Measuring body temperature
Body temperature is a vital sign and it is important to measure it accurately. This article reviews and compares the various methods available to nurses.
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In this article…
- The physiological processes that regulate body temperature
- The clinical indications for measuring temperature
- A review of various methods of temperature measurement
Louise McCallum is a lecturer at the University of the West of Scotland, Ayr; Dan Higgins is a freelance educator, resuscitation and critical care, and senior charge nurse, critical care, at University Hospitals Birmingham Foundation Trust.
McCallum L, Higgins D (2012) Measuring body temperature. Nursing Times; 108: 45, 20-22.
Body temperature is one of the four main vital signs that must be monitored to ensure safe and effective care. Temperature measurement is recommended by the National Institute of Clinical Excellence a part of the initial assessment in acute illness in adults (NICE, 2007) and by the Scottish Intercollegiate Guidelines Network guidelines for post-operative management in adults (SIGN, 2004). Despite applying in all healthcare environments, wide variations exist on the methods and techniques used to measure body temperature.
It is essential to use the most appropriate technique to ensure that temperature is measured accurately. Inaccurate results may influence diagnosis and treatment, lead to a failure to identify patient deterioration and compromise patient safety.
This article explains the importance of temperature regulation and compares methods of its measurement.
Keywords: Temperature/Vital signs/Monitoring
- This article has been double-blind peer reviewed
- Figures and tables can be seen in the attached print-friendly PDF file of the complete article in the ‘Files’ section of this page
5 key points
- Body temperature is one of the four main vital signs that must be monitored in a patient
- Wide variations in practice exist across the healthcare system for measuring body temperature
- Body temperature should be measured and recorded regularly with precision, consistency and diligence
- It is vital to measure temperature accurately as it has an impact on diagnosis and treatment
- Inaccurate temperature measurement may compromise patient safety
Body temperature represents the balance between heat production and heat loss (Marieb and Hoehn, 2010). If the rate of heat generated equates to the rate of heat lost, the core body temperature will be stable (Tortora and Derrickson, 2011).
All metabolising body cells manufacture heat in varying amounts. Therefore, body temperature is not evenly distributed across the body (Childs, 2011).
Core body temperature is found in the blood supplying organs such as the brain and those in the abdominal and thoracic cavities. Core temperature may be affected by intrinsic factors and, to a lesser degree, extrinsic (environmental) factors.
Peripheral temperature is recorded in tissues such as the skin, where environmental factors and a lack of insulating connective tissue influence temperature.
Box 1 sets out some intrinsic factors that influence temperature (see attached PDF).
A healthy body maintains its temperature within a narrow range using homeostatic thermoregulation mechanisms (Pocock and Richards, 2009). The normal range for core temperature in the literature varies, although 36°C-37.5°C is acceptable in clinical practice (Childs, 2011).
Extremes of temperature are easier to interpret (Box 2 - see attached PDF). It is estimated that accompanying every 1°C rise in body temperature is a 10% rise in the rate of enzyme-controlled chemical reactions (Marieb and Hoehn, 2010). At 43°C and above, cells are irreparably damaged and enzymes denatured, rendering death a certainty (Marieb and Hoehn, 2010). Conversely, as temperature drops, cellular processes become sluggish and the metabolic rate falls. Pocock and Richards (2009) suggest that consciousness is lost 33°C. The body is generally more tolerant of lower than higher temperatures (Marieb and Hoehn, 2010).
Maintenance of body temperature
The thermoregulatory centre is located in the hypothalamus in the brain. It consists of the heat-loss centre, the heat-promoting centre and the pre-optic region, which analyse and coordinate responses to maintain body temperature within the homeostatic range (Marieb and Hoehn, 2010).
Signals arrive at the thermoregulatory centre via afferent pathways from the sensory receptors in the skin and organs. The centre’s response is propagated along efferent neural pathways to the target organ or blood vessel to elicit a response to gain or lose heat (Childs, 2011).
Behavioural mechanisms, such as removing or adding layers of clothing, also play an important role.
Fig 1 outlines the autonomic physiological mechanisms that are activated through the thermoregulatory centre to keep body temperature within the normal range.
Indications for measurement
There are many clinical indications for measuring body temperature (Dougherty and Lister, 2011; Pocock and Richards, 2009). They include:
- To obtain a the baseline temperature to enable comparisons to be made with future recordings;
- To enable close observation in resolving hypothermia/hyperthermia;
- To observe and monitor patients for changes indicating an infection;
- To monitor the effect of treatment for antimicrobial therapy for infection;
- Before and during a blood transfusion to monitor for signs of a reaction.
