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Anesth Analg 2000;90:938-945
© 2000 International Anesthesia Research Society

REGIONAL ANESTHESIA AND PAIN MEDICINE

The Accuracy and Precision of Body Temperature Monitoring Methods During Regional and General Anesthesia

Christine G. Cattaneo, MD, Steven M. Frank, MD, Todd W. Hesel, Hossam K. El-Rahmany, MD, Lauren J. Kim, BA, and Kha M. Tran, BA

Department of Anesthesiology and Critical Care Medicine, The Johns Hopkins Medical Institutions, Baltimore, Maryland

Address correspondence and reprint requests to Christine G. Cattaneo, MD, Department of Anesthesiology and Critical Care Medicine, The Johns Hopkins Medical Institutions, Halsted 842, 600 N. Wolfe St., Baltimore, MD, 21287. Address e-mail to ccattaneo{at}jhmi.edu


   Abstract
Top
 Abstract
Introduction
 Methods
 Results
 Discussion
 References
 
We tested the hypotheses that accuracy and precision of available temperature monitoring methods are different between spinal anesthesia (SA) and general anesthesia (GA), and that patients receiving SA are at equal risk for hypothermia as those receiving GA. Patients scheduled for radical retropubic prostatectomy were enrolled. Either GA (n = 16) or SA (n = 16) was given according to patient and clinician preference. Temperatures were monitored with thermocouple probes at the tympanic membrane, axilla, rectum, and forehead skin surface. Tympanic temperatures were also measured with an infrared device, and forehead skin temperatures were monitored with two brands of liquid crystal thermometer strips. Accuracy and precision of these monitoring methods were determined by using tympanic membrane temperature, measured by thermocouple, as the reference core temperature (Tc). At the end of surgery, Tc was similar between SA (35.0 ± 0.1°C) and GA (35.2 ± 0.1°C) (P = 0.44). Accuracy and precision of each temperature monitoring method were similar between SA and GA. Rectal temperature monitoring offered the greatest combination of accuracy and precision. All other methods underestimated Tc. These findings suggest that patients receiving SA or GA are at equal and significant risk for hypothermia, and should have their temperatures carefully monitored, recognizing that most monitoring methods underestimate Tc.

Implications: Body temperature should be monitored during spinal anesthesia because patients are at significant risk for hypothermia. Rectal temperature is a valid method of measuring core temperature, whereas other methods tend to underestimate true core temperature.


   Introduction
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 Abstract
 Introduction
Methods
 Results
 Discussion
 References
 
Maintaining body temperature in the perioperative period has a significant impact on clinical outcome. Hypothermia (core body temperature <36°C) is associated with adverse clinical events, including myocardial ischemia (1), cardiac morbidity (2), coagulopathies (3,4), prolonged drug effect (5), wound infection (6), and patient discomfort (7).

Historically, body temperature has been monitored during general anesthesia (GA) to identify malignant hyperthermia. However, because regional anesthesia is not associated with malignant hyperthermia, temperature is often ignored in patients receiving this anesthetic technique (8,9). A recent survey found that only 33% of practicing anesthesiologists routinely monitor temperature during regional anesthesia, whereas 56% believe that temperature should be monitored routinely with regional anesthesia (8). Reasons for not monitoring temperature may include the misconception that regional anesthesia is not associated with a clinically significant risk for hypothermia, or the lack of convenient, reliable, and tolerable sites to monitor temperature in the awake or sedated patient. Although various skin-surface sites and methods are routinely used, no previous study has assessed the accuracy and precision of various temperature monitoring sites and methods during regional anesthesia. Because compensatory cutaneous vasoconstriction occurs above the level of regional block (10,11), and thermoregulatory vasoconstriction may also decrease skin temperature (12), it is possible that cutaneous temperature measured above this level is lower than core temperature (Tc) during regional anesthesia.

Thermoregulation is impaired during spinal anesthesia (SA) (13), epidural anesthesia (14), and GA (15). Studies comparing the incidence of hypothermia during regional and general anesthetic techniques report conflicting results. Some investigators report a similar risk of hypothermia with epidural anesthesia and GA (16,17), whereas others show an enhanced risk with epidural anesthesia compared with GA (18,19). Still another study reported a less frequent incidence of hypothermia with epidural than with GA (20). Whereas most comparative studies use epidural and GA, few studies have evaluated the relative risk of hypothermia with SA.

We therefore, determined the relative accuracy and precision of various temperature monitoring sites and methods during SA and GA. We tested the hypotheses that: 1) accuracy and precision of various temperature monitoring sites and devices are different during SA and GA; and that 2) patients receiving SA are at equal risk for perioperative hypothermia as those receiving GA.


