VOL: 101, ISSUE: 20, PAGE NO: 30
Jo Trim, MPhil, BSc, RN is nurse adviser at University Hospital Birmingham Foundation Trust
Core body temperature measurements are taken to assess for deviation from the normal range that may indicate disease, deterioration in condition, infection or reaction to treatment.
Anatomy and physiology
The body requires a stable core temperature, ranging from 36 to 37.5 degsC, to maintain cell metabolic activity. Core temperature is the balance between heat gain produced by cell metabolism and heat loss from variuos mechanisms, including respiration via the lungs and evaporation via the skin (Dougherty and Lister, 2004). Core temperature is controlled by the hypothalamus in the brain (Fig 1). This may impact on the ability to control core temperature following head injury.
Extreme temperature deviations, for example hypothermia or pyrexia, can lead to body dysfunction such as convulsions or, in extreme cases, death (Carroll, 2000).
Hypothermia is defined as a core temperature below 35 degsC that causes the metabolic rate to decrease. This can be caused by exposure to a cold environment, blood transfusion, surgery, or renal dialysis in the critically ill.
In extreme cases of hypothermia, the patient may need intravenous fluid to be warmed prior to administration and peritoneal or bladder lavage with warmed fluid. In less severe cases a warming blanket may be sufficient or blood warmer if a transfusion is required.
Pyrexia is a significant rise in core temperature. Vasoconstriction, shivering, increased oxygen demand and carbon dioxide excretion may occur. There are three grades of pyrexia (Dougherty and Lister, 2004):
- Low grade (normal to 38 degsC) that indicates an inflammatory response due to mild infection, allergy or disturbance of body tissue;
- Moderate to high grade (38 degs-40 degsC) pyrexia that can be caused by wound, or infection;
- Hypopyrexia (40 degsC and above) caused by bacteraemia, damage to the hypothalamus or high environmental temperatures.
Temperature reading sites
- Oral. Thermoreceptors in the posterior sublingual pocket of the mouth (under the tongue at the back of the mouth) respond to changes in core temperature (Fig 2). The thermometer probe must be placed in this pocket rather than under the front or on top of the tongue (Erikson, 1980). Factors affecting oral temperature readings include a respiratory rate of more than 18 breaths per minute (Dougherty and Lister, 2004), eating and drinking immediately prior to a reading, smoking within 15 minutes of the reading and patients moving the thermometer in their mouth (Stevenson, 2004; Carroll, 2000).
- Tympanic. These thermometers sense body heat through infrared energy given off by the tympanic membrane (Fig 3) (Carroll, 2000). Ear canal size, presence of wax, operator technique and the patient’s position can affect accuracy (Knies, 2003; Carroll, 2000).
- Axilla. Taking a patient’s temperature under their arm is considered unreliable.
- Rectal. Although rectal temperature readings are considered more accurate than oral or axillary readings, this method is not advocated due to its invasive nature.
- Check the equipment is in working order.
- Adhere to local infection control policy.
- Explain to the patient why temperature is being taken, and obtain informed consent.
- If an oral reading is required, ensure the patient has not eaten, drunk or smoked within the previous 15 minutes.
- Place a clean plastic protector over the chosen device (Fig 4).
- Oral thermometer: place the probe into the anterior sublingual pocket of the mouth. If able, ask the patient to hold the device in place.
- Tympanic thermometer: carefully insert the probe into the ear canal ensuring a snug fit, without causing patient discomfort (Fig 5).
- Press the start button and wait until the device indicates the reading is complete.
- Remove the probe and dispose of the plastic probe protector.
- Document the reading immediately (Fig 6) and compare against normal values.
Comprehensive training on equipment is essential to ensure correct use and adherence to the Clinical Negligence Scheme for Trusts. It is the responsibility of the individual practitioner to ensure both their practical skills and theoretical knowledge are maintained.
- This article has been double-blind peer-reviewed.
For related articles on this subject and links to relevant websites see www.nursingtimes.net