Translation of a paper from JAHRBUCH 2002
Published by the Austrian Association for Alpine and High Altitude Medicine
Authors: Hermann Brugger1, Markus Falk2
1Hermann Brugger, MD. (University of Vienna, Austria), emergency and mountain
rescue doctor for
the South Tyrolean Mountain Rescue Service. He is Chair of the Mountain
Medicine Committee –
International Commission for Mountain Rescue (ICAR), and lives in Brunneck,
South Tyrol, Italy.
2Markus Falk has a Masters degree in mathematics and information technology.
He lives in the Italian
South Tyrol.
Translation: Initial translation provided by Manuel Genswein; additional
grammatical wordsmithing by
CAA reviewers.
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Abstract
This review aims to evaluate and compare avalanche safety equipment for
downhill and off-piste skiers
on the basis of published rescue data. A new classification for avalanche
rescue devices is proposed.
With respect to the different mechanisms of action we distinguish the
following categories rescue
devices:
A) devices aiming to prevent or reduce the extent of burial (ABS Avalanche
Airbag, Avagear);
B) devices aiming to shorten the duration of burial (avalanche beacon,
K2 avalanche ball); and,
C) devices with the aim to prolong survival time during complete burial
(AvaLung TM).
The reduction of mortality is essential for the assessment of efficiency,
representing the main criterion
for the evaluation. The ABS Avalanche Airbag lowers mortality significantly
from 23.0% to 2.5%
(p=0.001) and is considered as acceptable, safe and useful (Class IIa,
the safety equipment of choice).
Avalanche beacons are marginally effective at reducing fatalities, (p=0.054)
(Class IIb, acceptable and
useful devices). Due to the lack of data, AvaLung, Avagear and the K2
Avalanche Ball are determined
to be “Class III, indeterminate efficacy”, as additional
confirmation is needed.
Keywords: Avalanche, classification, mortality, rescue device, safety
equipment
Introduction
In the last few years the options for safety equipment for the ski tourer
and off-piste skier have been
expanded by a few interesting possibilities. Although there is already
a retrospective evaluation for the
efficiency of avalanche rescue transceivers(1), there have not been any
evaluations for the efficiency of
the newer types of equipment. In this overview, we will analytically
compare and classify the available
safety equipment based on previously published data. Only those pieces
of safety equipment which are
ether available on the market or in the stage of final testing are included
in this comparison. Search
devices suitable only for organized rescue teams (e.g.; RECCO) were not
included.
Pathogen factors of an avalanche burial
The survival chances for persons caught in avalanches are dependant on
several factors. Apart from the
fact that a person carried by an avalanche can suffer fatal injuries
from mechanical snow pressure,
collision with obstacles in the avalanche path, or falls over cliffs
in the avalanche path, survival chances
after the avalanche comes to a stop depends on whether the victim is
able to breath, and in case of a
critical burial (head and the upper part of the body in the snow), how
fast the victim is dug out. If the
airway of the buried person is not clear and if there is no air pocket
round the victim, after 35 minutes
the victim’s fate is hopeless and any help will be too late(2).
However, with a clear air way and an air
pocket, it is possible to survive longer. Survival of totally buried
persons is also influenced by
hypothermia and other, up to now unknown co-factors(3). Data show approximately
75% of avalanche
fatalities are due to asphyxia, 15 to 20% due to deadly trauma and 5
to 10% due to hypothermia and
other factors.
Effect of safety equipment
The most deadly risk during avalanche burial is the danger of acute asphyxia.
The purpose of avalanche
safety equipment is to prevent suffocation of the buried person. This
purpose can be achieved by three
different mechanisms, each with different working principles and efficiencies.
The different approaches
will be discussed in detail.
A. Reduction of the degree of burial
The most efficient means of preventing avalanche fatalities is to avoid
complete burial. Of 1886 persons
which where caught in avalanches in Switzerland between 1981 and 1998,
735 persons (39%) where
fully buried, and 1151 (61%) were partly buried or stayed completely
unburied on the surface. 433
persons (23%) did not survive the avalanche. For the completely buried
persons, the mortality rate was
52%. For partly buried victims the mortality rate was only 4.2% (p >0.001,
Table 1) (3). Measures
which help to prevent complete burials reduce mortality. The ABS Avalanche
Balloon System and the
Avagear life vest provide floatation which keeps the victim on the surface
while the avalanche is in
motion. Swimming motions by the victim while the avalanche is in motion
can be life saving if it is
possible to stay on the surface
B. Reduction of burial time
Chance of survival over time in a complete avalanche burial is not linear.
(See Figure 1). In a complete
burial situation, there are phases where the survival rate drops very
rapidly - a high risk to die in the
first 18 to 35 minutes of burial (asphyxia phase) - and phases with an
almost stable survival rate – a
greatly reduced risk to die between 35 and 90 minutes of burial (the
latency phase)(5). In principle, by
reducing the burial time, mortality can be reduced. However, the efficiency
of this strategy depends on
which phase of the survival curve is affected. Safety equipment is especially
efficient when the steep
parts of the survival curve are affected. Transceivers and the K2 Avalanche
Ball are examples of safety
equipment that reduces burial time.
