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REGIONAL ANESTHESIA AND PAIN MANAGEMENT

Temperature Monitoring Practices During Regional Anesthesia

Steven M. Frank, MD^*, Judy M. Nguyen, MD^*, Christine M. Garcia, MD^*, and Rachel A. Barnes, MS

*Department of Anesthesiology and Critical Care Medicine, The Johns Hopkins Medical Institutions; and Medical Student, University of Maryland, Baltimore, Maryland

Address correspondence and reprint requests to Steven M. Frank, MD, Department of Anesthesiology and Critical Care Medicine, The Johns Hopkins Hospital, Carnegie 442, 600 N. Wolfe St., Baltimore, MD 21287. Address e-mail to sfrank{at}welchlink.welch.jhu.edu

	Abstract

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Monitoring and maintaining body temperature during the perioperative period has a significant impact on the risk of myocardial ischemia, cardiac morbidity, wound infection, surgical bleeding, and patient discomfort. To test the hypothesis that body temperature is inadequately monitored during regional anesthesia (RA), we randomly surveyed 60 practicing anesthesiologists to determine practice patterns for temperature monitoring. Only 33% of the clinicians surveyed routinely monitor body temperature during RA. Although skin temperature monitoring has limitations, it was the most commonly used method among the survey respondents. When temperature is monitored during RA, most clinicians use either liquid crystal skin-surface monitoring or axillary temperature probes. Of those surveyed, <15% use acceptable core temperature monitoring techniques (urinary bladder or tympanic membrane). In conclusion, it seems that body temperature is often not monitored in patients receiving RA.

Implications: The results of this survey of practicing anesthesiologists indicate that body temperature is often not monitored in patients receiving regional anesthesia. It is therefore likely that significant hypothermia goes undetected and untreated in these patients.

	Introduction

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Careful monitoring and control of the vital signs, including body temperature, during surgery are essential to safe anesthetic care. By monitoring temperature and preventing hypothermia, the risks of myocardial ischemia (1), cardiac morbidity (2), wound infection (3), surgical bleeding (4), and patient discomfort (5) can be reduced. Although temperature is frequently monitored during general anesthesia (GA), it is unclear how often temperature is monitored in patients receiving regional anesthesia (RA).

Intraoperative temperature monitoring became popular in the early 1960s when malignant hyperthermia (MH) was described as a rare but often fatal risk of GA (6). Although hyperthermia may occur late in the clinical course of MH, the high mortality rate prompted the routine measurement of body temperature during GA. Because RA is not associated with MH, temperature monitoring was not thought to be important in patients receiving RA. Other reasons for not monitoring temperature during RA may be the unfounded belief that RA does not alter body temperature, or the lack of a convenient site for placement of a temperature probe, because the usual monitoring sites (nasopharynx, esophagus, and oropharynx) are not well tolerated in awake or sedated patients.

Over the past decade, several studies indicate that RA significantly impairs thermoregulation and predisposes patients to hypothermia in the typically cold operating room environment (7). In fact, there is good evidence that the risk of hypothermia is equivalent during epidural and general anesthesia, especially when the block level is relatively high (T4-6) (7). Without temperature monitoring, hypothermia during RA will be neither recognized, prevented, nor treated. Even when conscious, patients receiving RA are often asymptomatic from hypothermia because autonomic (8) and behavioral (9) thermoregulatory responses are impaired. Patients are thus unable to report the sensation of cold during core hypothermia.

To determine practice patterns for body temperature monitoring and to test the hypothesis that body temperature is often ignored during RA, we surveyed practicing members of the ASA.

	Methods

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We surveyed 60 anesthesiologists who were selected randomly from the ASA directory of members. Of the 102 surveys mailed, 60 were returned (approximately 60% response rate). For the purposes of the survey, we specified that RA be defined as major conduction blockade (spinal or epidural techniques).

The questions used for the survey were as follows:

1. Are you in a private versus university practice setting?

2. Do you monitor body temperature during RA: yes, no, occasionally?

3. Should we monitor body temperature during RA: yes, no, occasionally?

4. At the end of the surgical procedure, do you think core temperature in patients receiving RA is less than, equal to, or greater than core body temperature in patients receiving GA?

5. If you do monitor temperature during RA, which site(s) (rectal, urinary bladder, axillary, forehead skin surface, tympanic, nasopharynx, esophageal) do you use? (more than one selection allowed)

6. For all cases you perform (regardless of anesthetic technique), what body site(s) (rectal, urinary bladder, axillary, forehead skin surface, tympanic, nasopharynx, esophageal) do you typically monitor? (more than one selection allowed)

	Results

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The respondents in our survey represented both private (40%) and academic (60%) anesthesia practices. Of the responders, 33% routinely practice temperature monitoring during RA, and 56% believe that temperature should be monitored during RA (Fig. 1). Most respondents (75%) believe that patients receiving RA develop hypothermia of equal or greater magnitude as those receiving GA, whereas 25% felt that hypothermia was more problematic during GA than during RA (Fig. 2). For the questions regarding preferred temperature monitoring sites, the survey allowed the respondent to choose more than one site. When body temperature monitoring is practiced during RA, the preferred site is most often axillary (40%) or forehead skin-surface (70%). Other core monitoring sites are much less likely to be used (Fig. 3). Relatively more accurate core temperature monitoring is practiced in patients receiving GA, during which nasopharyngeal and esophageal monitoring are more likely to be used (Fig. 3).

View larger version (60K): [in this window] [in a new window]   Figure 1. Temperature monitoring practice during regional anesthesia.

View larger version (37K): [in this window] [in a new window]   Figure 2. Percentage of responders who believe that core body temperature in patients receiving regional anesthesia is greater than, equal to, or less than core body temperature in patients receiving general anesthesia.

