Humans are homeotherms
- - need to keep the internal temperature environment as constant as possible.
- -As such, the human body will function most efficiently at a constant temperature of 37?C.
- -Any increase or decrease in body temperature will adversely affect many aspects of physiology, and ultimately human performance.
- hyperthermia >37.5
- Hypothermia ,35
- -About 75% of the metabolic energy produced is converted to heat, s
- - 25% of nutrient energy is available to do useful work
- - Even though the heat production is relatively high, it may not be sufficient in extremely cold conditions or water immersion to maintain the body?s core temperature.
Conversely, heat production may be excessive in temperate or hot conditions and must be dissipated to prevent the body from overheating.
The working environment in military aviation can be inherently hostile with summer temperatures on the tarmac in some deployed operations reaching in excess of 50?C.
- Given the long range of some aircraft, temperatures at the departure airfield may be very different to those experienced en?route or at the destination.
- The cold temperatures experienced at altitude have no affect on the body whilst enclosed in an air?conditioned cabin.
- However if air conditioning or pressurisation is lost, then occupants may be subjected to temperatures approaching ?60?C.
Processes body gains and loses heat?
- a) Radiation,
- b) Conduction,
- c) Convection, and
- d) Evaporation.
- -Everything with a temperature above absolute zero will radiate heat energy as electromagnetic waves.
- -Radiation is the transfer of heat through space from one body to another, usually in the form of infra?red radiation.
- -As propagation through matter is not required, heat energy can be transferred in this way through a vacuum.
- -Conduction is a transfer of kinetic energy in the form of heat between adjacent atoms in objects that are in contact with each other.
- The rate of heat transfer is dependent on the temperature differential between the two bodies.
- -Convection occurs in gases and liquids
- -movement (kinetic energy), transfer heat away from a solid object.
- As molecules are heated by contact with the solid object new unheated molecules replace them.
- Eventually in a closed system, all molecules of gas above the liquid will reach temperature equilibrium with the liquid and heat transfer will cease.
- -It requires 580 calories of heat to change 1 gram of water liquid into water vapour at body temperature.
- This heat is referred to as the latent (hidden) heat of vaporisation.
- The evaporation of body fluid as a result of sweating leads to heat loss from the body.
- -amount of water vapour in a given sample of air at a given temperature,
- expressed as a percentage of the maximum amount of water vapour that the sample could contain at that temperature.
- -majority of heat loss is by direct radiation to the environment surrounding the body
- -and is directly related to the environmental temperature.
- -Heat loss through conduction and convection includes direct contact with the air (10%), heating of inspired air in the lungs (2.5%) and heat loss from elimination of body waste products (1.5%). The bulk of this heat loss is by convection. Heat loss by evaporation may be ?insensible? loss at rest or obvious sweating. Insensible perspiration amounts to about 500ml per day and includes fluid loss from the skin (14.5%) and lungs (8%). A small amount of heat (3.5%) is lost from the chemical process of producing CO2 from carbonic acid (H2CO3) in the lungs.
PHYSIOLOGICAL RESPONSES TO HEAT?
- -Heat stroke will usually occur at core temperatures between 40?41?C.
- At this temperature, the thermoregulatory mechanism ceases functioning
- - which results in an uncontrollable temperature rise. Recovery from a temperature greater than 42?C is unlikely.
- Vasodilatation is the opening of the peripheral blood vessels facilitating blood flow to the skin to maximise heat loss by radiation. Vascular responses such as this are the body?s most effective mechanism for controlling temperature.
- Sweating: Sweating is the only effective method of heat loss at environmental temperatures above 35?C, when there is no thermal gradient between the skin and the surrounding air. This is achieved through latent heat which is utilised to change the sweat from a liquid to a vapour phase. The fluid that is left lying on the surface of the skin has a lower energy thus providing a cooling effect. Even in mild conditions, fluid loss is about 2.6 litres per day. To maintain proper fluid balance, fluid input must equal fluid output. Table 18?2 shows a comparison of total body water input and output, contributing to water balance.
The effectiveness of sweating depends on?
- a) Surface Area:
- b) Airflow:
- c) Humidity:
DEHYDRATION A good indicator of hydration levels is the presence of ?
-headache and the colour of urine.
These are consequences of heat stress ?
- -affect aircrew performance.
