Clinical studiesMild hypothermia, blood loss and complications in elective spinal surgery☆
Introduction
Strategies to prevent neurological injury during spinal surgery include the use of neurophysiologic monitoring and neuroprotection. No specific studies of the efficacy of neuroprotection in spinal surgery exist, and evidence obtained in other clinical settings has been extrapolated into the practice of spinal surgery. Because methylprednisolone has gained acceptance as a treatment for acute spinal cord injury, many spine surgeons use the Second National Acute Spinal Cord Injury Study (NASCIS II) [1] dosing protocol during “high risk” cases on a prophylactic basis or administer steroids if there is an apparent adverse change in monitored neurophysiologic variables. Using similar reasoning, hypothermia, which has an established neuroprotective efficiency to prolong the ischemic tolerance of the brain or spinal cord [2], [3], [4], is thus hypothesized to provide neuroprotection against potential mechanical injury during spinal surgery. Such injuries might involve accidental contusion, spinal cord or nerve root stretching or compression, or injury associated with the removal of intrinsic tumors. It is hypothesized that the neuroprotective agent, whether methylprednisolone or hypothermia, will reduce the extent of injury mainly by altering mechanisms of secondary cellular damage. However, both methylprednisolone and mild hypothermia have been associated with a risk for increased complications in some series of surgical patients [5], [6], and therefore data regarding risk/benefit should influence their use during elective spinal surgery.
Because general anesthesia impairs thermal regulation, mild hypothermia is commonly observed during surgery. Continuous monitoring of core temperature is a surgical standard and provides data that can be correlated with blood loss and complications. Experimental data have indicated that mild hypothermia (32.5C to 35.5C) can reduce some aspects of secondary spinal cord injury after spinal cord contusion [7], [8], [9], [10]; however, this efficacy of neuroprotective hypothermia is not clearly established in human spinal cord injury. Clinical and experimental studies have indicated that hypothermic neuroprotection from ischemia is markedly more effective if established before injury than if applied after injury as a therapy [11], [12]. Mild hypothermia has been studied as a treatment for brain injury [13], but only a few studies have reported its impact in elective neurosurgery [14], [15], [16].
Deep levels of hypothermia (less than 32.5C), clinically established for ischemic neuroprotection during high-risk cerebrovascular surgery, have been associated with altered hemostasis [17], [18], [19] and cardiac arrhythmia [20]. Mild hypothermia, considered safer, has also been associated with several potential complications, including wound infection [21], increased surgical blood loss [22], [23], perioperative myocardial infarction [24], and altered surgical wound healing [25], [26]. Mild hypothermia is therefore both a potential risk factor for perioperative morbidity [27], [28] and a potential neuroprotective strategy in spinal surgery. Furthermore, the hypothermic temperatures at which risks for complications increase and neuroprotective benefits occur may not be the same. As a risk factor, “mild” hypothermia is considered to exist at any temperature below 37C; however, experimental studies indicate that neuroprotection requires central nervous system temperatures below approximately 35.5C [29]. The threshold for hypothermic neuroprotection in humans is not known with certainty. The relative risks and potential neuroprotective benefits of hypothermia are likely to be related to the timing, duration, depth of hypothermia and rate of rewarming. Because of its neuroprotective potential, apparent simplicity, possible risks and inherent association with general anesthesia, it is important to determine the impact of mild hypothermia in the setting of spinal surgery.
This study assesses the association between incidental mild hypothermia, surgical blood loss and perioperative complications in a series of patients undergoing spinal procedures (ie, cervical or thoracic) that placed the spinal cord at risk for injury. A new measure to quantify subbaseline temperature, the time-temperature integral t1∫tx (TB−TO), is introduced, where TB = temperature baseline, TO = a patient's documented temperature, t1 = the first time point with a core temperature less than 37°C, and tx = the last time point with a core temperature less than 37°C.
Section snippets
Methods
We acquired data prospectively on 70 consecutive surgical patients in a single surgeon series at the Veterans Affairs Medical Center in Miami, Florida (institutional review board approval number 00242), a teaching hospital of the University of Miami. The population consisted of adult veterans, including patients with chronic and acute spinal cord injury. The surgical procedures included 60 cervical, 1 occipitocervical, 1 cervicothoracic, 7 thoracic and 1 thoracolumbar procedure. Indications for
Results
The surgical procedures performed are summarized in Table 2.
Time-temperature integral
We found that t1∫tx (TB−TO) is useful to represent intraoperative body temperature changes. It is strongly associated with duration of subbaseline temperature, and mean and nadir temperatures. Because t1∫tx (TB−TO) is measured using only the duration between time points when subbaseline temperature (<37C) is actually present, it differs from the mean in which all the temperature measurements are divided by the total anesthetic time. The mean could underestimate the occurrence of a brief but
Conclusion
Brief periods of mild hypothermia during elective spinal surgery were not associated with an increased rate of complications, but extended periods may increase risks of wound infection. Mild hypothermia was not associated with increased surgical blood loss. Studies to determine whether mild hypothermia is definitely neuroprotective for human spinal cord trauma are still needed. A randomized prospective study of the safety of mild hypothermia in elective adult spinal surgery is recommended.
Acknowledgements
The anesthesia staff at the Miami Veteran's Administration hospital, especially Dr. Michael Lewis and Dr. Gerard DeSousa. Elizabeth Owen and Janet Gabrin-Levy performed manuscript editing and figure preparation.
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