Inadvertent perioperative hypothermia: a literature review.
Abstract: Up to 70% of surgical patients develop hypothermia perioperatively. Inadvertent hypothermia can be caused by a cold operating theatre, anaesthetic effects, exposure to the environment and administration of cold intravenous or irrigation fluids. The adverse effects of unplanned hypothermia include increased blood loss, morbid cardiac events, impaired wound healing and increased mortality.

Preventing unplanned hypothermia increases patient comfort and prevents associated complications. It can be achieved by simple preventative measures (Burger & Fitzpatrick 2009, Lynch et al 2010).

KEYWORDS Hypothermia / Perioperative hypothermia / Prewarming
Article Type: Clinical report
Subject: Hypothermia (Care and treatment)
Hypothermia (Physiological aspects)
Hypothermia (Patient outcomes)
Author: Knaepel, Abigail
Pub Date: 03/01/2012
Publication: Name: Journal of Perioperative Practice Publisher: Association for Perioperative Practice Audience: Academic Format: Magazine/Journal Subject: Health; Health care industry Copyright: COPYRIGHT 2012 Association for Perioperative Practice ISSN: 1750-4589
Issue: Date: March, 2012 Source Volume: 22 Source Issue: 3
Geographic: Geographic Scope: United Kingdom Geographic Code: 4EUUK United Kingdom
Accession Number: 283274425
Full Text: Introduction

A recent audit within my care area highlighted that many patients entering the recovery area from theatres were hypothermic with temperatures below 36[degrees]C. The audit highlighted that both minor and major surgical patients were affected and were becoming hypothermic. As a result, the time spent in recovery had increased significantly whilst patients were re-warmed to 36 [degrees]C before being discharged to the ward.

NICE guidance (2008) advocates prewarming on the ward for patients that are hypothermic prior to surgery to assist in maintaining normothermia throughout the perioperative care pathway.

A specific goal has been set by our trust board to reduce patient related harm events by 50% by the end of March 2012 as part of the Leading Improvement in Patient Safety (LIPS) project. One of these harm events is post-operative hypothermia. A patient safety report is presented each month to the trust board and is used to measure progress against the stated patient safety aim and other key information related to patient safety (NHS Institute for Innovation & Improvement 2011).

The Royal College of Anaesthetists (RCoA 2006) has produced a document for minimum standards, and perioperative temperature management forms a section of this document. The document acknowledges that inadvertent hypothermia is preventable if frequent temperature monitoring and early warming therapy interventions are initiated. This review explores relevant research investigating inadvertent perioperative hypothermia, the effectiveness of forced air warming devices available, preoperative warming and reducing recovery time.

What is meant by inadvertent perioperative hypothermia?

Core body temperature is regulated in the conscious patient by the thermoregulatory system. In response to changes in the body's temperature, the hypothalamus acts as a thermostat, increasing body temperature by vasoconstriction or decreasing body temperature through vasodilatation (Weirich 2008).

Researchers estimate that 50-90% of surgical patients experience hypothermia during surgery. There are two contributing factors to inadvertent perioperative hypothermia: anaesthetic-induced and environmental. General anaesthetic inhibits the thermoregulatory system and as a consequence prohibits cellular metabolism; this results in the body being unable to produce heat. The environment (cold operating room temperature) determines the rate at which metabolic heat is lost by radiation and convection from the skin, and by evaporation from the surgical site (Kurz 2001).

'Redistribution temperature drop' develops immediately after induction of anaesthesia as a result of internal core to peripheral redistribution of body heat. This occurs due to anaesthesia-induced vasodilatation and reduced thermoregulatory vasoconstriction (Paulikas 2008).

Normothermia is defined as a core body temperature between 36 and 38[degrees]C; hypothermia is a core temperature below 36[degrees]C. A drop in core body temperature of just 1.5[degrees]C is associated with many post operative complications (Weirich 2008).

Effects of hypothermia

The major complications of perioperative hypothermia are as follows: surgical wound infection, increased length of stay, increased blood loss, increased blood transfusion, morbid cardiac events, duration of muscle relaxants, shivering, recovery stay and thermal discomfort (Kurz 2008).

