Mild hypothermia, blood loss and complications in elective spinal surgery

Spinal surgery carries risks of incidental spinal cord and nerve root injury. Neuroprotection, to minimize the extent of such injuries, is desirable. However, no neuroprotective strategies have been conclusively validated in nonvascular spinal surgery. Mild hypothermia resulting from general anesthesia is a readily achievable potential neuroprotective strategy. Mild hypothermia, however, has been associated with wound infection, increased operative blood loss and other complications. No previous studies have specifically evaluated whether mild hypothermia is associated with an increased risk of these complications in elective spinal surgery.

We investigated the association between incidental mild hypothermia, perioperative complications and operative blood loss.

This is a retrospective study employing cohort analysis, rank analysis and single and multivariate linear regression. The setting was the Veterans Administration Medical Center, a teaching hospital of the University of Miami.

Data on a total of 70 adult veterans aged 23 to 81 years undergoing complex spinal procedures in which passive cooling was employed during surgical decompression.

The variables measured were temperature, blood loss, mean arterial pressure (MAP) and duration of anesthesia. The outcome measured was the presence or absence of complications.

After 70 patients had been acquired, regression and rank analyses were performed to test for a link between mild hypothermia and blood loss. In addition, two cohorts, patients who experienced complications, and those who did not experience complications in the perioperative period, were compared for several variables including three measures of exposure to hypothermia. Surgical procedures included 60 cervical, 1 occipitocervical, 1 cervicothoracic, 7 thoracic and 1 thoracolumbar procedure. Hypothermia followed induction of anesthesia; esophageal or bladder temperature was monitored. Cooling was passive; warming utilized a forced air blanket. Temperature data from anesthetic records was used to derive mean intraoperative temperature, nadir intraoperative temperature and the rates of cooling and rewarming. The time course of hypothermia, the overall fluctuation in core temperature and the quantity of subbaseline temperature were determined. Medical and surgical complications were included. Two patients with complications considered irrelevant to hypothermia were removed from further analysis. Patients with and without complications were compared as cohorts for differences in mean values of age, comorbid risk factors, intraoperative MAP, intraoperative blood loss, anesthetic duration and temperature-related measures. Relationships between blood loss, anesthesia duration and temperature parameters were assessed in rank and regression analyses.

Patients with complications (n=12) had longer mean anesthetic durations (p=.0001) and larger mean surgical blood losses (p=.001) than patients without complications (n=56). Neither mean nor nadir intraoperative hypothermic temperatures were statistically associated with complications. However, large hypothermic integrals (p=.04) and the total quantity of recorded temperature fluctuation (p=.01) were both associated with complications. Comorbid risk factors, MAP and age were not statistically linked to complications. Finally, no relationship between any of the temperature measures and increased blood loss was found.

Operative blood loss was not linked to any index of the patient's temperature. Longer anesthesia durations were linked to complications and increased blood loss. Regarding mild hypothermia, neither mean nor nadir hypothermic temperatures were linked to complications, but the estimated total quantity of subbaseline temperature was linked, as was total fluctuation in temperature. Lengthy exposure to mild hypothermia appeared to be associated with wound infections. The use of mild hypothermia as a potential neuroprotective strategy during spinal surgery appears to be reasonably safe, but to avoid complications, the duration of hypothermic exposure should be minimized.