- -Reaction times, problem solving, memory, and arithmetic deteriorate
- -tolerance to G is reduced
- -Vibration has been shown to decrease tolerance to heat, and heat will amplify the effect of other stresses, generally increasing discomfort and fatigue.
This common condition is skin damage arising from excessive exposure to ultraviolet radiation. It can be extremely painful, and range from mild reddening of the skin through to severe blistering, similar to partial thickness burns. It can significantly erode the will to live in survival situations.
Fainting occurs due to massive peripheral vasodilatation often combined with dehydration, causing low blood pressure and then loss of consciousness.
Heat cramps are caused by excessive salt loss from the body due to sweating. As sweating increases, the sweat glands, which are normally good at retaining salt in body fluids, become less efficient. As a result, more and more salt is lost from the body. In normal temperate work environments, increasing salt intake with meals is all that is usually required. In extreme conditions such as desert operations, drinks containing electrolytes are recommended. In most conditions however, water is the best fluid replacement solution available.
- Heat exhaustion occurs as a prelude to heat stroke.
- Dehydration and a rise in core body temperature normally cause heat exhaustion.
- Thermoregulation fails, and core temperature can start to rise above 39?C.
- Fluid loss and a redistribution of blood flow and tissue fluids cause peripheral vasoconstriction,
- which further impairs heat loss.
- This condition is characterised by dehydration, circulatory failure, and collapse.
- It usually develops at about 5-10% dehydration with a temperature of 38-40?C.
Symptoms of HEAT EXHAUSTION?
- -are similar to those of shock and may include:
- a) faintness,
- b) rapid and weak pulse,
- c) reduced blood pressure, and
- d) pale clammy skin.
- These symptoms are caused by an inability of the circulatory system to cope with the decreased circulatory fluid volume and
- decreased peripheral vascular resistance, as a result of vasodilatation and fluid loss due to dehydration. Treatment for heat exhaustion is to lie the patient down, keep them cool, and give cool clear fluids to drink if the victim is conscious.
- - body's thermoregulatory mechanism breaks down with an uncontrolled increase in temperature.
- Signs of heat stroke are shown in Table 18?4. Neurological manifestations distinguish heat stroke from heat exhaustion.
- It is imperative to reduce body temperature immediately if heat stroke is suspected.
- eath is almost certain if the body temperature exceeds 42?C.
- If possible, the victim should be removed from the source of heat, loosen or remove clothing and seek medical attention as soon as possible. Cooling should be instituted rapidly but gently by spraying with water and fanning to promote heat loss by convection and evaporation. Too rapid cooling will lead to peripheral vasoconstriction, shivering followed by a further a rise in core temperature resulting in death.
PREVENTING HEAT DISORDERS?
- -Flight scheduling:
- -Sensible scheduling of flights can help to minimise the effects of heat stress.
- -Keeping ground standby to a minimum,
- -allowing sufficient recovery time between flights for rest and re-hydration, and
- - limiting the extent of operations are some strategies which can be used in extremely hot conditions.
Early recognition of heat stress?
- Recognising symptoms of heat stress in yourself is somewhat difficult.
- As with hypoxia, cognitive performance is reduced which may preclude an individual from recognising the onset of heat stress.
Conserve sweat not water?
-fluid should not be consumed until experiencing the first signs and/or symptoms of dehydration.
Acclimatise where possible?
- -whereby the body adapts to a new hot environment by living in that environment for some time.
- -The process can take a few days, or up to a week.
- - Initially unacclimatised individuals feel very uncomfortable and lethargic.
- - Individuals normally experience irritability, lethargy and have difficulty doing mental or physical tasks.
- -They have a higher than normal resting pulse rate, and poor exercise tolerance.
- As acclimatisation occurs after a few days these symptoms progressively lessen as physiological changes occur which enable the body to better withstand the heat.
- -Their heart rate decreases, exercise tolerance improves, sweat glands produce more sweat and it becomes more dilute, and a hormone called aldosterone is secreted by the adrenal glands which helps the kidneys to retain salt. It is possible to artificially acclimatise over a two to three week period by a daily one?hour exercise programme (while wearing insulated clothing to raise the skin temperature), or using a thermal chamber. This is called ?acclimation?, as opposed to acclimatisation, which occurs naturally upon exposure to a hot environment. Additionally, hard physical exercise should be avoided until fully acclimatised.