C. Prolonging survival time during complete burial
The survival chance of a fully buried victim also depends on the existence
of free airway and an air
pocket in front of the face when the avalanche comes to a standstill(2).
If the victim is able to keep the
airway free and have an air pocket, survival time can be prolonged. This
means more time available for
the rescue of the victim, and this increases the survival chances. The
rescue vest AvaLungTM(6-8) is a
device which creates an artificial air pocket .
Figure 1: Chance of survival for persons who have been caught and completely
buried in an avalanche
in open terrain (Switzerland 1981-1998, n=735) as a function of burial
time in minutes. Reducing the
degree of burial raises the chance of survival, especially when the steep
sections of the survival curve
are included. Modified according to(3).
Avalanche safety equipment
Depending on the function of the avalanche safety equipment, it can be
categorised into one of three
different classes. Type A minimizes the degree of burial, type B the
burial duration and type C extends
the time a fully buried avalanche victim can survive within the avalanche.
The types B and C assume a
full burial situation and therefore, in order to be efficient, must always
be combined with a tool like a
shovel which helps to uncover the victim. Note from CAA – both
probe and shovel required for effective
location and recovery.
Type A. Safety equipment which minimizes the extent of burial
The ABS Avalanche Balloon System(9) is the only commercially available
avalanche safety device
designed to reduce the extent of burial. It consists of one or two balloons
which are integrated into a
special back pack. Pulling the emergency handle inflates the balloons
with 150 litres of a nitrogen / air
mixture within 2 to 3 seconds. Buoyancy due mainly to the effects of
the inverse segregation(10, 11)
causes the person caught in the avalanche to be held on the snow surface.
All of the known accidents where persons have been caught by avalanches
and used the ABS system
(N=40) have been documented by the Swiss Institute for Snow and Avalanche
Research(12). Thirty nine
persons (97.5%) using the ABS system survived the avalanche and one person
(2.5%) did not (Table
2). In eight cases (20%), the balloon was not inflated because the release
handle was not pulled or the
inflation system did not function properly. Five persons (16%) were fully
buried even though their
balloon was inflated. Despite this failure rate, the ABS avalanche balloon
reduces the likelihood of
complete burial from 39% to 16.2% (Fisher’s Exact Test, p=0.006)
and lowers the mortality rate from
23% to 2.5% (Fisher’s Exact Test, p=0.001). Since the study data
was collected, the mechanical
triggering mechanism has been replaced by a pyro-electric system and
the single balloon system has
been replaced with a double balloon design. Those modifications to the
ABS have increased reliability.
Even though the efficacy of the system is very high, in Europe the ABS
system has not been accepted
by the backcountry skiers and off-piste skiers in the expected way.
The Avagear rescue jacket is being developed in the United States and
is in its trial phase(11). In contrast
to the ABS balloons contained in a backpack, the Avagear balloons are
contained in the collar or
shoulders of a jacket or vest, and inflate around the neck and the shoulders
of the victim. With this
design the head of the victim should stay above the snow surface more
often, and the balloon protects
the victim’s head and cervical spine during the flow of the avalanche.
In addition, one hopes that with
this system it is more likely to have an air pocket around the head and
face of the victim if a full burial
does occur. During the three tests which were made up to now, the victim’s
face remained free.
Avagear seems to offer a further development of the avalanche airbag
principle which makes sense.
Type B. Safety equipment which reduces the burial time
The avalanche rescue transceiver was developed in 1968 by John Lawton
(USA) and is currently the
most efficient device to reduce burial duration. A retrospective analysis
of 328 fully buried avalanche
victims has shown a highly significant decrease of the median burial
duration from 120 min to 35 min
(p<0.001)(1). The mortality rate was reduced from 75.9% to 66.2% (p=0.054).
Reduction of the
mortality rate is not as significant as the reduction of the burial duration.
This is due to the fact that the
reduction of the median burial time mainly affects the part of the survival
curve which is relatively flat,
and does not significantly affect the part of the survival curve between
18 and 35min after the avalanche
occurred where the survival rate is rapidly decreasing. (See Figure 1).
Since newer avalanche rescue
transceivers use digital technology with a useful direction indicator,
it is anticipated that the survival
data for this new generation of transceivers will be better. However,
there are no data available yet
which prove that conclusion.
Based on one survey, the percentage of European ski and snowboard tourers
who are equipped with
avalanche rescue transceivers has increased from 29 % in the period 1970-1979
to 74% in the period
1990-1999 (p=0.039)(4). Today the avalanche rescue transceiver is the
most generally accepted type of
avalanche safety equipment used by back country ski tourers and off-piste
skiers.
