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Keeping the right tempereature during surgery – anaesthetists are warming to advice on inadvertent hypothermia

28 July, 2010  

The UK’s clinical watchdo hypothermia, and following it need not involve too much expense or effort, as it represents flexible advice. But accurate thermometers remain a trickier issue

C Mark Harper
Consultant anaesthetist
Royal Sussex County
Brighton, UK

Abhijoy Chakladar,
Research fellow in

The National Institute for Health and Clinical Excellence’s (NICE) guidance on the management of inadvertent perioperative hypothermia (IPH) in the UK is now more than a year old and has a significant impact on the day-to-day workings of anaesthetists and theatre staff. It amalgamated international research spanning more than a decade and provided succinct guidance to the clinical practitioner.[1] Its guidance is by no means exclusive and serves as a primer for reducing  hypothermia.
The significance of IPH and its role in perioperative morbidity has been known for some time and it is a common consequence of hospital admission, anaesthesia and surgical exposure.
Prevention of IPH or reducing operative heat loss has been shown to decrease bleeding during operations and postoperatively reduce recovery time from general  anaesthesia,[2-4] reduce length of hospital stay, improve  wound healing and reduce wound infection.[5] There have been discussions in the anaesthesia press about impending capital investments and financial implications,[6] and the inappropriate applications of the NICE guidance.[7] Guidelines are suggestions toward better practice and capital investments may not be required to fulfil their aims. As such, NICE issues a disclaimer;[1] it says it is just common-sense advice, giving clinicians the opportunity to audit their own practice, apply only guidelines that are appropriate to their patient populations and reduce targetbased political pressures. For instance, an audit
at a specialist ear nose and throat (ENT) hospital showed that its patients are not at high risk from IPH, with low rates of hypothermia.[6] It is therefore  not appropriate to warm this patient group routinely, as simple warming methods are  effective, thus not requiring the purchase of additional warming equipment.
Much of the focus is given to intra-operative warming, but prevention is better and often less resource-intensive than cure, and this is emphasized in the pre-operative phase of the NICE guidance which recommends temperature monitoring and management on the ward. The guideline also suggests that, should a patient’s temperature be less than 36.0°C, unless the operation is life- or limb-saving, active warming should be initiated until such time as the patient is normothermic. The  potential effect of this on theatre efficiency has been noted,[7] but clinicians must evaluate each case individually as to whether to proceed or not. Nevertheless, there is an argument for active warming prior to anaesthesia. Much of the heat lost after induction of anaesthesia is from the body’s core to its peripheries, caused by vasodilatation after general and neuroaxial anaesthesia, and active skin warming may prevent this.[8]  As guidelines mature, discussions about their efficacy move from ones of applicability and cost to what the guidelines have left out. Much of the concern surrounded increased cost relating to the purchase of disposable forced-air warming blankets (FAWB) and associated devices. The clinicians involved in creating the NICE guidelines understand that they were limited by a lack of direct evidence and the need to extrapolate data on the effectiveness of warming devices.[9]
Forced-air warmers are the only devices used in published studies where complications have been reduced with warming, and so were the only devices that could be recommended by NICE at that time. Small studies have shown that warmed intravenous fluids are effective in reducing temperature decreases,[10] and warming mattresses using carbon polymer technology and air-free blankets employing resistive polymer semi-conductive fabrics are just as efficacious as forced-air warming blankets.[11,12] Simpler methods such as warmer ambient temperatures and extra blankets may prove equally effective, and the NICE guidance does not preclude their use. The warming method used must fit the clinical situation and be palatable to the patient and clinical teams.
Obstetric procedures
Obstetric procedures were not covered by the NICE guidance. There is a paucity of research in obstetric perioperative warming and what research has occurred has involved small numbers of patients. Some studies have shown important trends after neuro-axial anaesthesia, prior to elective Caesarean section, such as reduced hypothermia and shivering with forced-air preand perioperative warming.[13] More recently, reduced maternal temperature decrease with pre-warmed and actively warmed intravenous fluids has been demonstrated. [10] However, a national survey has shown that the majority of obstetric units in the UK do not routinely warm patients undergoing elective section.[14] Our own (unpublished) audit has shown that 10% of patients undergoing elective Caesarean section become hypothermic and 25% suffer from shivering.
A recent audit at another institution showed that 42% of patients undergoing elective Caesarean section were hypothermic (as defined by NICE) on admission to the recovery room after their operation (additional data obtained from authors),[15] and analysis of previous audits has suggested that all patients undergoing Caesarean section with spinal or epidural anaesthesia should receive intra-operative warming.[16]
Alternative warming methods and their relation to maternal and foetal outcomes must be researched, as although FAWBs reduce maternal heat loss and are useful in general anaesthesia, they may be obstructive to the awake mother holding her baby.
The guidance did not consider technologies used to measure core temperature, perhaps due to a lack of good evidence. Thermometers in use range from glass-alcohol to infrared tympanic and temporal to thermistor or thermocouple-based tympanic, naso-pharyngeal, oesophageal, urinary catheter-bladder and pulmonary artery catheter (PAC) thermometers. For the NICE guidance to be appropriately  applied, tools used to measure temperature must be reliable and accurate.

