Aggressive Warming vs. Routine Thermal Management During Surgery
- Previous small studies demonstrated adverse clinical impact from failure to correct intraoperative hypothermia, including prolonged recovery, increased transfusion requirement, impaired wound healing, surgical site infection, and postoperative cardiovascular events.
- The PROTECT study included 5,013 patients undergoing major noncardiac surgery, randomized to active warming to a core temperature of 37.0°C versus routine thermal management with targeted temperature of 35.5°C, to determine the impact on the primary composite outcome of myocardial injury, cardiac arrest, and all-cause mortality at 30 days and secondary outcomes including deep surgical site infection, transfusion requirement, hospital length-of-stay, and 30-day hospital readmission.
- No statistically significant difference in primary or secondary outcomes was observed between the two groups. Aggressive rewarming to 37.0°C was not protective against adverse outcomes compared to a more moderate thermal target of 35.5°C.
Does aggressive intraoperative warming to 37.0°C, compared to routine thermal management with a target temperature of 35.5°C, decrease the risk of adverse cardiovascular outcomes and all-cause mortality among patients with ≥1 cardiovascular risk factor undergoing major noncardiac surgery?
Background: Without active measures to counteract temperature loss, most patients become hypothermic during surgery, initially from redistribution of lower-temperature peripheral blood to core tissues after anesthetic induction, followed by anesthetic-induced widening of thermoregulatory thresholds in the cold operating room environment.
In the PROTECT study, patients ≥45 years of age planning to undergo major noncardiac surgery lasting 2-6 hours at 1 of 12 hospitals in China or at Cleveland Clinic were randomly assigned to a protocol of aggressive intraoperative warming to 37.0°C versus routine thermal management with a target of 35.5°C. Operating room temperature was maintained at 20°C, and temperature was measured in the nasopharynx or distal esophagus.
Patients in the aggressive warming protocol received: 1) prewarming with a forced air cover 30 minutes prior to anesthetic induction to mitigate redistribution-induced temperature loss, 2) application of two forced air covers during surgery as needed to maintain a target core temperature of 37.0°C, and 3) warming of all intravenous (IV) fluids to body temperature prior to administration. Patients in the routine thermal management arm received: 1) no prewarming prior to induction, 2) use of forced air warming during surgery only as needed to prevent core temperature from falling below 35.5°C, and 3) use of room temperature IV fluids.
The primary outcome was a composite of myocardial injury, nonfatal cardiac arrest, or all-cause mortality within 30 days. Secondary outcomes were deep surgical site infection within 30 days, red blood cell transfusion, duration of hospital stay, and hospital readmission within 30 days of surgery. Exploratory outcomes included reduction in hemoglobin on postoperative day 1, mean Quality of Recovery-15 score on postoperative day 3, 30-day superficial wound infection, and myocardial infarction according to the third universal definition.
Between March 2017–March 2021, 5,013 subjects were enrolled and included in the intention-to-treat analysis, with 2,507 assigned to active warming and 2,506 to routine thermal management. Thirty-day follow-up was complete in 99% of subjects. Intraoperative time-weighted core temperature was 36.8°C in the aggressive warming group versus 35.8°C in the routine thermal management group. Temperatures at the end of surgery were 37.1°C versus 35.6°C.
At least one component of the primary outcome (myocardial injury after noncardiac surgery [MINS], cardiac arrest, or mortality) occurred in 9.9% of aggressively warmed patients versus 9.6% of patients receiving routine thermal management (relative risk [RR], 1.04; 95% confidence interval [CI], 0.87-1.24; p = 0.69).
No impact of temperature management was observed on any of the individual components of the primary outcome. In the aggressive rewarming versus routine thermal management groups: MINS occurred in 9.4% versus 9.0% (RR, 1.05; 95% CI, 0.85-1.30), cardiac arrest occurred in 0.2% versus 0.6% (RR, 0.40; 95% CI, 0.13-1.27), and all-cause mortality occurred in 0.5% versus 0.7% of subjects (RR, 0.76; 95% CI, 32-184). No significant impact of aggressive warming was observed on any secondary or exploratory outcome.
Among patients undergoing major noncardiac surgery with ≥1 risk factor for a postoperative adverse cardiovascular event, aggressive intraoperative warming to a core temperature of 37.0°C did not decrease risk of adverse cardiovascular outcomes compared to maintenance of a thermal target of 35.5°C. Likewise, no significant difference in infection, transfusion requirement, or hospital readmission was observed.
The investigators cited hypothermia-induced vasoconstriction and increased catecholamine release from shivering and thermal discomfort as potential mediators of increased myocardial oxygen demand, which in turn might have led to greater risk of postoperative adverse cardiovascular events, as concerns motivating this study. These concerns were supported by earlier, small studies in which temperatures in the control arm may have been allowed to drift significantly lower than 35.5°C. Based on these new findings from the PROTECT cohort, there is no apparent clinical benefit to aggressive intraoperative warming to 37.0°C compared to maintenance of core temperature to a more moderate target of 35.5°C. It is still important to emphasize that failure to measure temperature or mitigate thermal loss in the operating room may lead to profound hypothermia with adverse clinical consequences.
Keywords: Anesthetics, Blood Transfusion, Body Temperature, Erythrocyte Transfusion, Heart Arrest, Heart Disease Risk Factors, Hemoglobins, Hypothermia, Intraoperative Period, Length of Stay, Myocardial Infarction, Operating Rooms, Patient Readmission, Rewarming, Risk Factors, Secondary Prevention, Surgical Procedures, Operative, Surgical Wound Infection, Temperature
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