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PEDIATRIC ANESTHESIA

Brain Magnetic Resonance Imaging Increases Core Body Temperature in Sedated Children

Yvon F. Bryan, MD^*, Thomas W. Templeton, MD, Todd G. Nick, PhD, Martin Szafran, BS, and Avery Tung, MD^¶

*Department of Clinical Anesthesia and Clinical Pediatrics, Department of Pediatrics, Center for Epidemiology and Biostatistics, Cincinnati Children's Hospital Medical Center and University of Cincinnati, College of Medicine, Cincinnati, Ohio; Department of Pediatric Anesthesia and Critical Care, Wake Forest University School of Medicine, Winston-Salem, North Carolina; Pritzker School of Medicine, ¶Department of Anesthesia and Critical Care, University of Chicago, Chicago, Illinois

Address correspondence and reprint requests to Yvon F. Bryan, MD, Associate Professor of Clinical Anesthesia and Clinical Pediatrics, Department of Anesthesia, Cincinnati Children's Hospital Medical Center, MLC 2001, 3333 Burnet Avenue, Cincinnati, OH 45229. Address e-mail to yvon.bryan{at}cchmc.org.

	Abstract

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An increasing number of children now undergo magnetic resonance imaging (MRI) under sedation. MRI requires a cool environment. Because children have a larger surface area to body weight ratio than adults and because active warming devices are not MRI compatible, hypothermia as a result of passive heat loss is a risk. Absorption of radiofrequency radiation generated by the scanning process, however, may partially offset this heat loss. To determine the effect of absorbed radiofrequency radiation on body temperature during MRI, we measured pre-MRI and post-MRI tympanic temperatures in 30 children who underwent brain MRI while sedated with chloral hydrate and covered with a hospital gown and blanket. The mean (± sd) age was 14.9 ± 8.6 mo, and weight was 9.8 ± 2.8 kg. During an average scan duration of 42 ± 13 min, mean tympanic temperatures increased 0.5°C from 36.9°C ± 0.4°C to 37.4°C ± 0.3°C; (95% CI difference, 0.3°C to 0.7°C; P < 0.001). Our findings suggest that children sedated with chloral hydrate for brain MRI did not become hypothermic but rather had increased body temperature despite minimal barriers to heat loss and no active warming. These results imply that aggressive measures to prevent passive heat loss during MRI studies may not be needed in all patients.

	Introduction

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The number of children undergoing magnetic resonance imaging (MRI) scans has increased significantly in recent years (1). Because many young children cannot tolerate the confined spaces and sounds generated by magnet operation, sedation or general anesthesia is often required (2–4). The specialized MRI environment, however, represents a significant thermal challenge to anesthetized children. The cool temperatures and low humidity required for proper magnet function predispose to radiant and convective heat loss (5,6). Excessive use of blankets can obscure the patient and increase the difficulty of remote monitoring. Sedation and/or general anesthesia limit intrinsic thermoregulation (7). Finally, active warming devices are generally not MRI compatible. The MRI scan, however, generates radiofrequency radiation (RFR), which is absorbed by the patient and may offset heat lost to the environment (8).

We hypothesized that children sedated for MRI procedures would not become hypothermic because of absorption of RFR generated by the MRI process. To test this hypothesis, we prospectively measured tympanic temperatures before and after brain MRI procedures in 30 children ranging in age from 2 to 33 mo.

