From: amirza@bronze.ucs.indiana.edu (Anmar Caves)
Subject: Re: reheatable handwarmers
Organization: Indiana University
Date: Wed, 16 Dec 1992 22:28:31 GMT

A MEDICAL INTRODUCTION TO THE USE OF HEATPACKS IN CAVING
Anmar Mirza, EMT-D

INTRODUCTION:

	There are several methods for warming cold cavers.  Here we
will discuss the relative merits of two types of heat packs.  The
reduction/oxidization pack will be referred to as the Iron pack, and the
Sodium Acetate phase change pack will be referred to as the Acetate
pack.  Both packs have proven to be useful in general caving and
cave emergencies.  Cavers should be aware of the potential problems
and advantages of each type.  


HYPOTHERMIA

	Hypothermia is defined as a lowering of body temperature.
It is a major problem facing cavers in colder climates.  A person 
at rest without protection from heat loss in 70F(21C) calm air will 
develop hypothermia.  Two types of hypothermia, chronic and acute, 
can affect cavers.  Each has certain characteristics of which cavers 
need to be aware.
	Chronic hypothermia has a long onset time, generally
several hours.  This is likely to occur in cavers who have gotten
wet during a trip, or in cavers who have been pushing for many hours 
with little to eat.  Chronic hypothermia can be insidious because the 
body adapts to the lower core temperature.  Metabolism has time to
change to compensate for lowered temperature.  You may not
show any symptoms, or your friends may not notice any signs until
the onset of hypothermia is quite advanced.
	Acute hypothermia, in contrast, has a relatively quick onset
time.  It happens so quickly that the body has little or no time to
to adapt.  This can happen to cavers in northern areas after immersion
in cold water with inadequate protection.  Its onset time can be as
long as a couple of hours, or as short as a few minutes.  The
major problem here is that you may get out of the water and feel ok, 
cold but functional, and then get hit by severe afterdrop as the cold 
blood in your extremities is suddenly dumped into the core.  This can 
result in ventricular fibrillation, typically fatal quickly if 
untreated. 
	As with most medical conditions, there are no sharply defined
lines separating each type, and the conditions vary for different 
people.  Large people tend to take longer to develop hypothermia than 
smaller people. Conversely they will tend to take longer to warm up.  
The state of ones health and mental well-being also plays an important
factor.  A strong, healthy person will take longer to develop 
hypothermia than will a sick or weaker person.  A stronger,
healthier person will rebound more quickly from hypothermia.
	Chronic hypothermia is far more common in the context of caving,
and since the purpose of this article is to compare the types of heat
packs used in treating or preventing hypothermia, we will stick with
it.  Our bodies do a marvelous job in balancing heat production and
heat loss to maintain a core temperature of appr. 98.6F(37C).
In caving there are many conditions where heat loss overtakes heat 
production.  In a typical cave environment of 45F(7C)-60F(16C), a 
caver will be dressed to maintain a comfortable temperature while 
moving.  When you stop moving you will produce less heat, and unless 
measures are taken to prevent heat loss, you will lose heat faster 
than you produce it.  

PREVENTION OF HYPOTHERMIA

	This is where heat packs are very useful.  Unlike insulation, 
heat packs actually add heat.  The problem with merely insulating
yourself from the cave environment is that it is difficult to
completely balance out heat loss with heat production. This is 
especially true in colder caves.  As your body temperature lowers, 
your body will attempt to produce more heat by shivering.  This is a 
good method of heat production - as long as you have energy to do it.
Your body uses energy to produce the muscular action needed to keep
warm.  In an extended wait or transport, you may run out of the 
energy necessary to keep up muscular action.  This is especially 
important because the brain and other vital organs also need energy 
to survive.  This is where external heat application becomes
important.  Often, cavers waiting around for any period will use a 
"Palmer Furnace" or any of the many variations.  This is huddling over
a candle or a carbide lamp, trapping heated air under an extended
shirt or jacket.  This technique works fairly well but, has the 
disadvantage of requiring that you balance yourself over the flame
source.  This tends to cut off circulation to various body parts, 
as well as being difficult to do while sleeping.  Additionally, 
this method does not work for someone strapped into a litter and
moving.  Heat packs are excellant for those roles, for they do not 
require you to balance over a candle or lamp, and can be placed
exactly where needed.