   Methods
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 Abstract
 Introduction
 Methods
Results
 Discussion
 References
 
After receiving approval from our joint committee on clinical investigation and obtaining written informed consent, 32 ASA physical status II and III men patients scheduled for radical retropubic prostatectomy were enrolled. The sample size was chosen to achieve a power (ß) of 0.80, with a confidence interval of 95% ({alpha} = 0.05) and to detect a correlation coefficient ({rho}) of 0.9. No patient had a history of significant cardiovascular or pulmonary disease. At the discretion of the anesthesia care team and the patient’s preference, patients received either SA (n = 16) or GA (n = 16). Patients were enrolled in the study consecutively until a total of 16 for each of the anesthetic techniques were studied. Percent body fat was measured by the infrared interactance method (21).

Patients receiving SA were given IV premedication with midazolam 1–3 mg immediately before the spinal injection. Spinal anesthesia was started in the operating room (OR) with 20 mg bupivacaine (0.75%), 20 mcg fentanyl, and 200 mcg epinephrine, delivered through a 25- or 27-gauge Whitacre needle. A T3–T8 sensory block level was achieved as detected by loss of pinprick sensation. The patients were sedated intraoperatively with additional IV midazolam (1–5 mg). Regression of spinal block in the recovery room was assessed by pinprick test every 20 min by trained postanesthesia care unit nurses. Duration of spinal block was defined as the time required for resolution to a level of L1.

General anesthesia included IV thiopental or propofol, an opioid (fentanyl, morphine or hydromorphone), a nondepolarizing muscle relaxant, and approximately 1 minimum alveolar anesthetic concentration of isoflurane. All patients receiving GA had tracheal intubation for the intraoperative course and were extubated in the OR. All patients had postoperative analgesia delivered IV by patient-controlled analgesia with hydromorphone.

In the preoperative holding area, two liquid crystal thermometry strips (LCT) (OmniTherm, Inc.; St. Louis, MO and Sharn, Inc., Tampa, FL) and a skin-surface thermocouple probe (Mon-a-thermTM; Mallinckrodt, St. Louis, MO) were placed on the forehead to monitor forehead skin temperature. An infrared tympanic thermometer (Thermoscan; San Diego, CA) was used to measure tympanic membrane temperatures.

On arrival in the OR, a thermocouple probe was placed in the aural canal by one of the investigators to measure Tc at the tympanic membrane. Placement was verified with the patient reporting detection of a scratching sound when the thermocouple wire was rubbed. Then, the probe was taped securely in place. Thermocouple probes were also placed on the skin in the axilla, 10 cm into the rectum, and near the patient, without contact to measure ambient temperature. Temperature measurements taken by the thermocouple probes were recorded at 2-min intervals by using the electric thermometer (Isothermex; Columbus Instruments, Inc., Columbus, OH) linked to a laptop computer, and then, stored on a hard disk. Forehead skin temperature taken by the LCTs and tympanic membrane temperature taken with the infrared thermometer were recorded at 15-min intervals by one of the investigators. Temperatures were monitored immediately after initiation of anesthesia and continued until discharge from the recovery room, approximately 2–3 h postoperatively.

Temperatures in the two anesthetic groups were compared before the induction of anesthesia, at 15-min intervals intraoperatively, and immediately before leaving the OR. Postoperatively, temperatures were compared on arrival in the recovery room, at 30-min intervals in the recovery room, and on discharge from the recovery room.

Patients in both anesthetic groups received similar thermal care both in the OR and in the recovery room as per protocol. All patients in both groups (GA and SA) received prewarmed (38°C) lactated Ringer’s solution and blood products through an in-line fluid warmer. Ambient OR temperature was 21–23°C. No forced air warming devices were used at any time during the study. Patients had a single cotton blanket across their upper bodies intraoperatively and one or two cotton blankets over their entire bodies postoperatively.

Demographic variables were compared between the two groups by Student’s t-test. The incidence of postoperative core hypothermia in the two groups was compared by {chi}2 analysis. By using tympanic temperature measured by thermocouple as the reference Tc, accuracy was defined as the difference from this reference (in °C) for each monitoring method. Precision was defined by linear regression of the reference Tc and temperature measured by each monitoring method. The Bland and Altman statistical method (22) was used to assess accuracy and precision of the temperature monitoring methods from the reference Tc. All determinations of accuracy and precision were made by using measurements taken in the OR at the end of the surgical procedure. Significance was defined as P < 0.05 and all data are given as mean ± SEM. Temperature differences of 0.5°C were defined a priori as clinically significant.