The study by Kurz (1996) highlighted that in 200 patients studied there was a 19% rate of surgical wound infection in hypothermic patients as opposed to a 6% rate in normothermic patients. The same study showed that hypothermic patients had an average hospital stay of two days longer (cited in Kurz 2008).

Shmied et al's (1996) study showed that hypothermic patients had an average of eight units of blood transfused compared to one unit in the normothermic group (cited in Kurz 2008).

Frank et al's (1997) study highlighted an 8% rate of ventricular tachycardia in hypothermic patients compared to 2% in the normothermic group, and a 6% rate of morbid cardiac events in the hypothermic group compared to 1% in the normothermic group (cited in Kurz 2008).

Methods used to combat hypothermia

There are various methods available to combat perioperative hypothermia and their efficacy has been tested in the research. Yet there still continues to be confusion about the 'best' method for maintaining normothermia in the perioperative environment.

Some of these methods include:

* Forced air warming

* Mediwrap

* Heated gel pads

* IV fluid warming

* Warming irrigation fluids

* Theatre temperature

* Prewarming


Literature search

I conducted my literature search through CINAHL. I used the key headings 'hypothermia', 'perioperative hypothermia' and 'prewarming' to find a range of research papers that were current (2005-2011) and appropriate to the topic being studied. From these papers, I also searched their references for previous yet still applicable research that dated back to 1996.

Summary of the results

Research papers

Smith et al (2007) found that the temperature decreased after induction in both actively warmed patients and controls, but to a greater extent in the control group. The final temperature at the end of surgery was higher in the actively warmed group, and more patients in the control group were hypothermic at the end of surgery than in the actively warmed group. In the control group, the patients that were actively warmed intra-operatively had higher temperatures at the end of surgery than those that were not warmed at all. The hypothermic patients spent on average approximately 11 minutes longer in recovery than the actively warmed patients. This study found the Equator forced air warmer to be effective for arming and maintaining normothermia.

Andrzejowski et al (2008) found that the core temperature of the prewarmed group was greater than that of the control group. Eight percent of patients in the control group were hypothermic at the start of induction. Patients in the prewarmed group (68%) maintained normothermia throughout surgery to a greater extent than in the control group (43%). This study found the Bair Paws warming system to be effective for prewarming.

Rathinam et al (2009) found that prewarming and intra-operative warming with the Mediwrap insulating blanket was comparable to the use of the Bair Hugger forced air warming device. The study found that when the Mediwrap blanket was used for a minimum of 30 minutes for prewarming, there was no difference in core temperatures between the groups during the pre- and intra-operative period. However, this study also found that the Mediwrap group had an incidence of higher temperatures in the post-operative period compared to the forced air warming group.

Melling et al (2001) found that core temperatures were increased in the prewarmed groups compared to the control group. This study also highlighted that the incidence of surgical wound infection was significantly lower in the prewarmed groups (5% compared to 14% in the control group). Fewer patients in the prewarmed groups were given postoperative antibiotics than in the control group. By prewarming for 30 minutes, the authors showed reduced surgical wound infection rates from 14% to 5%. Prewarming was delivered via a forced air warming device.

Fossum et al (2001) found that there was a statistically significant increase in core body temperature in the treatment (prewarmed) group. This study highlighted that the treatment group maintained higher temperatures upon entry to the recovery area than the control group. Interestingly this study found that the older the patient the lower the initial postoperative temperature, and that there was no association between temperature and the incidence of postoperative nausea and vomiting. Prewarming was delivered via a forced air warming device.

Clinical guidelines

NICE (2008) issued guidance for best practice on the care of the adult surgical population undergoing general, regional or combined anaesthesia. The guidance advocates the need for prewarming however only mentions the use of forced air warming. This guidance details the care that should be implemented in order to prevent unplanned hypothermia.

ASPAN's (2010) Evidence-based clinical practice guideline also advocates prewarming and states that without prewarming a period of hypothermia is typical even if active warming is applied intraoperatively. This guideline offers best practice advice integrated with current research in this area of care.

The Royal College of Anaesthetists' guideline (2006) advocates prevention of inadvertent hypothermia rather than treating the adverse effects. The guideline advocates prewarming and suggests that active warming should be initiated for all patients with a length of surgery greater than 30 minutes (which includes anaesthetic time).