The K2 Avalanche Ball is a further development of the avalanche cord,
an old concept which is out of
date. After pulling the trigger handle, the spring loaded Avalanche Ball
pops out of a small pocket on a
backpack, and opens itself. The low density ball stays attached to the
person by a cord, and floats on the
surface of the avalanche. When the avalanche stops, companions must visually
locate the ball and then
follow the cord leading to the buried person. The presumption is that
prompt location of the victim
should lead to faster recovery, reducing the duration of burial. Up to
now several successful tests have
been conducted, but no “real life” uses of this device have
been documented (11).
Type C. Safety equipment which prolongs the survival time for a fully
buried avalanche victim
The AvaLung (TM) survival jacket was patented by MD Thomas Crowley
and developed by Black
Diamond Equipment Ltd. in Salt Lake City, Utah, USA(13). The AvaLung
is a sleeveless synthetic
jacket which is worn when travelling in the backcountry. In an avalanche
the victim needs to get the
mouthpiece (which comes out of the collar) in his mouth and keep it there
during the flow of the
avalanche. A valve separates the inhaled air from the exhaled air. During
inhalation, the air is extracted
from the avalanche snow through a special textile filter on the front
side of the victim; during
exhalation, the air is directed and vented to the back side of the victim.
Due to the separation of the
exhaled air, the re-inhalation of CO2 is prevented. The latest version
of the AvaLung system is
integrated into a light weight chest harness.
The goal of the AvaLung system is to prolong survival time in a full
burial situation by at least one
hour(6-8). The AvaLung concept assumes a full burial situation, therefore
a transceiver and a shovel must
always be carried as well. To be successfully rescued, the victim depends
on third party help. The
efficiency of this safety equipment has been proven in 33 experimental
tests where the human test
subjects were buried at a 1m depth. There are 3 documented cases where
the AvaLung was used in real
avalanche situations. The question of whether or not the mouthpiece can
be properly positioned and
kept in the victim’s mouth through the turbulence of the avalanche
can not be conclusively answered
yet(14). Compared to the ABS avalanche airbag system, the AvaLung has
the advantage of lighter weight
and lower price.
Valuation and classification of the safety equipment
The statistical proof of a significant decrease of mortality is the
most important criteria for valuation of
avalanche safety equipment. User friendliness and acceptance are additional
criteria, which can
indirectly influence the efficiency and mortality, but were not taken
into account in this study. The
analysis of the present data allows one to assign an evidence level (Table
3) to the safety equipment,
following the international guidelines 2000 for cardiopulmonary reanimation(15).
Table 3: Safety equipment and their mechanisms of action. The safety
equipment is classified after its
evidence level. The classification was made according to the international
guidelines for cardiopulmonal
reanimation 2000(15).
A. The ABS avalanche balloon system highly significantly reduces mortality.
The class IIa can be
assigned to it, which means a good to very good evidence level - acceptable
and wise to use it - the
safety equipment of choice. The further development of the balloon system
leading to a jacket lik
system which produces an air space around the face as well as a higher
reliability rate in the triggering
of the system could make the efficiency even higher.
B. The efficiency of the avalanche rescue transceiver is documented
in several studies and they show a
marginally significant reduction of mortality. The device is only efficient
in connection with a shovel.
The class IIb means medium to good evidence – acceptable and wise
to use it. It has to be noted that the
analysis of the efficiency is based on data of the years 1981-1994. It
is possible that a new analysis
based on current data shows a better significance.
C. AvaLung, Avagear and the K2 Avalanche Ball are all in the class
III category “indeterminate”,
which means that the evidence level can not be determined due to the
lack of data.
Conclusions
There is sufficient data to determine the evidence level for two categories
of avalanche safety
equipment. As expected, there is no avalanche safety equipment which
meets the class I criteria. Class I
classification could only be proven through controlled studies, and it
would not be reasonable to
intentionally put live persons into harmful avalanches.
All this analysis was only possible thanks to the precisely collected
data of Frank Tschirky and other
employees of the Swiss Institute for Snow and Avalanche Research. Frank
Tschirky died of a heart
attack in spring 2001 on a trekking tour in Nepal - we lost one of the
best avalanche experts.
Reliable analysis of avalanche safety equipment in the future will only
be possible if the data is
collected by an independent institute.
A common characteristic of all avalanche safety equipment is that devices
can improve survival
chances, but can never completely eliminate the risk of dying in an avalanche.
Risk compensation theory (a propensity to take higher risks because of
perceived protection from
avalanche safety equipment) suggests human factors could negate some
of the technical improvements
in avalanche safety equipment. In the back country we are only safe when
we act in a defensive manner
so that we avoid triggering avalanches. Technical devices will never
replace this mountaineering skill.
Recognition and respect for natural hazards in the mountains will always
remain our most reliable
partner.
Disclosure
This study has not been financially supported. The authors do not have
patent or financial interests on
the production or the sales of the described safety equipment.
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