives had been recorded with 20 cases of DCI,
mainly after repetitive dives with nitrox and
using reverse profiles (15).
A recent French paper (16) investigated
the use of deep stops in three protocols tested
in the wet compartment of a decompression
chamber. In Profile I eight subjects dived to 60
msw (192 fsw) and in Protocol I used a deep
stop beginning at 27 msw (86 fsw) followed
by many other stops to the surface. Protocol
II was a repetitive dive to 50 msw (160 fsw),
a 3 hr surface interval followed by the second
dive with the deep stop at 18 msw (58 fsw).
Protocol III went to 60 msw (192 fsw) but
used a single shorter stop at 25 msw (80 fsw).
Using PDDB scoring, it was concluded that
these experimental deep stop profiles provided
no benefit compared with the standard MN 90
French Navy decompression table. Though no
times are given for the stops, from the graphs
they appear to be only about 1 min. If so, these
could have been ineffective, as was the case
with a similar too short deep stop from our
25 msw (82 fsw) dives. The depth of 60 msw
(192 fsw) is also much deeper than our present
work and may require different stop times and
stop depths to significantly reduce the PDDB
scores.
As a result of our deep stop research
(3), the Italian Recreational Diver Federations
have recommended the use of a 2.5 min deep
stop at 15 msw (50 fsw) from a 25 msw (82
fsw) dive and similar dives during ascent so it
will soon be possible to compare diving with
and without the deep stop and the incidence
of DCS resulting from its use for correlation
with PDDB. However, reduction of PDDB
may nevertheless help to make recreational
diving safer. This would be consistent with
the comments by Nishi et al. (8) who state “the
incidence of DCS is higher when many bubbles
are detected and that the incidence of DCS is
low when few or no bubbles are detected. Thus,
when evaluating decompression profiles, dives
which produce many bubbles in a majority of the
divers can be considered stressful with a higher
risk of DCS and should be avoided. Conversely
dives which produce few or no bubbles in the
majority of divers can be considered safe.”
CONCLUSIONS
We conclude that 2.5 min at 15 msw (50
fsw) is the optimal deep stop time following
25 msw (82 fsw) dives for 20 to 25 min for
preventing PDDB. Shorter or longer times are
not as effective. The shallow stop at 6 msw (20
fsw) for 3-5 mins normally recommended does
not seem as important. However, longer times
do not afford additional benefit in reducing
PDDB.
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UHM 2007, Vol. 34, No. 6 – Decompression deep stop times
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ACKNOWLEDGMENTS
We would like to thank the enthusiasm and
professional recreational divers of the Italian Dive Club
– “Sub Novara Laghi” led by Daniele Pes and Carlo
Bussi for carrying out the many open water profiles and
Doppler recordings with great accuracy and without
whom this research would not have been possible.
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