How clinical guidelines compare with research findings

Having reviewed the relevant primary research papers investigating how perioperative hypothermia can be avoided, I will now review the associated clinical best practice guidelines to ascertain if what the guidelines advocate is consistent with the findings from the research studies.

The NICE (2008) guideline states that 'if the patient's temperature is below 36[degrees]C, forced air warming should be initiated on the ward'. However, all of the research studies discussed above that looked into prewarming conducted the study on all patients regardless of their pre-operative temperature, so long as it was not beyond the normal temperature parameter of 38[degrees]C. This highlights that the best practice guideline is inconsistent with recommendations made from primary research in the area of practice.

It is well documented that all patients that have a general anaesthetic will have an immediate temperature drop of at least 1[degrees]C due to the redistribution shift in temperature from the core to periphery (Weirich 2008). NICE (2008) only advocates prewarming for patients with a temperature below 36[degrees]C, however the patients that are for example 36.2[degrees]C on the ward will not have prewarming initiated but will lose up to a degree as soon as the general anaesthetic has been induced. Therefore, the patient would have a potential temperature of 35.2[degrees]C after induction of anaesthesia and staff would then have to aggressively re-warm the patient in the intraoperative phase of care.

The NICE guidance (2008) is also contradictory to the RCoA guidance (2006) which states that 'patients should be prewarmed to 36[degrees]C or above before induction of anaesthesia'. This is more consistent with the research findings from the studies included in my literature review as the RCoA have acknowledged that patients may need to be prewarmed to a temperature above 36[degrees]C prior to induction of anaesthesia. The RCoA guidance goes on to say that if patients are not prewarmed on the ward, 'active warming should be initiated in the anaesthetic room for all procedures where the total operative time (from first anaesthetic intervention to arrival in recovery) is greater than 30 minutes'. This would be indicated for most cases.

The RCoA guidance is therefore stating that patients need to be actively warmed prior to induction of anaesthesia to prevent (or minimise) the effect of redistribution temperature drop following general anaesthesia; where NICE are stating that only patients with a pre-operative temperature below 36[degrees]C should be prewarmed. The ASPAN guideline also advocates prewarming although they do not specify what the patient temperature should be in order to commence active warming therapy, (ASPAN 2010). This could be interpreted that prewarming should be initiated regardless of the patient temperature preoperatively.

These inconsistencies in best practice guidance make implementing a preventative regime extremely difficult in the clinical area. This in some part is due to a lack of knowledge among care providers about the available research; NICE guidance is more accessible and is often used to guide practice in the health care setting.

There is also some discrepancy between NICE and RCoA guidance with regard to the frequency of temperature measurement throughout the care pathway. NICE recommend monitoring the patient temperature from the induction of anaesthesia, then every 30 minutes until the end of surgery. ASPAN recommend frequent temperature measurement but do not state the timings when this should be done. The RCoA guidance however recommends temperature being measured throughout the operation and in recovery until such time as it reaches 36[degrees]C. Therefore, if the temperature is 36[degrees]C at the start of surgery, this could be interpreted that it does not need to be monitored again until the patient is admitted to recovery.

A consistent recommendation among the guidance is that all IV fluids >500mls should be warmed to 37 [degrees]C. It is interesting that this is a consistent recommendation among the guidelines and research but in practice it is not always followed. If the trust were to implement warming of IV fluids >500mls for all patients as standard, this would also have a cost implication for the trust. Fluid warming giving sets are approximately [pounds sterling]21 each and, based on 270 patients per month (intermediate surgery), this would be an added cost pressure of [pounds sterling]5,670 per month which is currently not budgeted for in the annual non-pay budget.

Recommendations for practice

Following my literature review and critical appraisals of research papers and clinical guidelines, I have come to the conclusion that the recommendations for practice with an aim to maintain normothermia throughout the perioperative environment are as follows:


All of the research has indicated that prewarming contributes to the maintenance of normothermia in this patient group. Even though there are some inconsistencies with the amount of time required for prewarming to be effective and the preoperative temperature classification prior to prewarming being initiated, all of the papers and the clinical guidelines advocate prewarming for surgical patients with a forced air warming device.