RELATIVE MERITS OF APPLICATIONS OF THE TWO HEAT PACKS

	Which type of heat pack to use?  This will depend on many
factors. First some pros and cons.

ACETATE

Pros:
Can be reactivated in the field by boiling in water.
This same process will add to the total heat delivered if the pack is
then used as a hot-water bottle soon after reactivation.
Can be used when wet.

Cons:
Low heat production for given weight and volume.
Freezing will often activate the pack.
Packs can be accidently activated before needed.
Heavy and bulky.
Fairly fast heat dumping, can be slowed by insulation.

IRON

Pros:
Relatively high heat output for weight and volume.
Widely available.
Small and lightweight.
Not affected by cold.
Easy to place on a patient (can be taped in place if needed)

Cons:
Will not work as well when wet.
Definite shelf life.
Cannot be reused.

	The Acetate pack will dump its heat as fast as it can after
activation.  The rate of heat output can be controlled somewhat by
insulating the packs, for example with a neoprene pouch.  Such a
pouch will also reduce the risk of accidental activation due to
freezing or shock.  At best, an Acetate pack will only be useful for a
couple of hours before needing to be reactivated.  The process of
reactivation involves boiling the pack in water. 
	The Iron pack will produce its heat more slowly; the rate can 
be controlled somewhat by decreasing its available oxygen supply.
Typical life is from 6-12 hours depending on the size of the pack, 
the available oxygen, and the construction of the pack.  Oxygen
depletion should not be a primary concern, as a 19 gram pack will
consume 2.8 liters of oxygen over several hours.  That volume of
oxygen would be used in only a couple of minutes breathing.  It can 
tolerate getting wet to a degree, but its effectiveness is reduced.  
It should not be considered useful if it will be immersed.

DISCUSSION:

	Given the relative pros and cons, which heat pack should be 
used for a given situation?  It would seem that under wet conditions
or long transport conditions (several days) the Acetate packs would be
better.   For most situations the Iron packs will be better, the main 
advantages being heat output vs size and weight.  A typical caver could
carry several of the large variety for the same given volume as an
acetate pack, at lower weight.  The decision of what to use will be
up to the individual caver.  
	It should be remembered that for mild hypothermia, where the 
patient is still conscious and coherent, rewarming is usually not a 
problem and should be encouraged.  If the patient cannot move, or is 
exhausted and out of energy, with none being readily available, then 
heating should be liberal and aggressive.  The best method of rewarming 
for a mildly hypothermic patient is removal of heat-robbing conditions, 
and exercise.  The latter not always being feasible due to injury or
exhaustion. It has been shown that the best method for warming a mild 
hypothermia victim under these conditions is direct skin to skin contact 
with another person, preferrably in a sleeping bag or somesuch.  Heat 
pack placement should be on the torso, groin and hands.
Monitoring of the packs is important, if they become too warm they
should be insulated from the skin.  
	For severe hypothermia the treatment is radically different.  
Here the idea is to prevent further heat loss.  Do not try to field 
rewarm this patient.  Heat pack application to the torso is still
indicated as the output from even several heat packs isn't enough to
significantly rewarm someone.  If any rewarming effect is noticed, the
patient should be closely monitored.  If the effect is pronounced
then some of the heat packs should be removed.  This is unlikely as a
severly hypothermic person can be several hundred kilo-calories heat
deficient and each heat pack will only add tens of kilo-calories.
The important effect here will be prevention of further loss of heat.

Permission to reprint this article in entirety granted.
-- 
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