Measuring body temperature
The measurement of core body temperature may seem simple, but several issues affect the accuracy of the reading. These include the measurement site, the reliability of the instrument and user technique (Pusnik and Miklavec, 2009). Practitioners must understand the advantages and disadvantages associated with the chosen method so they can explain the procedure to patients and obtain valid consent (Nursing and Midwifery Council, 2008).
True core temperature readings can only be measured by invasive means, such as placing a temperature probe into the oesophagus, pulmonary artery or urinary bladder (Childs, 2011). It is not practical, nor indeed necessary, to use such sites and methods in all cases; they tend to be reserved for patients who are critically ill.
Non-invasive sites such as the rectum, oral cavity, axilla, temporal artery (forehead) and external auditory canal are accessible and are believed to provide the best estimation of the core temperature (Pusnik and Miklavec, 2009). The temperature measured between these sites can vary greatly, so the same site ought to be used consistently and recorded on the chart with the reading (Davie and Amoore, 2010).
The oral cavity temperature is considered to be reliable when the thermometer is placed posteriorly into the sublingual pocket (Hamilton and Price, 2007). This landmark is close to the sublingual artery, so this site tracks changes in core body temperature (Dougherty and Lister, 2011).
Electronic or disposable chemical thermometers may be used. Chemical thermometers should be avoided if the patient is hypothermic (<35°C) because their range of operation is 35.5°C-40.4°C (Fulbrook, 1997). Low-reading thermometers may be of some use. Mercury-in-glass thermometers can no longer be bought because of European Council rules (Medicines and Healthcare products Regulatory Agency, 2011).
Care must be taken to avoid the anterior region immediately posterior to the lower incisors because the temperature here is substantially lower (Dougherty and Lister, 2011).
Factors affecting accuracy include recent ingestion of food or fluid, having a respiratory rate >18 per minute and smoking (Dougherty and Lister, 2011). Oxygen therapy, particularly with high-flow rates, may influence temperature but this claim has been refuted by Stanhope (2006).
The tympanic thermometer senses reflected infrared emissions from the tympanic membrane through a probe placed in the external auditory canal (Davie and Amoore, 2010). This method is quick (<1 minute), minimally invasive and easy to perform. It has been reported to estimate rapid fluctuations in core temperature accurately because the tympanic membrane is close to the hypothalamus (Stanhope, 2006).
Although its accuracy and reliability have been questioned in many studies in the past decade, with differing outcomes. Tympanic thermometry continues to be used. Operator error and poor technique are frequently cited problems (Farnell et al, 2005), so training is recommended. Ear wax is known to reduce the accuracy of readings, so it is recommended that the ear is inspected before measurement (Farnell et al, 2005).
Advantages of this site are that the measurement does not appear to be influenced by oral fluids or diet, environmental temperature or other extraneous variables (Robb and Shahab, 2001). If patients have been lying with their ear on a pillow, allow 20 minutes to elapse so the temperature can normalise (Bridges and Thomas, 2009).
Temperature is measured at the axilla by placing the thermometer in the central position and adducting the arm close to the chest wall.
The literature suggests that this is an unreliable site for estimating core body temperature because there are no main blood vessels around this area (Sund-Levander and Grodzinsky, 2009). These authors also argue that the axillary temperature can be affected by the environmental temperature and perspiration.
Fulbrook (1997) produced convincing evidence indicating that chemical thermometers are clinically unreliable for measuring axillary temperature. Giantin et al (2008) suggested that electronic digital thermometers can be used at this site as a reliable alternative in older people.
Rectal temperature is said to be the most accurate method for measuring the core temperature (Lefrant et al, 2003). However, obtaining this is more time consuming than other methods and might be considered unfavourable for some patients (Dzarr et al, 2009). Practitioners should pay particular attention to issues of privacy.
The presence of faeces prevents the thermometer from touching the wall of the bowel and may generate inaccurate readings (Sund-Levander and Grodzinsky, 2009). Sund-Levander and Grodzinsky (2009) suggested this method does not track immediate changes to core temperature because of the low flow of blood to the area, so core temperature may be under- or overestimated at times of rapid flux.
Temporal artery temperature
The temporal artery thermometer is quick to use. It is held over the forehead and senses infrared emissions radiating from the skin (Davie and Amoore, 2010). However, its reliability and validity have not been widely tested. A single-centre study comparing it with other methods found that, despite the infection control advantages of this non-touch method, it underestimated body temperature compared with the control (Duncan et al, 2008).
Body temperature should be measured and recorded regularly with precision, consistency and diligence. Practitioners should be acutely aware of patients’ temperature because it serves as a useful indicator of change in their clinical condition.
A review has illustrated that none of the methods for measuring temperature at the bedside are perfect (Davie and Amoore, 2010). Once a site and a method have been selected, they must be used consistently to ensure accuracy and patient safety.
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