   Results
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 Abstract
 Introduction
 Methods
 Results
Discussion
 References
 
The SA and GA groups were similar with regard to age, weight, height, percentage body fat, and preoperative Tc (Table 1). There were no significant differences in estimated blood loss, crystalloid administration, number of units of blood transfused, duration of surgery or average ambient temperature (Table 2). The temperature of the irrigating fluid used in the OR was approximately 37°C. The average sensory block level was T6 ± 1 dermatome.


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  		Table 1. Preoperative Demographic Data
 

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  		Table 2. Intraoperative and Postoperative Data
 
There was no significant difference in Tc between SA or GA groups at any time intraoperatively (Fig. 1). Mean Tc at the end of the surgical procedure was similar in the SA (35.0 ± 0.1°C) and GA (35.2 ± 0.1°C) groups (P = 0.44). On admission to the recovery room, there was no difference in Tc between the anesthetic groups. Rewarming rates, however, were significantly different from 30 to 120 min postoperatively (Table 3). At 30, 60, 90, and 120 min after admission to the recovery room, the incidence of hypothermia was less in the GA group than in the SA group (P < 0.05). The duration of spinal block in the recovery room was 70 ± 10 min. Ambient temperature in the recovery room was 21–22°C.



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  		Figure 1. Core temperature measured at the tympanic membrane during and after anesthesia and surgery for radical retropubic prostatectomy. Core hypothermia occurred similarly during general anesthesia and spinal anesthesia. Patients receiving spinal anesthesia remained hypothermic longer into the postoperative period. End OR = end of surgery. Data are given as mean ± SEM. *P < 0.05 versus spinal anesthesia.

 

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  		Table 3. Incidence of Postoperative Hypothermia (Core Temperature <36°C)
 
Accuracy of each temperature monitoring method was similar in patients receiving SA or GA (Fig. 2 & 3, Tables 4 & 5), with the exception of forehead skin temperature, which was closer to Tc during GA. All sites except the rectum underestimated Tc, regardless of anesthetic technique (Fig. 2, Table 4). Precision of each temperature monitoring method was also similar in patients receiving SA or GA, with the exception of axillary skin temperature which had greater precision during GA (Fig. 3, Table 5).



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  		Figure 2. Accuracy of core temperature of various monitoring methods shown as Bland and Altman (22) plots. Core temperature measured by thermocouple probe at the tympanic membrane was used as the reference core temperature. A negative difference indicates a measurement less than the reference temperature. Spinal and general anesthesia are compared.

 

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  		Table 4. Differences Between Temperature Measurements at the Time of Admission to the Recovery Room in °C
 


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  		Figure 3. Precision of various temperature monitoring sites shown as linear regressions of each monitoring site compared with the reference core temperature measured at the tympanic membrane by thermocouple probe. Spinal and general anesthesia are compared.

 

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  		Table 5. Correlation Between Different Temperature Monitoring Methods Shown as r2 Values from Simple Linear Regression
 

   Discussion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
References
 
The similar incidence and magnitude of perioperative hypothermia in patients receiving SA and GA suggest that body temperature should be carefully monitored, regardless of anesthetic technique. In addition, the prolonged duration of residual postoperative hypothermia in patients receiving SA may put them at greater risk for hypothermia-related adverse events. There were no clinically significant differences for accuracy or precision of temperature monitoring methods between SA and GA.

Given the proximity to the internal carotid artery, the tympanic membrane temperature, measured by thermocouple probe, is considered to represent true Tc. This does not imply that other methods of measuring tympanic membrane temperature are equally reliable for clinical use. Intraaural infrared thermometers are a relatively poor method for intraoperative temperature monitoring (23,24). In the current study, the infrared tympanic thermometer was less accurate and precise than either the rectum or the Omni LCT method.

Studies investigating the adequacy of LCTs for temperature monitoring have reported different conclusions. One study found LCTs adequate for monitoring an increase in temperature (25), whereas others found them either adequate (26,27) or inadequate (28) to follow body temperature trends perioperatively. However, one study (25) evaluated patients during cardiopulmonary bypass, which involved large Tc changes and therefore may not apply to patients with smaller Tc changes. Because patients receiving regional anesthesia are typically vasoconstricted above the level of sympathetic block (10,11), we hypothesized that skin surface monitoring on the upper body would underestimate true Tc. In the current study, skin-surface monitoring methods generally underestimated Tc during both SA and GA. This underestimation was especially apparent at the lower range of Tc. This may reflect the decreased cutaneous blood flow at lower Tc because of thermoregulatory vasoconstriction. Although rectal temperature was greater in accuracy and precision compared with other methods, rectal was the only method that appeared to overestimate Tc. This may reflect fecal insulation and delayed changes during the development of hypothermia.