	Methods

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This study was approved by the IRB of the University of Chicago. After written parental/guardian consent, we prospectively studied 34 children scheduled for brain MRI procedures in which anesthesia coverage was requested. All scans were performed from 2002-2003. Before the procedure, all patients were sedated with chloral hydrate at a target dose of 75 mg/kg orally. Once an adequate depth of sedation had been achieved (approximately 20 min after sedative administration), tympanic temperature was measured in the right ear (FirstTemp Genius; Sherwood Medical, St. Louis, MO) and ear plugs were placed in both ears. The sedation and temperature monitoring process was performed in a holding room at the same temperature as the MRI suite and was complete in 30 min in all patients. During this period, children were clothed in hospital gowns. Immediately before being brought into the magnet room, all children were additionally covered up to their necks with a single cotton blanket. After positioning in the MRI scanning device, a MRI-compatible monitor (In Vivo Research CO, Orlando, FL) was applied to continuously monitor oxygen saturation, electrocardiogram, capnogram, and arterial blood pressure. As per routine operation, the magnet room was kept at a temperature of 20°C–22°C and a relative humidity <50%. The ambient temperature was measured and recorded before each scan. The fan that circulates air in the magnet bore during the scan was left on for all patients.

The MRI scanner used was a 1.5T GE Sigma LX (GE Medical Systems, Milwaukee, WI) with a quadrate (transmit and receive) head coil. Brain sequences did not require contrast and represented normal protocols used for children at our institution. Specific absorption rate (SAR) values for a typical 10-kg child receiving a standard brain MRI scan ranged from 0.4 to 1.3 W/kg over a duration of 3 to 6 min depending on the sequences performed. On completion of the scan, the patients were brought out of the magnet room and the ear plugs were removed. Tympanic temperatures were then immediately measured in the same ear from which the pre-scan measurement was taken. Measurements of both pre-scan and post-scan temperature were made by the same observer in all patients.

Data collected included age, weight, body surface area, gender, diagnoses for all children, total dose of chloral hydrate, pre-tympanic and post-tympanic temperatures, total scan duration, and time spent in the magnet room. Differences in pre-scan and post-scan tympanic temperatures were analyzed using a paired Student's t-test. Simple linear regression analysis was used to predict change in temperature values from pre-scan temperatures. To assess possible confounding relationships with pre-scan temperature, change in temperature was also analyzed by using analysis of covariance, where pre-scan temperature, weight, seizure history or seizure medications, and time in the bore of the magnet with and without active scanning were used as covariates. For all statistical tests, 95% confidence intervals (CI) and P values are reported using two-tailed tests and a 0.05 significance level.

	Results

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Thirty patients (17 males, 13 females) underwent the entire protocol. Three patients were excluded because chloral hydrate was ineffective and general anesthesia was necessary, and one was excluded because his pre-scan temperature was 38.8°C. The most common diagnosis was a history of seizures, found in 12 of the 30 patients. Eight were concurrently taking antiseizure medications (phenobarbital, Topamax, Tegretol). All patients were sedated with chloral hydrate at doses ranging from 41-108 mg/kg (mean ± sd, 72 ± 18 mg). Descriptive statistics are reported in Table 1.

The mean (sd) pre-scan tympanic temperature was 36.9°C ± 0.4°C and the mean post-scan temperature was 37.4°C ± 0.3°C (P < 0.001). The median pre- and post-scan temperatures were the same as their respective means, with a pre-scan range of 36.0°C to 37.8°C and post-scan range of 36.5°C to 37.9°C. The 0.5°C increase in average temperature from pre-scan to post-scan was significant by paired Student's t-test (95% CI, 0.3°C to 0.7°C; P < 0.001). Figure 1 shows a dot plot of the temperature readings for pre and post-temperatures with vertical lines marking the means.

View larger version (17K): [in this window] [in a new window]   Figure 1. Dot plot of temperature readings for pre and post-scan temperatures. Vertical lines represent means for pre-scan and post-scan groups.