Pros and cons of different thermometers
All thermometers have advantages and disadvantages. PAC thermometers may be seen as the gold-standard method of measuring core temperatures, but their associated complications precluded routine use.[17] Alcohol thermometers are cheap and reliable, but are prone to breakage and have a response time too slow for efficient intraoperative use. Though accurate when proximal to the tympanic membrane, most available infrared tympanic devices are too large to fit far enough into the external auditory meatus to retain this accuracy,[18] and a local study found that, on average, these thermometers under-read by 0.74°C.[19] Naso-pharyngeal and oesophageal probes are useful when the patient is  anaesthetised, but they are limited to intra-operative use, require careful positioning and cannot be comfortably used in awake patients. The accuracy of bladder thermometers with respect to PAC temperature measurements has been demonstrated and can be used throughout the operative period,[20] but specialised urinary catheters and monitoring equipment are required. Although most patients undergoing operations with a neuro-axial block have urinary catheters inserted, this is not essential and infection risks preclude their use in every patient. Temporal artery infrared thermometers may be a good alternative, as they are non-invasive, give reproducible results with simple training and are comfortable for awake patients. A recent pilot study has shown that infrared tympanic thermometers can give an accurate peripheral
representation of core temperature measured via a urinary catheter,[17] and may prove to be a more useful device.  However, larger trials comparing the various monitoring methods are required. The level of allowable inaccuracy from peripheral estimation of core temperature has not been formally defined, but it has been suggested that it should not vary from the true core temperature by more than 0.5°C.[18] The  limitations of current thermometer technologies must be appreciated by the anaesthetist and these limitations must be factored into decisions surrounding perioperative temperature management. There is a clear need to ‘establish the least inaccurate’ non-invasive thermometer.[19]
New guidelines often lead to reviews of current practice and good guidance aimed at interested professionals generates much debate, and the NICE guidance has done just that. National guidance can only be successful if its applicability to local populations is established through audit and then only applied where it is appropriate. An audit recipe, incorporating the NICE guidelines, can be found on the UK’s Royal College of Anaesthetists’ website with a data-collection form and spreadsheet tool to facilitate anaesthetic departments in auditing, interpreting and presenting their current performance in managing IPH.[21]
We must continue to monitor and audit the prevalence of IPH, institute appropriate measures and actively research new methods to prevent it. The NICE guidance on IPH offers perioperative physicians unprecedented evidenced-based influence in perioperative patient care and the ability to introduce positive, effective and patient-orientated changes. The prevention of hypothermia and subsequent beneficial outcomes to patients is the key message; the means employed are  secondary.

Competing Interests
C.M.H. has received support for research and loans of equipment from Inditherm, Arizant, The Surgical Company, and Augustine Medical. A.C. has no competing interests.

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5. Kurz A et al. N Engl J Med 1996;334:1209-15.
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9. Harper CM et al. Br J Anaesth 2008;101:293–5;  doi:10.1093/bja/aen233.
10. Woolnough M et al. Int J Obstet Anesth 2009;18(4):346-51.
11. Harper CM. Anesthesiology 2007;107:A92.
12. Kimberger O et al. Anesth Analg 2008;107(5):1621-26.
13. Horn EP et al. Anesth Analg 2002;94:409–14.
14. Woolnought MJ et al. Anaesthesia 2009;64:50–3.
15. Petsas A et al. Anaesthesia 2009;64:921-2.
16. Harper CM et al. Anaesthesia 2006;61:612.
17. Harper CM et al. Eur J Anaesthesiol 2008;25:3AP1–4.
18. Sessler DI. Anesthesiology 2008;109:318–38.
19. Harper CM. Anesth Analg 2009;109(1):288.
20. Horrow JC. Anesthesiology 1988;69:986–9.
21. Royal College of Anaesthetists, UK. Raising the Standard: A compendium of audit recipes.,– appendix1.doc, docs/ARB_2.7-appendix2.xls. A.C. has no competing interests.