Why do we warm patients in theatre?

In the first of a series of the most influential randomised controlled trials in anaesthesia, Dr Jeremy Fabes appraises the trial that kickstarted intra-operative patient warming.


Intra-operative Hypothermia

Perioperative normothermia to reduce the incidence of surgical wound infection and shorten hospitalization, Kurn, A. et al. NEJM;334:1209-15 1996.



Hypothermia was known to be undesirable with animal studies showing increased wound infections

Little warming employed – hypothermia common in colorectal surgery



The trial included adults having elective colorectal resection for cancer or inflammatory bowel disease.

It excluded patients taking corticosteroids, immunosuppressive drugs, chemotherapy, malnutrition, recent infection

It was powered for 90% chance of detection with alpha of 0.01, based on previous audit. It was calculated that 400 patients would be required and an interim analysis was planned a priori.

Patients were randomised to control (warming to 36.5°C) and intervention (passive cooling to 34.5°C core).

They minimised sympathetic confounders with aggressive hydration, post-operative oxygen protocolisation, and opiate post-operative patient-controlled analgaesia.

Patients were assessed for risk of infection (to compare groups) using National Nosocomial Infection Surveillance System – Type of surgery, ASA score and expected duration of surgery.

The surgeons were blinded and they made all the ‘subjective’ decisions such as transfusion, antibiotics, feeding, follow-up and discharge.

There was an objective definition of infection: presence of pus and growth in culture

There was also an objective definition of collagen deposition – in-growth into implanted tube



Core temperatures (34.7 vs. 36.6) with a difference that persisted for five hours postoperatively

The trial was stopped after 200 patients due to the groups meeting significance criteria


Hypothermic patients, despite comparable NNISS scores required:

– More transfusions of allogeneic blood (P = 0.01)

– More infection (19% vs. 6%)

– More collagen deposition

– Slower to eat, remove sutures and be discharged – even in those without an infection


Multivariate Analysis

– Wound infection was associated with hypothermia independent of predictable risk factors like age, NNISS and higher risk surgical site

– Hypothermia increases wound infection with an odds ration of 5

– Smoking increases it even more – the odds ration is 10



Intra- and immediate post-op periods are key for establishment of wound infection

Hypothermia around bacterial contamination decisive for establishment of infection

Hypothermia has an impact beyond wound infection – impact on healing, ileus, transfusion and LOS

Further support for negative impact of smoking – wound healing, infection and discharge



Generally accepted clinical efficacy, monitoring and thermoregulation now routine

National Institute of Clinical Excellence inadvertent hypothermia guideline

Intra-operative warming is now a standard in the Royal College of Anaesthetists audit recipe book and is a marker for quality of care/recovery in many departments.


Summary by Dr Jeremy Fabes

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