Tympanic temperatures increased in 24 of the 30 patients (80%), and decreased or was unchanged in 6 patients (20%). These groups were similar in age, weight, body surface area, duration of scan, use of antiseizure medications, and dose of chloral hydrate. In the 24 patients with increased temperature, the mean (sd) increase was 0.64 ± 0.34°C and the range from 0.1°C to 1.3°C. Six of the 24 patients (25%) were receiving antiseizure medications, and their average increase was 0.45°C. Five patients had temperature increases equal to or more than 1.0°C. None of these five were receiving antiseizure medications and their ages (3–21 mo), weights (5–13 kg), dose of chloral hydrate (51.7 to 98.4 mg/kg), and diagnoses (hypotonia, seizures, GE reflux, hydrocephalus, microphthalmia) were varied. Both their magnet bore times (39.6 ± 11.2 min) and pre-scan temperatures (36.4°C ± 0.3°C) were less than the overall group average, suggesting that increased scan times were not predictive of large temperature increases. Overall, two patients had a large amount of sweat on their foreheads and wet hair post-scan, suggesting the presence of a partially intact, intrinsic thermoregulatory response. The observed increases in tympanic temperatures in these 2 patients were 0.6°C and 1.1°C, respectively, with one reaching a peak post-procedure temperature of 37.9°C.

Of the 6 patients whose temperatures did not increase during the MRI scan, 2 had no change in core temperature, 1 had a decrease of –0.1°C, 2 had a decrease of –0.2°C and 1 had a decrease of –0.3°C. The patient with the largest decrease in temperature (–0.3°C) was a 9.4-kg, 14-mo-old child with a seizure disorder who spent a longer time in the magnet room than average (12 min compared with the 7-min average) without being scanned. The two patients who experienced decreases of –0.2°C were a 10.6-kg, 25-mo-old ex-premature infant and a 7.7-kg, 13-mo-old toddler with periventricular leukomalacia.

Linear regression showed a significant relationship between the magnitude of temperature change during MRI and pre-scan temperature. Specifically, patients with lower pre-scan tympanic temperatures had larger intra-scan increases in temperature (Fig. 2). For each 0.5°C decrease in baseline (pre-scan) temperature, the increase in temperature resulting from MRI scanning increased 0.36°C (95% CI, 0.21°C to 0.50°C; P < 0.001). Overall, pre-scan temperature explained 46% of the total variability of the overall temperature change.

View larger version (22K): [in this window] [in a new window]   Figure 2. Scatter plot of relationship between pre-scan temperature and change in temperature. Regression and 95% confidence interval lines are marked.

This relationship between pre-scan temperature and overall temperature change persisted after adjustment for potential confounding variables. An analysis of covariance with the following covariates: weight, duration of magnet operation, time in the bore of the MRI magnet without active scanning, and presence/absence of seizure history and/or antiseizure medications did not alter the relationship between pre-scan and change in temperature. After adjustment, the change in temperature increased 0.37°C for each 0.5°C decrease in the patient's baseline (pre-scan) temperature (95% CI 0.23°C to 0.51°C; P value <0.001). Pre-scan temperature was the only significant predictor of temperature change (Table 2).

	Discussion

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In our study of children sedated with chloral hydrate for brain MRI scans, we found that tympanic temperatures increased during the MRI scan. This increase occurred despite a magnet room temperature of 20°C–22°C, a relative humidity <50%, no use of active heating devices, and use of only a single cotton blanket to reduce passive heat loss. Of 30 patients, temperatures increased in 24 patients (80%), remained the same in 2 patients (7%), and decreased in only 4 (13%). The largest increase in temperature was 1.3°C in a 49-minute scan, whereas the largest downward change was only –0.3°C, occurring during a 44-minute scan.

This finding contrasts with the decrease in body temperature that occurs in children with induction of anesthesia and exposure to a cool operating room environment (9). Like the operating room, the MRI magnet room is usually kept cool and at a low relative humidity. In addition, children have a large surface area to weight ratio compared with adults (10) and altered thermoregulatory thresholds (11). The magnetic field of the MRI environment also prohibits the use of active heating devices and mandates the use of specialized equipment to monitor temperature continuously during active scanning. Finally, because children are often monitored remotely via video from outside the magnet room, excessive swaddling with blankets can obscure the patient and make visualization difficult. Taken together, these features of the MRI suite suggest that children undergoing MRI scanning are predisposed to hypothermia from passive heat loss and that aggressive measures should be taken to minimize heat loss, particularly during prolonged scans. Such measures are advocated by current textbooks of pediatric anesthesia (6,12).