View larger version (52K): [in this window] [in a new window]   Figure 3. Preferred temperature monitoring sites in anesthetized patients. Skin-surface refers to forehead skin monitoring with liquid crystal thermometry. Responders were allowed to choose more than one site.

	Discussion

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The following sites for core temperature monitoring are listed in order from most to least accurate for correlation with true core (blood) temperature (10,11): pulmonary artery, tympanic membrane, esophagus, nasopharynx, oropharynx, urinary bladder, rectum, axilla, skin. Accessible sites that are tolerated during RA are the tympanic membrane, urinary bladder, rectum, axilla, and skin-surface. Urinary bladder and rectal temperatures are considered intermediate sites rather than core sites because when core temperature changes, temperatures in the bladder and rectum lag behind because of isolation from the central core. These sites should thus overestimate core temperature during shorter surgical procedures. Another potential limitation of rectal and bladder sites during RA is that vasodilation redistributes heat toward the lower body, which may artificially increase the temperatures measured at these sites. Axillary temperature monitoring requires correct probe placement over the axillary artery and even then is likely to underestimate core temperature (11).

The most commonly used single monitoring technique during RA is liquid crystal thermometry on the forehead skin-surface. These devices are manufactured with a built-in offset, because skin temperature is 2–3°C lower than core during steady-state conditions, but the actual difference depends on vasomotor tone and ambient temperature (12). It is likely that, during RA, the core-to-skin gradient is increased because of compensatory vasoconstriction above the level of the block (7); thus, liquid crystal thermometers placed above the block level may significantly underestimate core temperature during RA. This hypothesis, however, has not yet been tested. Thus, there are three potential reasons why skin temperature estimates of core temperature may be inaccurate (12). The internal redistribution of body heat is accompanied by an increased mean skin temperature from systemic vasodilation. Skin blood flow can be altered significantly by thermoregulatory vasoconstriction during the perioperative period. Finally, changes in ambient temperature may contribute to altered skin temperature.

The forehead skin is a common site for measuring cutaneous temperature. Reasons for selecting this site go beyond convenience and accessibility (12). There is little variability among individuals because forehead subcutaneous tissue insulation is minimal. Compared with some other areas, the forehead skin has fewer thermoregulatory arteriovenous shunts that could dramatically alter skin temperature without comparable central temperature changes. A study comparing liquid-crystal monitors with forehead thermistors under clinical conditions simulating intraoperative hypothermia found that the two types of monitors were comparable in both rapidity and linearity of responses, but not in accuracy (13). Changes in liquid crystal thermometry have been shown to accurately reflect core temperature trends in adults only during the very immediate postoperative period, but not 15, 30, 45, and 60 min postoperatively (14). When large changes in body temperature are induced (e.g., cardiopulmonary bypass), liquid crystal skin surface thermometry provides a reasonably accurate assessment of core temperature (15).

Sessler et al. (9) have described the effects of RA on thermoregulatory control in human volunteers. As with GA (16), RA causes an initial redistribution of heat in the first 30–60 min, when heat is transferred from the core to the peripheral thermal compartment (17), and core temperature decreases by approximately 1°C. In addition to redistribution, there is a reduction in the thresholds for both vasoconstriction and shivering because of an increased apparent temperature, which fools the hypothalamus by sending warm signals from the lower body (18). This results in an expanded interthreshold range over which core temperature can vary and no efferent thermoregulatory responses are triggered (8,19). Even the ability to sense hypothermia is impaired during RA because the relatively warm input from the lower body overrides the sensation of core hypothermia.

There is evidence that the incidence of inadvertent hypothermia is just as great in patients receiving RA as in those receiving GA (7). Some data suggest that low regional blocks decrease the risk of hypothermia (19,20). Other studies demonstrate mixed results, with either GA (21) or RA (22,23) presenting the greatest risk of hypothermia. Risk factors for hypothermia also include cold operating rooms (20), open body cavities, significant fluid and blood administration, lean body habitus (24), and extremes of age (very young and very old) (7).

Perioperative hypothermia has been associated with adverse clinical outcomes. In high-risk patients, mild hypothermia (35.0–35.5°C) is associated with increased myocardial ischemia (1) and cardiac morbidity (2) in the early postoperative period. This effect seems to be adrenergically mediated based on increased norepinephrine, increased vasomotor tone, and hypertension in mildly hypothermic patients (25,26). Hypothermia-induced postoperative shivering increases total body oxygen consumption and increases patient discomfort (27). Postoperative wound infection occurs more often in patients who develop intraoperative hypothermia due to impaired oxidative killing by macrophages and lower tissue oxygen concentrations in the presence of hypothermia-induced vasoconstriction (3). Both platelet function and coagulation factors are impaired by hypothermia, thus increasing the potential for bleeding during surgery (28). Furthermore, hypothermia-related coagulopathy most likely goes undetected in the clinical setting because routine coagulation testing (prothrombin and partial thromboplastin time) is performed at a standardized temperature of 37°C (29). Because hypothermia-related adverse outcomes can occur regardless of anesthetic technique (RA versus GA), it is evident from the results in the current study that there is room for improvement in the level of care that is delivered to patients receiving RA.

There are recognized limitations to our study. It is possible that, with a larger number of survey respondents, we may have seen different results. The sample was, however, relatively well balanced between academic and private practice physicians. The surveyed physicians also covered a wide demographic area in the United States.

In summary, despite the potential for and the consequences of hypothermia, body temperature is often ignored during RA. When body temperature is monitored during RA, the methods for monitoring are often less than optimal.

	Acknowledgments

 
This study was supported in part by the Anesthesia Patient Safety Foundation.

The authors acknowledge assistance from Susan Kelly for data collection and from Neeraj Gupta for data analysis and manuscript preparation.

	References

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