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,
,§
*Department of Anesthesiology, Washington University, St. Louis, Missouri;
Department of Anesthesia and General Intensive Care, University of Vienna, Vienna, Austria;
Outcomes ResearchTM Institute, University of Louisville, Louisville, Kentucky; and
§Ludwig Boltzmann Institute for Clinical Anesthesia and Intensive Care, Vienna, Austria
Address correspondence to Andrea Kurz, MD, Department of Anesthesiology, Washington University, 660 S. Euclid Ave., St. Louis, MO 63110. Address e-mail to kurza{at}msnotes.wustl.edu
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Abstract |
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Implications: In this observational study, we evaluated core temperatures and intraoperative thermal management in patients undergoing spinal or epidural anesthesia. Hypothermia was common, however, rarely detected either by temperature monitoring or estimates by anesthesiologists. In addition, it was not treated with active warming. Consequently, temperature monitoring and management have to be done during neuraxial anesthesia.
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Introduction |
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Hypothermia during neuraxial anesthesia develops initially from a core-to-peripheral redistribution of body heat (6), the same mechanism that causes the initial hypothermia after the induction of general anesthesia (7). Redistribution of body heat during spinal or epidural anesthesia typically decreases core temperature 0.5°-1°C (6) with the amount depending on numerous factors including the patient’s previous thermal environment (8) and medication use (9). In most patients, redistribution is the major cause of hypothermia. Subsequent hypothermia occurs because of heat loss exceeding heat production. The extent to which core temperature decreases during this phase depends largely on ambient temperature (10,11), the magnitude and duration of the surgical procedure (12), and the amount of unwarmed IV fluids given (13).
A recent survey concluded that temperature monitoring is rare during neuraxial anesthesia (14). A limitation of this study, however, is that the authors did not evaluate the magnitude or duration of surgery, use of active warming, or patient temperatures (15). Consequently, the authors were unable to determine whether unmonitored patients actually become hypothermic.
Temperature monitoring may be rare during neuraxial anesthesia because clinicians restrict monitoring to cases where they believe there will be a risk of hypothermia. However, they may use active warming even without monitoring core temperature in patients at high risk of hypothermia, especially in those undergoing large and long procedures. Clinicians assume that they are able to maintain normothermia in the entire patient population undergoing neuraxial anesthesia by using temperature monitoring and thermal management selectively in high-risk patients. This assumption also implies that anesthesiologists can estimate patient thermal status without monitoring core temperature.
We thus tested the hypotheses that during neuraxial anesthesia: 1) core hypothermia is related to extent and duration of surgery; 2) temperature monitoring and thermal management is used selectively in patients at high risk of hypothermia, namely those undergoing large and long procedures; and 3) anesthesiologists can estimate patient thermal status.
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Methods |
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On patient arrival in the postanesthesia care unit, we recorded the type of neuraxial anesthesia (epidural versus spinal), duration of surgery, whether body temperature was monitored intraoperatively, and whether forced-air warming was used. Patients were asked to rate their thermal comfort on a 100-mm-long visual analog scale, in which 0 mm was designated the worst imaginable cold, 50 mm thermoneutrality, and 100 mm identified the worst imaginable heat. We also asked the anesthesiologists to rate patient thermal status as normothermia (core temperature >36°C), hypothermia, or unable to estimate. We also recorded the type and magnitude of surgery. Operations were divided into small (arthroscopy, cystoscopy etc.), medium (knee arthroplasty, lumbar decompression etc.), and large (hip arthroplasty, major vascular surgery etc.).
Postoperative core temperature was measured at the tympanic membrane with a thermocouple (Mon-a-Therm; Mallinckrodt Anesthesiology Products, St. Louis, MO). The aural probe was inserted until the thermocouple touched the tympanic membrane. The aural canal was occluded with cotton, the probe secured, and a gauze bandage positioned over the external ear. We also measured tympanic temperature with an infrared thermometer (Ototemp 3000, HTTS-3000-SD, Exergen, Newton, MA). Both anesthesiologists and patients were told that the purpose of the study was to compare infrared aural canal measurements and thermocouple determined tympanic membrane temperatures.
Patients were initially divided by the extent of their operations. Within these categories, we evaluated the fraction of patients who were given forced-air warming versus passive insulation and whose temperature was monitored intraoperatively. Normothermia was defined as a tympanic membrane temperature >36°C. Then, we evaluated the number of normothermic and hypothermic patients in each monitoring category. Finally, patients were divided by the anesthesiologists’ impression of their thermal status. Within each category, we evaluated the fraction of hypothermic and normothermic patients.
The fraction of patients within various designated groupings were evaluated with 
2 statistics. The relationship between duration of surgery and core temperature was evaluated with linear regression. Core temperatures in patients having small, medium, and large operations were compared with a one-way analysis of variance and Scheffé’s f-test. Results are presented as means ± SD; P < 0.05 was considered statistically significant.