One aspect of the MRI environment that differentiates it thermodynamically from the operating room is that radiofrequency radiation (RFR) is introduced during the scanning process. The quantity of RFR absorbed during MRI procedures is typically described as the specific absorption rate (SAR), expressed as watts/kg and defined as a "time derivative of incremental energy absorbed by and in turn dissipated in an incremental mass contained in a volume element of a given density" (13). Federal guidelines limit SAR values to a whole body average of 4 W/kg for 15 minutes or 3 W/kg for 10 minutes averaged over the head (14).

Such values, however, were developed using awake adult volunteers and may not be applicable to the sedated or anesthetized child with an increased surface area/weight ratio, potentially altered thermoregulation from anesthetics or other medications, and possible dehydration from prolonged pre-scan fasting. These patients may be at increased risk for clinically significant absorption of RFR. Although typical SAR values for the scanning protocols used in our study were consistently less than those described above, they were able to cause clinically significant heat absorption. One 2004 case report noted an increase in axillary temperature from 35.6°C to 38°C during a 95-minute cardiac MRI in a 16-month-old, 10-kg anesthetized infant (15). The authors suggested that prolonged scanning times and high radiofrequency energy sequences typical of cardiac MRI may have caused this temperature increase. In a retrospective review of 250 children undergoing cardiac MRI studies using a variety of general anesthetics, however, routine measures were taken to prevent hypothermia and no incidences of abnormal heating were described (16) (personal communication, Dr. Kirsten Odegard, 9/25/05).

Unlike patients undergoing cardiac MRI under general anesthesia, we examined children who underwent brain MRI imaging sequences with significantly less radiofrequency energy than generated during a cardiac MRI scanning sequence. Although we did not specifically measure SAR values, estimates for our study patients were likely less than those used during cardiac MRI. Nevertheless, we found tympanic temperature increases exceeding 1°C/hour in some cases. Our findings suggest that in infants, absorption of RFR could possibly be clinically significant, and that aggressive measures to counteract passive heat loss to the environment can instead result in potentially dangerous increases in core body temperature during common scanning sequences using 1.5T magnets.

In our study, we chose to measure tympanic temperatures because the measurement can be rapidly obtained, does not require prolonged equilibration times, and does not disturb the sedated child (17). This approach, however, may have resulted in a discrepancy between tympanic and actual "core" body temperature. Because our studies involved only brain sequences, preferential absorption of RFR by the head may have resulted in differential heating of the head relative to the rest of the body. Temperatures measured at the ear may thus have reflected head temperature and not actual core body temperature. We felt this possibility to be unlikely, however. Although we did not measure blood temperature, several patients exhibited thermoregulatory responses consistent with heating rather than cooling. Nearly all patients in our study felt warm in their torso and extremities after the scan, and two patients were observed to be sweating profusely. In addition, it is likely that rapid blood flow rates through the head would act to equilibrate temperature differences throughout the body and minimize the size of any temperature gradient generated by differential heating.

Our patients also were not completely anesthetized; instead they were given chloral hydrate. Although the thermoregulatory effects of chloral hydrate are unknown, sedation with midazolam in adults produces only mild impairments in thermoregulation (18). Because patterns of heat loss in these patients may be different from patients under general anesthesia, our results cannot be generalized to anesthetized patients undergoing MRI. Nevertheless, chloral hydrate is commonly used by most pediatricians and radiologists for MRI scans in infants and children less than 20 kg when contrast is not required (19,20). Our data suggest that RFR-induced increases in temperature are relevant even when only chloral hydrate is used.

