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47
Almost all experts in dive medicine agree that divers should
ascend slowly following dives whether recreational, working or
technical. The reality is that very little direct evidence exists on
what ascent rate is safest. Most of the recommendations come
from observational studies of bubble grade found using Doppler
ultrasound or are based on anecdotal or theoretical concerns.
In the early days of diving, John Haldane and Leonard Hill
experimented with a variety of linear ascent rates, some as slow
as 3 to 5 feet per minute, but the rate itself was not sufficient to
prevent decompression sickness (DCS) when exposures were
sufficiently great. Haldane realized there must be an optimal
ascent rate that would reduce unnecessary exposure to depth
and provide a sufficient decrease in pressure to allow offgassing
but be slow enough to protect divers from DCS. This ascent rate
is thought to vary depending on the depth, tissue saturation and
breathing gas. In saturation diving, the decompression rate is on
the order of several feet per hour, while in short, deep diving it
is on the order of feet per minute — faster at depth and slower
close to the surface. In recreational diving, where the depth and
exposure are limited, a maximum ascent rate may be specified
without regard to depth.
U.S. Navy frogmen generally wanted to ascend from their
dives and exit the water quickly, but such quick ascents were
impractical for hardhat divers. Thus, a compromise of 60 feet
per minute was reached. This ascent rate remained in place
for many years even as dive tables continued to be refined. It
was not until about 20 years ago that the U.S. Navy changed
their recommended ascent rate to 30 feet per minute.
In 2009 a study published in the journal Aviation, Space and
Environmental Medicine looked at 47 recreational divers using
ascent rates of 30 feet per minute and 60 feet per minute. At
various intervals following the dives the divers were checked
with Doppler ultrasound devices, and the group that had a
faster ascent rate was shown to have higher bubble grades.
This gives credence to the theory that slower ascents help
reduce decompression stress on the body after diving.
Perils of Quick Ascents
Decompression stress is defined as the amount of inert gas
dissolved in various tissues throughout the body. During
ascent, bubbles increase in size and are released by tissues
into the veins. These venous bubbles then travel to the lungs,
where they are offgassed through normal breathing. Faster
ascent rates are thought to have an impact on decompression
stress by not allowing sufficient gas bubbles to be offgassed
through breathing.
DCI is one of the greatest concerns associated with fast
ascents. When bubbles arise or become trapped in tissues or
vessels they begin to cause traumatic injury to the body, which
is known as decompression sickness. Air can enter the arterial
circulation and result in an arterial gas embolism (AGE),
which can cause a rapid onset of strokelike symptoms. AGE
may occur subsequent to lung injuries but may also happen in
the absence of any apparent injury to the lungs. On the other
hand, rapid ascents can occur with no ill effects at all.
Besides not allowing for sufficient offgassing during
decompression, rapid ascents can have other implications.
Expanding gas in air spaces such as middle ears and sinuses
may cause local injuries known as reverse blocks or reverse
squeezes. These pressure injuries (barotraumas) occur when
gas expands in these spaces faster than the body can equalize
them. Congestion is often a factor in reverse blocks. The
lungs also need time to release expanding gas. During slow
ascents this is achieved through normal breathing as the
lungs are constantly equalizing to ambient pressure. If the
ascent is too fast and the lungs cannot accommodate the
expanding gas, pulmonary barotrauma can result.
keePing it under control
So how can divers control their ascent rates so as not to
exceed the limit on which they have decided? One way is
by practicing buoyancy control skills. BCDs and drysuits
rely on air inside them while at depth to control buoyancy.
During ascent, this air expands, causing the diver to ascend
more and more rapidly if it is not vented, which can lead to
an out-of-control ascent. Improperly maintained inflator
or deflator mechanisms on BCDs and drysuits can cause
more air to enter the system than intended and may not
allow the diver to vent air quickly enough to stay in control.
Proper weighting before the dive is also essential in helping
to eventually control ascent rate since the diver will not need
to add as much air to the BCD while at depth. Keep in mind
that equipment changes such as switching from aluminum
to steel tanks have an impact on proper weighting and
buoyancy control during the dive and the ascent.
The most common method of controlling speed during
ascent is to simply vent excess air from the BCD or drysuit
during ascent. Body positioning can also help by increasing
drag in the water. Monitor your computer, and know what
ascent rate it has been set to. If the rate is faster than you
prefer, many computers allow you to change it. Use of a
bottom slope, wall, ascent line or other visual or tactile
reference is helpful. When practicing ascents in open water
without a slope or ascent line, closely monitor the time and
depth during your trip to the surface. Safety stops are a great
way to slow and pause your ascent in the shallow depths
where the greatest changes in pressure occur.
Despite the lack of definitive consensus on what ascent rate
divers should use, “slow” is a good way to go. The U.S. Navy
and the National Oceanic and Atmospheric Administration
(NOAA) use a rate of 30 feet per minute, and recreational
dive-training-agency recommendations range from 30 to 60 feet
per minute. Regardless of the ascent rate you choose, it is most
important that your ascents be well under control.
AD
46-47_Safety101_Winter2012.indd 47
12/21/11