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Results |
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Twenty-six patients (22%) had small operations; 71 patients (59%) had medium operations; and 23 patients (19%) had large operations. Among the patients who were not given forced-air warming, initial postoperative tympanic membrane temperatures did not differ significantly as a function of operation size: small, 35.5 ± 0.7°C; medium, 35.3 ± 0.7°C; and large, 35.4 ± 0.8°C (P = 0.82). Furthermore, there was no correlation between duration of surgery and initial postoperative core temperature in the unwarmed patients: slope = -0.002, r2 = 0.02.
Fluids were warmed in only 15% of the cases, and there were no differences among the patients having small, medium, and large operations. Forced-air warming was used in only 37 patients (31%). The fraction of patients in whom core temperature was monitored and in whom forced-air warming was used did not differ as a function of operation size (Fig. 1). Body temperature was monitored in 27% of the patients and the only intraoperative monitoring site was upper chest skin temperature. Unmonitored patients were as likely to be hypothermic as those who were monitored (Fig. 2). Anesthesiologists were unable to reliably estimate patient thermal status (Fig. 3). Patients whose core temperature exceeded 36°C felt significantly warmer than those with core temperatures between 35°C and 36°C, 65 ± 10 vs 49 ± 24 mm (P = 0.002). Interestingly, thermal comfort of patients whose core temperatures were <35°C and whose core temperatures were between 35°C and 36°C was comparable (48 ± 24 vs 49 ± 24 mm, respectively).
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Discussion |
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Neuraxial anesthesia impairs behavioral thermoregulation with the result that patients often do not consciously perceive that they are hypothermic (4,5). Our data were consistent with previous observations in that patients were unable to identify when they became hypothermic. Additionally, the anesthesiologists were unable to reliably estimate patient thermal status. This presumably resulted, in part, because patients did not perceive being hypothermic, and, in part, from the lack of correlation between the amount of hypothermia and the duration or magnitude of surgery. These results suggest that core temperature should be monitored in patients having neuraxial anesthesia because hypothermia is common and cannot be predicted.
Generally, core temperature can be evaluated reliably in the pulmonary artery, distal esophagus, tympanic membrane, or nasopharynx (17) and can be estimated with reasonable accuracy by using oral, axillary, and bladder temperatures except during extreme thermal perturbations (18–20). Among these temperatures, only a few are routinely suitable during neuraxial anesthesia. Tympanic measurements are accurate and safe when a thermocouple is positioned adjacent to the membrane and the ear canal occluded with cotton. However, placement of the probe requires some time and consequently might not be optimal in an busy clinical setting. Most infrared aural canal temperatures are insufficiently accurate or precise for clinical use (21,22). Axillary temperatures are a good alternative during conduction anesthesia—so long as the probe is positioned carefully over the artery and the arm maintained at the patient’s side. Intermittent oral temperature is another good method (23). Forehead skin surface temperature is another potential measurement site. It estimates core temperature with a 2°C offset remarkably well (24). Core temperature during neuraxial anesthesia can thus be estimated from the tympanic membrane, axilla, mouth, or forehead skin surface. One of these sites should be suitable in most any patient. There is thus, no compelling technical reason to avoid temperature monitoring in this patient population.
The potential limitation of our study is that it was performed in only two centers. Presumably the results would differ somewhat had we included a larger number of study centers, although the frequency of temperature monitoring was similar to that reported in a recent survey of members of the American Society of Anesthesiologist (14). It appears unlikely that the amount of hypothermia in unwarmed patients given neuraxial anesthesia will differ much from center to center or that anesthesiologists in other centers can better estimate patient thermal status. It is similarly unlikely that other patients would better estimate their own thermal status.
In summary, there is substantial potential for hypothermia during neuraxial anesthesia. Temperature monitoring and thermal management were not applied selectively in the patients we studied. However, there was also no correlation between the amount of hypothermia and the magnitude or duration of surgery. Hypothermia was common, however, rarely detected during neuraxial anesthesia because of the lack of temperature monitoring and the inability of anesthesiologists to reliably estimate patient thermal status. Because perioperative hypothermia is associated with numerous complications, precautions to reduce heat loss and active warming should be taken to prevent intra and postoperative hypothermia. Therefore, intraoperative temperature should be monitored routinely in all patients.
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Acknowledgments |
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We appreciate the assistance of Kathy Martin-Bredahl, RN, of the Department of Anesthesiology, Washington University, St. Louis, MO, and Thomas Scheck and Alex Kober of the Department of Anesthesia and General Intensive Care, University of Vienna, Vienna, Austria.
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Footnotes |
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References |
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