Frank et al. (8) reported that body temperature is not regularly monitored during regional anesthesia, although most practitioners felt that it should be monitored. When body temperature is monitored during regional anesthesia, most clinicians report using either forehead skin surface with LCT devices (70%) or axillary skin temperature (40%) (8). It should be recognized that these techniques are likely to underestimate Tc, especially at lower Tc values, which may result in undue concern about hypothermia. The accuracy of LCT monitors is dependent on the built-in offset which is usually between 2°C and 4°C. This offset allows an estimate of Tc derived from skin temperature. Because surface warming methods may directly heat the skin temperature monitors, these methods of monitoring would be unreliable during active warming.

Previous studies have shown impaired thermoregulation with epidural anesthesia (14,16,29,30), SA (13,29,31,32), and GA (15,16,33). Vaughan et al. (17) reported a similar magnitude of postoperative hypothermia in patients receiving regional anesthesia and GA; however, that study did not specify which type of regional anesthesia (epidural or SA) was used, the level of regional block, or the specific surgical procedure. A later study reported an equivalent risk of postoperative hypothermia in patients receiving either epidural anesthesia or GA (10). Yet, other investigators have reported conflicting results. Epidural anesthesia was associated with greater hypothermia than GA in two studies (18,19), whereas GA was associated with greater hypothermia than epidural anesthesia in another study (20). We found an equal magnitude of intraoperative hypothermia in patients receiving either SA or GA for the same surgical procedure, and a longer duration of postoperative hypothermia in the SA group. The significant hypothermia with SA may be explained by the relatively high level of spinal block, which is associated with greater thermoregulatory inhibition (13,34).

The mechanism of impairment of thermoregulation during SA has been delineated in previous studies. In 1972, Roe and Cohn (31) described a rapid decline in Tc in patients receiving SA and postulated that this hypothermia was likely caused by vasodilation in the lower extremities and an impaired hypothalamic response with a lowered shivering threshold. Subsequent studies determined that hypothermia during SA is caused by both decreased shivering and vasoconstriction thresholds (32) and that the degree of thermoregulatory inhibition during regional block is proportionate to the number of dermatomes blocked (13,34). Given these mechanisms, the conclusions of previous investigations, and the results of the current study, it is clear that patients receiving regional anesthesia are at significant risk for hypothermia and should be monitored and treated accordingly.

We describe a significant difference in postoperative rewarming rates between SA and GA. On admission to the recovery room, patients in both groups had equivalent Tc. However, over the first 90 minutes postoperatively, patients in the GA group rewarmed significantly faster than patients in the SA group. A similar finding has been reported; however, the regional anesthetic technique and block height were not specified (17). Other investigators reported similar rewarming rates between patients receiving SA or GA (35) and similar rewarming rates in those receiving epidural anesthesia and GA (10). The difference in rewarming rates between these studies may be explained by the short duration of surgery in our study relative to the duration of the spinal block. Thus, the residual sympathetic block may have been a source of continuing cutaneous heat loss and slower postoperative rewarming in the SA group.

One limitation to our study was the lack of randomization for anesthetic technique. This is unlikely to have had a significant effect on our findings or conclusions because the two groups were similar for age, weight, percentage body fat, and duration of surgery, which are major determinants of thermoregulation and temperature changes. A possible second limitation is the assumption that tympanic temperature represents a valid reference Tc. Other investigators have proposed that distal esophageal temperature is more representative of Tc; however, it is well documented that esophageal and tympanic temperatures are closely correlated and reliable measurements of Tc (36–38).

In conclusion, patients receiving SA are at a similar risk for developing perioperative hypothermia as are patients receiving GA during radical retropubic prostatectomy. Those receiving SA remain hypothermic longer into the postoperative course. These findings indicate that temperature should be monitored in patients receiving either SA or GA for major surgical procedures. Between the routinely used temperature monitoring methods, rectal temperature monitoring has the most accuracy and precision for temperature monitoring during regional or GA, whereas other methods tend to underestimate true Tc.


   Acknowledgments
 
Supported, in part, by the Anesthesia Patient Safety Foundation and an educational grant from Abbott Laboratories.


   References
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 

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Accepted for publication December 29, 1999.




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Lippincott, Williams & Wilkins Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins with the assistance of Stanford University Libraries' HighWire Press®. Copyright 2006 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999 HighWire Press