HEAT STRESS INDICES?
- Military flying often exposes aircrew to high levels of thermal stress.
- Flying clothing reduces the surface area available for effective sweating,
- and this combined with inadequate cockpit ventilation and cooling inhibits the body?s ability to keep cool.
- All the different parameters used to measure the thermal environment can be combined into derived scales used to describe how comfortable or uncomfortable a person may feel under certain conditions. One such scale is called the Wet Bulb Globe Temperature (WBGT). The ?Player Comfort Level? used on sporting fields is another. For example, a player may feel quite comfortable with an air temperature of 27 degrees, 20% humidity and a slight breeze. They may be very uncomfortable with a temperature of 25 degrees, 90% humidity and still conditions.
An aircraft specific scale is called the Fighter Index of Thermal Stress (Table 18-5).
- This was developed to estimate how stressful environmental conditions might be for a pilot flying a single seat fighter with a bubble canopy.
- If the FITS falls within dangerous levels, restrictions on operations may be imposed to protect the crew.
PHYSIOLOGICAL RESPONSES TO COLD?
EFFECTS OF EXPOSURE TO COLD?
- Local cold injury is classified as either freezing or non-freezing. Freezing injury refers to frostnip and frostbite.
- Non-freezing cold injury is called immersion or trench foot.
Frostnip occurs when the tissue fluid in peripheral parts of the body (such as the fingers, toes, ear lobes and nose) freeze but the blood vessels and cells remain undamaged. It is thus a reversible injury. The affected parts become tingly then numb with a pale appearance and feel firm to the touch. The affected area will be painless until warmed. Treatment for this type of cold injury is slow warming of the affected area using body heat. It is important not to expose the frozen tissue to direct heat or rub it vigorously as tissue damage may result.
Frostbite is irreversible damage to tissues when freezing damages the blood vessels that supply the tissue with oxygenated blood, and ice crystals form within cells so that the tissues die. As with frostnip, the affected area will become numb, pale, firm and waxy in appearance. Thawing should not be attempted until medical help is close by as this will often result in severe pain. In addition, the tissue may turn gangrenous resulting in systemic infection which has a high mortality rate.
- Immersion foot is a non-freezing injury resulting from chronic exposure to cold and wet conditions.
- Tissue hypoxia results in damage to nerves and muscles but the blood vessels and skin do not suffer irreversible changes.
- The affected area will be pale and without a pulse, and when re-warmed will show localised swelling, redness and pain.
- Hypothermia is defined as a core temperature less than 35?C.
- Shivering is the initial response, but this is lost below 32?C.
- Non-shivering responses include peripheral vasoconstriction, hormone release, increased heart rate initially, then slowing below 32?C.
- There is an initial rise in respiratory rate, followed by a decrease, causing hypoxia and hypercapnia.
- There is a ?cold diuresis? and fluid depletion, and a linear decrease in cerebral metabolism.
- - (32 - 35 degrees C).
- -Shivering, lethargy, loss of motor coordination, tachycardia, diuresis, mild confusion, nausea, dizziness.
Moderate Hypothermia (28 ? 32 degrees C). Loss of shivering, more confusion, muscle rigidity, decreased level of consciousness, low BP, weak slow pulse, cold to touch, ?paradoxical undressing?.
- - (< 28 degrees C).
- -Absent reflexes, coma, fixed dilated pupils, muscle rigidity, the heart may stop, flat EEG (measuring brain activity).
- The person may look dead. Below 28?C, there is a risk of ventricular arrhythmias.
- Asystole (cardiac arrest) occurs at about 19?C.
- Brain activity also ceases at 19?C.
Prevention of hypothermia?
- a) Avoid cold conditions,
- b) Seek shelter,
- c) Dry/waterproof layered clothing,
- d) Dress for the worst possible conditions likely to be encountered, and e) Consume food and warm sweet drinks.
- DO NOT drink alcohol as this causes vasodilatation increasing heat loss.
Treatment for hypothermia includes?
- a) Removing the person from the cold environment,
- b) Removing wet and cold clothing,
- c) Wrapping the victim in blankets or other material to raise temperature slowly, and
- d) Use of body heat to warm the victim (inside a sleeping bag).
- DO NOT place the victim in hot water or use an electric blanket to warm them rapidly as this may cause excess cooling of the core and shock by peripheral vasodilatation.