Our observations raise important questions for anesthesiologists regarding the monitoring and management of temperature for anesthetized children undergoing MRI procedures. Although our results are in accordance with the findings of Kussman et al. (15), other studies (16) have not reported overall increases in core body temperature from heat absorption. Taken together, these data suggest that some, but not all, patients are at risk for temperature increases as a result of an overall increase in heat content during MRI procedures. Although younger age, lower body weight, and longer scan duration might seem to be likely risk factors for heating, we found no relationship between these factors and increased temperature change during MRI. In addition, there was no correlation between disease state or medications and magnitude of temperature change.

We did observe an inverse relationship between pre-scan (but not post-scan) tympanic temperature and MRI-induced temperature change. This finding is poorly understood but suggests a possible interaction among environmental temperature, intrinsic physiologic thermoregulatory mechanisms, and radiofrequency energy absorption. It may be that in a cool environment, radiofrequency energy absorption fails to completely suppress the patient's own thermoregulatory mechanisms. Such an inability to adjust intrinsic thermoregulation in response to external radiofrequency heating has been previously observed in animals (21). Patients with intrinsic heat-generating mechanisms triggered by lower prescan temperatures and exposed to extrinsic radiofrequency energy may thus generate larger temperature increases than those who begin their scans at a more normal temperature. Clinically, this finding implies that "precooling" patients to offset the increase in temperature caused by RFR absorption is unlikely to be useful, as cooler patients merely heat up to a greater degree during MRI scanning. Overall, little is known about how thermal information is integrated and transformed into a physiologic response. As MRI magnets become increasingly more powerful, the interaction between RFR absorption, intrinsic thermoregulation, and the anesthetized or sedated state may require further inquiry.

Although some patients in our study did experience a decrease in temperature, we were unable to identify factors specific to these patients before scanning. Anesthetized children may be so thermodynamically fragile that small shifts in the balance of passive heat loss and RFR absorption can result either in heating or cooling of the patient during MRI. Administration of IV fluids, for example, may reduce heat gain by two mechanisms: preventing dehydration, which can impair body heat loss, and introducing fluid at "MRI room temperature" into the body. Unfortunately, because routine devices used for active warming and continuous temperature monitoring are not MRI-compatible, clinicians are limited to making an initial assessment of the likelihood of heating or cooling during MRI procedures, and acting accordingly to facilitate or prevent heat loss to the environment. Such an assessment can be difficult to make without detailed knowledge of the anticipated SAR, the thermoregulatory stability of the patient, and other potential risk factors that may determine whether the patient will heat up or cool down during MRI procedures. Nonetheless, our data and those of others suggest that clinically significant heating or cooling may occur in a relatively short time span, and that anesthesiologists should consider both effects when caring for sedated neonates, infants, and children in the MRI suite.

In conclusion, we report that children sedated with chloral hydrate for standard brain MRI protocols experienced increased tympanic temperatures after an average scan duration of <1 hour. This increase in temperature occurred despite minimal efforts on our part to reduce passive heat loss and without use of active warming devices. Our data demonstrate that heating occurs during routine MRI procedures, and can exceed 1°C per hour in some cases. Although we identified only one risk factor predisposing to temperature increases during MRI (pre-scan temperature), it is likely that extended, high energy sequences such as those required for cardiac imaging also increase heat absorption. As a result, physicians performing sedation or general anesthesia for children undergoing MRI procedures should consider the possibility that either heating or cooling may occur. If possible, we recommend continuous temperature monitoring during the MRI scan, particularly if the scans are prolonged or use high-energy sequences such as those used for spine and cardiac studies. Care must be taken in applying devices that warm children or decrease their ability to dissipate heat. Further studies on the balance between RFR absorption and heat loss during MRI scans are required to better understand the effect of the MRI scans and environment on heat exchange and thermoregulation of sedated and anesthetized children.

	Footnotes

 
Accepted for publication January 31, 2006.

Presented, in part, at the American Society of Anesthesiologists annual meeting on October 21, 2003, San Francisco, California.

	References

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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