The term, "Hypothermia" refers to deep body or core cooling. The body core consists of the vital organs, which includes the heart, brain, lungs and abdominal organs. In health these are kept at a constant temperature of 36.9 degrees Celsius. When the core temperature drops to 35 degrees Celsius a person is considered to be suffering from hypothermia.

Immersion Hypothermia results from immersion in cold water. The high thermal conductivity of water results in rapid body cooling.

The extremities and surface of the body are its shell. The shell temperature is lower than the core temperature at about 33 degrees celsius producing a temperature gradient and heat is transferred from the core to the shell and thence to the environment by the physical processes of conduction, convection and radiation.

When the body is cooled, cold receptor input from the skin stimulates a special centre in the brain(the hypothalamus), which initiates a shutting down of the skin blood vessels (vasoconstriction). This has the effect of conserving heat. Also, shivering, which is an involuntary contraction of muscles, is initiated, and when maximal can produce an amount of heat equal to five times basal heat production. In addition there is a release of hormones which increases cellular metabolism resulting in more heat being produced. Despite these compensatory mechanisms, heat losses exceed heat production in water temperatures below 20 degrees celsius and the body cools rapidly. The lower the water temperature, the faster the rate of cooling.

Initial immersion results in a sensation of cold, heart, respiratory and metabolic rates are increased at first, and then shivering commences becoming maximal at a body temperature of 35 deg C. As the body cools below 35 deg C mental confusion occurs; manual dexterity and strength are reduced to a point at which a victim might be unable to grab and hold a rescue line. Between 33 deg C and 30 deg C shivering is replaced by muscular rigidity. Unconsciousness occurs at about 30 deg C. Below 30 deg C pupils become fixed and dilated, reflexes are lost, heart , respiratory, and metabolic rates slow and muscles become flaccid. Cardiac rhythm changes occurs between 28 and 26 deg C and cardiac arrest about 24 deg C.

Survival times vary according to a number of factors. For a person of average build, the following Table is a rough guide.

Port Phillip water temperature varies from 9.5 deg C to 23 deg C. In October the traditional start of the 'snapper' season, the water temperature is only 11 deg C.

Age - Due to their larger ratio of body surface to mass , very young children cool exceedingly rapidly and boys more rapidly than girls.

Alcohol - it has the effect of dilating or opening up the skin blood vessels allowing heat to be lost more rapidly. It can also effect brain function and the shivering mechanism is inhibited. Survival time is greatly diminished.

Body build - The fat layer beneath the skin acts as an important insulator against heat loss. Consequently a fat person will survive much longer than a thin person. Experienced swimmers also build up their fat layers and develop an adaptation to the cold which helps them to survive longer than a person of average build.

Exercise - Surprisingly, exercise is not effective in maintaining heat when immersed in cold water. Exercise causes the skin blood vessels to dilate (open up) resulting in a greater heat loss than production.

Research into "areas of high heat loss" has resulted in the development of postures in the water that can decrease heat loss and thereby increase survival time. Also a jacket has been designed with particular reference to these areas of high heat loss.

The areas of high heat loss are the head and neck, lateral sides of chest, axillae or armpits, and groin areas. Forty per cent of heat loss is from the head and neck.

The "Thermofloat" jacket is designed to insulate these areas of high heat loss. Wearing one will increase survival time fourfold. See Survival Aids for more information on the Thermofloat Jacket.

Heat Escape Lessening Posture (HELP).
This is achieved by placing the arms across the chest to protect the axillae or armpits, and flexing the legs to protect the groin area, and wearing headgear, and remaining still in the water. It will result in a 50% increase in survival time.

Huddle
It is also possible for three people to adopt the "HUDDLE" position to conserve heat loss.(See diagram)

"HUDDLE"

"H.E.L.P."

This refers to a drop in core temperature and can be as much as 3 deg C and can result in the death of a person suffering from hypothermia who has just been rescued. "After drop" results from the return of cold blood from the extremities to the core. There is always some "after drop", but the aim is to minimize it. It can be minimized by not massaging or warming the limbs and also by preventing the person from walking around immediately after rescue. The person should remain still for at least 30 minutes and be closely observed for signs of deterioration.
Regrettably, there have been a number of reported cases of sailors being rescued conscious from the sea, but who have been allowed to walk around and subsequently become unconscious and died. It is thought to have been due to the effects of "after drop".

Put on as much clothing as possible. (although clothing when wet is not a good insulator, it will reduce the rate of cooling). Put on a life-jacket, adopt the "HELP" posture, hold still and STAY WITH YOUR BOAT until rescued.

What should I do if I'm close to land?
The decision to strike out for land would depend on a number of factors, such as, water temperature, swimming experience, body build, tide and wave conditions, and proximity to land. Research has shown that at a water temperature of 10 deg C, about 0.85 miles would be the greatest distance that a person of average build is likely to survive before being overcome by hypothermia. After consideration of these factors if there are any doubts, hold still and STAY WITH YOUR BOAT until rescued.

* If there are a number of people in the water, children, adults with slight body build, and the very quiet, have top priority in order of rescue. The very quiet are likely to be suffering a more severe hypothermia than the noisy.

* A victim suffering from moderate hypothermia may not have the manual dexterity to grab hold of a rescue line or buoy and may have to be physically rescued from the water.

* To minimize "after drop" do not allow the victim to walk around after rescue from the water - keep the victim still for at least 30 minutes. Do not massage or warm the limbs.

* The profoundly hypothermic victim who is unconscious may have the signs of being dead with fixed dilated pupils and undetectable breathing and pulse. If there is a short search and rescue history,(hours and not days), it may be possible to resuscitate the victim. CPR and urgent transport to hospital for rewarming is required. (There are case reports of profoundly hypothermic victims who arrived at hospital with no recordable electrocardiogram, absent pulse and breathing who after warming and CPR have made a recovery without neurological damage).

* Remove the victim from the cold-inducing environment - be aware of the cooling effect of wind(wind chill) . Space blanket may be helpful in preventing further heat loss.

* Minimize "after drop" by not allowing the conscious victim to walk around after rescue for at least 30 minutes, and by not massaging or warming the limbs. If possible warm the areas of high heat loss, or by core warming using warmed oxygen if this device is available. Body to body warmth has also proved a successful method of rewarming. If the victim's condition deteriorates, perform CPR if necessary.

* Perform CPR on the profoundly hypothermic victim who appears dead if the search history is short.(CPR should be at a reduced rate with profound hypothermia)

* Transport victim to hospital in the slightly head down position to maintain blood pressure, and handle victim gently to avoid inducing a fatal cardiac arrythmia (ventricular fibrillation).

It depends on the degree of hypothermia. A person suffering mild hypothermia, i.e. someone who is cold, shivering, conscious and alert and whose core temperature is not less than 35 deg C would benefit from a warm drink, and a warm shower, and would recover quickly. However DO NOT give any fluids to a victim suffering from moderate hypothermia (core temperature 34 - 31 deg C) due to the instability of this degree of hypothermia and the possibility of the victim lapsing into unconsciousness as a result of "after drop". Under these circumstances giving the victim oral fluids could result in aspiration of fluid into the lungs leading to a fatal outcome.