VOL: 97, ISSUE: 09, PAGE NO: 36
Mathew Kelly, BSc, RGN, is practice development nurse, ITU, Manor Hospital, Walsall Hospitals NHS Trust
Beverley Ewens, BSc, RGN, is consultant nurse, ITU;Phil Jevon, BSc, RGN, PGCE, is resuscitation training officer, Manor Hospital, Walsall Hospitals NHS Trust
Hypothermia can be a life-threatening condition. Nurses need to be able to identify those at risk, know how to monitor the condition accurately, be familiar with re-warming procedures and understand the physiological complications of the condition.
The cells of the body function at their optimum level when the body’s core temperature remains constant, at around 37°C. Heat production in the body is a byproduct of metabolism, which is measured by the Basal Metabolic Rate (BMR).
BMR is a measurement of the rate of energy use in the body during basal conditions - in other words, while the patient is awake but the body is resting. BMR is also based on oxygen consumption, hormonal factors and exercise and reduces with the age of the patient. If patients are ill and/or pyrexial, a small amount of energy expenditure will raise BMR, increasing heat production and temperature. It is vital to maintain a balance between heat production and heat loss.
The central control of body temperature occurs in the hypothalamus, in the diencephalon portion of the cerebrum, commonly known as the body’s ‘thermostat’. This serves as a reflex control mechanism, which means that as body temperature rises, messages are relayed back to the hypothalamus via peripheral receptors, which in turn activate heat-lowering mechanisms (Tortora et al, 1996). No single body temperature can be considered ‘normal’ because measurements on many patients have shown a range of temperatures. In a healthy person, the range of core temperature is maintained between 36°C and 37.5°C.
Definition of hypothermia
Hypothermia is defined as a core temperature of less than 35°C (Chan et al, 1998). It can occur when the body loses too much heat or cannot maintain its normothermic state. There are three different classes of hypothermia:
- Mild hypothermia (32°C-35°C);
- Moderate hypothermia (28°C-32°C);
- Severe hypothermia (below 28°C) (Jolly et al, 1992).
Those at risk of hypothermia include:
- Children - due to their immature regulatory systems;
- Elderly people - due to their altered thermoregulatory mechanisms.
Risk factors for hypothermia include:
- Poor accommodation/inadequate heating (Turney et al, 1998);
- Exposure to a cold environment, for example immersion in cold water;
- Burns, which can lead to excessive heat loss:
- Overdose of medication that lead to coma and immobility. Those that may cause or exacerbate hypothermia include: phenothiazines, benzodiazepines, morphine, barbiturates and vasodilators (Chan et al, 1998);
- Underlying illness, for example hypothyroidism, diabetic ketoacidosis, hepatic encephalopathy and cerebrovascular accident;
- Alcohol abuse - ethanol is a vasodilator that produces anaesthesia and depresses the central nervous system (Carson, 1999).
Common reasons for falling temperature
All patients who receive general anaesthesia experience some degree of heat loss. The temperature and humidity in the operating theatre, the number of room air exchanges occurring per hour and cool air currents all contribute to loss of body heat (Dennison, 1995).
Hypothermia is induced during certain types of cardiac surgery. The patient’s temperature is lowered using a cardiopulmonary bypass machine or by surface cooling with ice in order to reduce oxygen and metabolic demands. This helps to protect vital organs during low blood flow periods (Foldy et al, 1989). Cardiopulmonary bypass surgery is carried out on the cold, ‘stilled’ heart, and the patient is maintained on the bypass machine. After surgery, the patient is re-warmed and the heart restarted (Dennison, 1995).
Mechanisms of heat loss
There are four mechanisms of heat loss:
- Radiation - the flow of heat from a high temperature in the body to a lower temperature in the surrounding environment;
- Convection - heat transfer by movement or flow of air;
- Conduction - heat transfer via direct contact with cooler surfaces;
- Evaporation - perspiration, respiration and breaks in skin integrity (Tappen et al, 1996).
In hypothermic states, heat production increases as a result of shivering, but this can cause oxygen consumption to increase by up to 140%. In shivering, the body increases heat production from skeletal muscle. When young children are exposed to cold, they conserve heat by cutaneous vasoconstriction (Carson, 1999). In the newborn and young children, brown fat provides a further resource for the generation of body heat (Casey, 2000). This is the main system of heat generation in babies before they are able to shiver.
In elderly people, the vasoconstrictive response to cold is delayed so that a lower core temperature is needed to trigger it. Both responses to temperature stress are less efficient in elderly people. Piloerection - commonly known as ‘goose pimples’ - is another method of heat production and retention. This occurs when body hair stands on end, particularly on the arms and upper body, and traps a thin layer of insulator air, which is then warmed to body temperature.
Close, accurate monitoring of the patient’s core temperature is imperative as this allows the nurse to note temperature changes quickly and to take action to prevent further heat loss as well as observe the effects of treatment.
While they are still in widespread use, the traditional oral/rectal mercury thermometers have been superseded by electronic models, which are faster, safer and provide continuous readings (Tortora et al, 1996). The oesophagus, nasopharynx, tympanic membrane, bladder and pulmonary artery reflect core body temperature (Soukup, 1998).
Re-warming should not exceed increases of 0.3°C-1.2°C per hour in cases of mild hypothermia, but rapid re-warming of 3°C per hour is essential if there is severe hypothermia and cardiovascular instability (Carson, 1999). As a rule, the management of hypothermia depends on the degree of its severity. Passive and active re-warming methods are also available.
Simple passive re-warming measures, such as providing warm dry clothes and warm drinks, may suffice in cases of mild hypothermia. However, the patient’s thermoregulatory mechanism must be intact for passive re-warming to be effective and these methods will be inappropriate for patients whose cardiovascular systems are compromised (McCance et al, 1997).
External methods or re-warming include:
- Warm baths;
- Heated pads/blankets;
- Warm-air blankets (Murphy, 1998).
These methods are more effective than passive ones, but there is potential for peripheral vasodilation, which may induce hypotension and cause core temperature to drop further, increasing the risk of arrhythmias (Murphy, 1998). Studies comparing circulating water blankets and warm-air blankets conclude that both warm the patient adequately, but the water blanket will restore normal temperature more quickly (Sanford, 1997).
- Inhalation of warm, humidified gases (Murphy, 1998);
- Administration of warmed intravenous fluids (Chan et al, 1998);
- Peritoneal/haemodialysis, gastric lavage and extra-corporeal blood-warming (Chan et al, 1998).
These methods of treatment are invasive, but they may be needed to treat profound hypothermia and its complications.