Patients kept at a body temperature of 37°C during major surgery had no fewer cardiac complications than patients kept at 35.5°C, according to recent research in a collaboration between US and Chinese scientists. There were also no differences in the number of infections or required blood transfusions in patients kept at cooler body temperatures.
Body temperature generally decreases during surgery, largely because anaesthetic medications interfere with the body’s processes for regulating temperature. While practices vary around the world, nursing staff in Western countries typically use forced-air heaters to keep patients warm during surgery, with a target temperature of 36°C, or 96.8°F.
This trial, one of the largest to date, sought to determine whether warming patients even more – to 37°C, or 98.6°F – would reduce the risk of cardiac complications, which are a leading cause of death in the first 30 days after major surgery.
Results showed no significant differences between groups for the trial’s primary endpoint, a composite of troponin elevation due to ischaemia (an indicator of heart injury), non-fatal cardiac arrest or death from any cause within 30 days after surgery. Researchers also reported no differences for any of the trial’s secondary endpoints.
“This trial tells us that there is no benefit to aggressively warming patients to 37°C during surgery. It is simply unnecessary, and it doesn’t improve any substantive outcomes,” said Dr Daniel Sessler, Michael Cudahy professor and chair of the Department of Outcomes Research at Cleveland Clinic and the trial’s lead author.
“Also, the results show that 36°C should not be considered the threshold for defining mild hypothermia since there was no harm at 35.5°C.”
The researchers enrolled 5,050 patients who had surgery at 13 medical centres, mostly in China. Participants had various major non-cardiac surgical procedures, with a minimum duration of two hours and an average duration of four hours. Half of the patients were randomly assigned to receive routine care, with a target body temperature of 35.5°C, while half received aggressive warming, with a target body temperature of 37°C.
For patients assigned to routine care, nursing staff put a warming cover in position but did not activate it until the patient’s body temperature decreased to less than 35.5°C, resulting in an average group body temperature of 35.6°C. With the more aggressive warming protocol, nurses covered patients with a heated blanket for 30 minutes before surgery and then used two forced-air heaters to keep patients warmed to a mean of 37.1°C during surgery.
In addition to seeing no benefit in terms of the composite primary endpoint, the trial reported no significant differences between groups in serious wound infections, length of hospitalisation, hospital re-admissions or the need for blood transfusions.
The investigators were surprised that rates of wound infections and transfusions were similar to previous studies, which suggested that both were more common in patients maintained at lower body temperatures.
Nearly all patients were enrolled in China, Sessler said, but the results should be generalisable to patients and health care settings in other countries.
“This study shows that it is reasonable to keep patients warm, but we saw no evidence whatsoever that it makes a difference if they’re just above or just below 36°C,” he said. “Surgical patients should still be warmed, but there’s no need to be super-aggressive about the warming.”
The study did not assess less serious or non-medical outcomes, such as patient comfort or shivering. Sessler said that patients maintained at a lower body temperature may shiver or feel cold after surgery, but both are temporary and unlikely to have a meaningful health impact.
Study details
Aggressive intraoperative warming versus routine thermal management during non-cardiac surgery (PROTECT): a multicentre, parallel group, superiority trial.
Daniel Sessler, Lijian Pei, Kai Li, Shusen Cui, Matthew Chan, et al.
Published in The Lancet on 4 April 2022
Summary
Background
Moderate intraoperative hypothermia promotes myocardial injury, surgical site infections, and blood loss. Whether aggressive warming to a truly normothermic temperature near 37°C improves outcomes remains unknown. We aimed to test the hypothesis that aggressive intraoperative warming reduces major perioperative complications.
Methods
In this multicentre, parallel group, superiority trial, patients at 12 sites in China and at the Cleveland Clinic in the USA were randomly assigned (1:1) to receive either aggressive warming to a target core temperature of 37°C (aggressively warmed group) or routine thermal management to a target of 35.5°C (routine thermal management group) during non-cardiac surgery. Randomisation was stratified by site, with computer-generated, randomly sized blocks. Eligible patients (aged ≥45 years) had at least one cardiovascular risk factor, were scheduled for inpatient non-cardiac surgery expected to last 2–6 h with general anaesthesia, and were expected to have at least half of the anterior skin surface available for warming. Patients requiring dialysis and those with a body-mass index exceeding 30 kg/m2 were excluded. The primary outcome was a composite of myocardial injury (troponin elevation, apparently of ischaemic origin), non-fatal cardiac arrest, and all-cause mortality within 30 days of surgery, as assessed in the modified intention-to-treat population.
Findings
Between March 27, 2017, and March 16, 2021, 5,056 participants were enrolled, of whom 5013 were included in the intention-to-treat population (2507 in the aggressively warmed group and 2506 in the routine thermal management group). Patients assigned to aggressive warming had a mean final intraoperative core temperature of 37.1°C (SD 0.3) whereas the routine thermal management group averaged 35.6°C (SD 0.3). At least one of the primary outcome components (myocardial injury after non-cardiac surgery, cardiac arrest, or mortality) occurred in 246 (9.9%) of 2,497 patients in the aggressively warmed group and in 239 (9.6%) of 2,490 patients in the routine thermal management group. The common effect relative risk of aggressive versus routine thermal management was an estimated 1.04 (95% CI 0.87–1.24, p=0.69). There were 39 adverse events in patients assigned to aggressive warming (17 of which were serious) and 54 in those assigned to routine thermal management (30 of which were serious). One serious adverse event, in an aggressively warmed patient, was deemed to be possibly related to thermal management.
Interpretation
The incidence of a 30-day composite of major cardiovascular outcomes did not differ significantly in patients randomised to 35.5°C and to 37°C. At least over a 1.5°C range from very mild hypothermia to full normothermia, there was no evidence that any substantive outcome varied. Keeping core temperature at least 35.5°C in surgical patients appears sufficient.
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