The Effects of Ambient Temperature and Forced-air Warming on Intraoperative Core Temperature

The Effects of Ambient Temperature and Forced-air Warming on Intraoperative Core Temperature

The purpose of this study is to determine:

1. the effect of ambient temperature on the rate of core temperature change from 1 to 3 hours after induction of anesthesia (linear phase of the hypothermia curve) in major operations lasting at least a couple of hours and
2. whether the relationship between ambient temperature and rate of core temperature change is different for patients who are or are not warmed with forced-air.

Each patient will be randomly assigned to ambient temperature of 19°C, 21°C, or 23°C. Using a factorial approach, patients will also be randomly assigned to passive insulation or forced-air warming, stratified by the three types of surgery listed below. Group allocation will be based on computer generated codes (randomly permuted block sizes) prepared by Department of OUTCOMES RESEARCH statisticians using SAS statistical software. Allocation of consented patients to designated ambient temperature and forced-air vs. passive insulation will be via a web site that will be accessed by investigators in Bejing about 90 minutes before surgery.

Patients assigned to passive insulation will be covered as usual with a cotton gown and single layer of cloth surgical draping. Patients assigned to forced-air warming will also be covered with a gown and surgical drapes, but a forced-air cover (Bair hugger 63500, 3M) will be inserted between the gown and the skin surface. A lower-body cover (about 91 by 221 cm) will be positioned so the lower end of the forced-air segments extend from the ankles upward for the entire length of the cover in thoracic and abdominal cases. The cover's foot drape will extend over the feet, and in turn be covered by the surgical drape. An upper-body forced-air cover will similarly be applied for patients having hip arthroplasties. The forced-air cover will be connected to a Bair Hugger blower and #635 covers set to "high" (≈43°C).

Ambient temperature will be adjusted to the designated temperature about an hour before a patient enters the operating room and adjusted as necessary to maintain the designated temperature throughout surgery. General anesthesia will be induced as per usual clinical routine. Neuraxial (epidural or spinal) and other regional blocks are permitted.

Any patients whose core temperature decreases to less than 34.5°C will be actively warmed with forced air and the ambient temperature increases to the extent practical.

Inclusion Criteria:

• Adults scheduled for major open thoracic surgery or video assist thoracic surgery or major abdominal surgery under general anesthesia expected to last at least two hours

Exclusion Criteria:

• Special risk for bleeding or myocardial infarction (as determined by the attending anesthesiologist)
• Patients who would otherwise have been actively warmed