Regular temperature measurement

Again, there were inconsistencies in the frequency required for temperature measurement throughout the perioperative pathway, but if we are to prevent inadvertent hypothermia rather than treat it, we can only achieve this if we take regular temperatures and act accordingly depending on the results.

Publication of trust 'Maintaining Normothermia' guideline

A guideline has been developed based on NICE (2008), RCoA (2006) and researched guidelines which now needs to be implemented if staff are to comply with the available guidelines and best practice research.

Ambient theatre temperatures of at least 22[degrees]C

In all of the research studies and in two of the clinical guidelines (RCoA 2006, ASPAN 2010) there is a requirement for the ambient theatre temperature to be maintained at a minimum of 22[degrees]C. When we have audited our theatre temperatures, we have found them to be running at as little as 19[degrees]C which will contribute to the patient becoming hypothermic. The ambient temperature is now monitored daily by theatre staff and is increased if needed.

A minimum of one sheet and two blankets for each patient

As advocated by NICE (2008), the trust has adopted the two blanket policy. Often patients would arrive in theatre with only one hospital blanket and this could have been another contributing factor to the rise in numbers of hypothermic patients. Now, if patients arrive to theatre with less than two blankets, an incident form will be generated and reported to the ward manager.

Active warming (including IV fluid warming) for all major and intermediate surgery

Again, as consistent with the clinical guidelines and research, all theatre cases that are longer than 30 minutes, inclusive of anaesthetic time, should have active warming initiated. I have discussed this in detail during the Hypothermia Steering Group meetings as the theatre budget needs additional funding to accommodate the increase in cost to implement this regime.

Increased numbers of tympanic temperature probes and warming devices

I undertook a quick audit to ascertain if the theatre department and the wards had enough equipment to enable staff to increase temperature measurement and active warming therapies. I found that both areas required additional equipment. The equipment was sourced and should now enable staff to comply with the best practice guidelines.

Patient education

When we first started conducting audits on postoperative temperatures, it was evident that the new treatment centre had a higher percentage of hypothermic patients. Due to this patients were given information leaflets at preoperative clinic to inform them to bring a dressing gown and slippers to try to keep themselves warm prior to and after surgery. This action was not replicated in the main hospital, but after members of the Hypothermia Steering Group spoke with the preoperative nurses, patients will now receive the same information prior to surgery across both of the hospital sites.

Shared responsibility for patient care on a multi-disciplinary level

Since the development of the Hypothermia Steering Group, many processes have been adapted to try to maintain normothermia. This had to have a multi-disciplinary approach due to the nature of the perioperative pathway and this is often neglected in practice. A failing at any stage of the pathway could ultimately cause hypothermia and so it is the responsibility of all of the staff involved to play an active role in the process.


From the research papers, I have ascertained that the prewarming regime was instigated on all surgical patients regardless of their preoperative temperature, so long as they were not hyperthermic with a temperature of 38[degrees]C or more. I have discussed how this is inconsistent with some of the clinical guidelines that are available and what we have proposed to do as a trust to work around this.

All of the research papers found that prewarming contributed to the maintenance of normothermia which is supported by previous research findings. However, the only conclusive evidence for the method of prewarming is via forced air warming devices and here I have assessed which devices were used and if there would be any cost implications if a trust were to adopt these methods.

None of the prewarming studies tested two or more warming devices for efficiency which would have added depth to the comparison; NICE (2008) suggests that the comparison should be repeated for several different warming interventions. The forced air warming devices used in the studies are the same as those used in the trust in which I work and so the findings can reasonably be generalised to my local practice.

All of the research papers and the guidelines are consistent in that they recommend warmed IV fluids. As discussed previously, this is an area of practice which needs greater attention. Current practice is that IV fluids for major surgeries are warmed as standard, however there is inconsistent practice among clinicians for intermediate surgeries. Implementation of some measures will also need to be discussed at higher management level as they do have a cost implication.


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Correspondence address: Hinchingbrooke Healthcare NHS Trust, Hinchingbrooke Park, Huntingdon, Cambridge, PE29 6NT. Email:

Provenance and Peer review: Unsolicited contribution; Peer Reviewed; Accepted for publication November 2011.

About the author

Abigail Knaepel Dip HE, BSc (Hons)

Operating Department Practitioner, Hinchingbrooke Healthcare NHS Trust

No competing interests declared
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