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PEDIATRIC ANESTHESIA

Iatrogenic Hyperthermia During Cardiac Magnetic Resonance Imaging

Barry D. Kussman, MBBCh FFA(SA)^*,¶, Robert V. Mulkern, PhD^,, and Robert S. Holzman, MD^*,¶

Departments of *Anesthesiology, Perioperative and Pain Medicine, and Radiology, Children’s Hospital Boston; and Departments of ¶Anaesthesia and Radiology, Harvard Medical School, Boston, Massachusetts

Address correspondence and reprint requests to Barry D. Kussman, MBBCh, FFA(SA), Department of Anesthesiology, Perioperative and Pain Medicine, Children’s Hospital, 300 Longwood Ave., Boston, MA 02115. Address e-mail to barry. kussman{at}childrens.harvard.edu

	Abstract

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We report the occurrence of accidental hyperthermia in a young child undergoing anesthesia for cardiac magnetic resonance imaging. Although the tendency during anesthesia is to develop hypothermia, the absorbed radiofrequency energy from magnetic resonance scanning is added to metabolic energy and must be balanced by appropriate heat loss to maintain normothermia. In addition to stressing the clinical importance of temperature monitoring, this report suggests that the recommended specific absorption rates to prevent excessive patient heating may need to be revised for infants and young children.

IMPLICATIONS: Radiofrequency energy absorbed during magnetic resonance imaging (MRI) is added to metabolic energy and must be balanced by appropriate heat loss to maintain normothermia. Although hypothermia is more likely during anesthesia for MRI, this report of hyperthermia stresses the importance of temperature monitoring, particularly for long scans with high-energy sequences.

	Introduction

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Cardiac magnetic resonance imaging (MRI) is valuable in the anatomic evaluation and functional assessment of congenital heart disease (CHD) (1). For young children, general endotracheal anesthesia with muscle relaxation achieves immobility and faster image acquisition by permitting breath-holding sequences (2). Although children are at increased risk of hypothermia during general anesthesia in this cool, dry environment, substantial radiofrequency (RF) energy-induced heating of tissues may occur (3). We report a case of iatrogenic hyperthermia occurring in a young child during cardiac MRI.

	Case Report

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A 16-mo, 78-cm, 10.4-kg girl with known supravalvar aortic and pulmonary stenosis (non-Williams) underwent anesthesia for cardiac MRI. She was developmentally normal with no cardiac symptoms, was not receiving any medications, and had no known drug allergies. Apart from sedation for an echocardiogram, she had no previous anesthetic history, and there was no family history of anesthesia problems. Preoperative evaluation revealed a well, active child with normal vital signs (axillary temperature, 35.6°C), no evidence of cardiac failure or infective endocarditis, and a normal clinical examination apart from a precordial examination consistent with aortic and pulmonary stenosis. The electrocardiogram was normal, and an echocardiogram showed supravalvar aortic stenosis (maximum instantaneous gradient = 45 mm Hg) and pulmonary stenosis (maximum instantaneous gradient = 60 mm Hg), thickened pulmonary and aortic valves, and qualitatively good biventricular function.

After an 8-h fast, the patient was orally premedicated with midazolam (7.5 mg) and acetaminophen (150 mg). Anesthesia was induced with etomidate (4 mg) and fentanyl (20 µg), followed by rocuronium (10 mg) to facilitate tracheal intubation, and was maintained with sevoflurane (end-tidal concentration, 0.6%) in nitrous oxide and oxygen (fraction of inspired oxygen = 0.3) and intermittent doses of fentanyl and rocuronium. A single, warmed blanket was placed over the patient. She was scanned with a head coil for 19 consecutive series over 95 min with a General Electric Twin Speed 1.5 Tesla magnet with MR software version 10.0 (Fairfield, CT). The pulse sequences included fast spin echo sequences, which are associated with the highest specific absorption rates. The heart rate varied between 105 and 120 bpm, and end-tidal CO₂ was within the expected range. Ringer’s lactate solution 455 mL was infused over 2 h. Ondansetron (1 mg) was given prophylactically, and after reversal with neostigmine (0.5 mg) and glycopyrrolate (0.1 mg), the trachea was extubated. The patient appeared flushed and felt very warm. In the postanesthesia care unit, she was crying and irritable, with a heart rate of 200 bpm, room air saturation of 98%, and an axillary temperature of 38°C. Acetaminophen 100 mg orally was given (4 h after the first dose), but she was still febrile (37.7°C) and irritable 2.5 h later. Because it was already late in the day and the patient lived far from the hospital, she was admitted for overnight observation. Ibuprofen 100 mg was given orally, and her temperature returned to normal (3.5 h from end of anesthesia). The fever did not recur, and she had an uneventful night. The white cell count the next day was normal, and the patient did not develop a viral-type illness.

	Discussion

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Transmitted RF power during MRI is transformed into heat as a result of resistive losses within the patient’s tissue (3). Models of human exposure to RF fields predict that the rate of change of deep body temperature depends on skin blood flow, ambient temperature, relative humidity, and clothing insulation (4). Absorbed RF energy is added to metabolic energy and must be balanced by appropriate heat loss to maintain normothermia. Counteracting this RF energy-induced heating is the tendency to develop hypothermia triggered by general anesthesia-induced impairment of thermoregulatory responses (5), as well as the low ambient temperature and humidity in the bore of the magnet.

The specific absorption rate (SAR) is the rate of RF power coupling to biological tissue and is expressed as watts per kilogram (W/kg). Safety standards define SAR values less than 1.5 W/kg as the normal operating mode and between 1.5 and 4 W/kg as a level requiring patient temperature monitoring (6). Although the scanner used on this patient did not permanently record the SAR, we determined that this patient had a whole body average of 1.87 W/kg.¹ To avoid excessive patient heating, MR scanners have built-in safety features that will cease scanner operation if (a) the predicted SAR of the planned sequence exceeds the acceptable programmed limit for the patient being scanned, (b) if the ambient temperature in the bore of the magnet increases, or (c) there is a difference in the ambient temperature measured by the sensors at each end of the bore. The scanning log confirmed that the fans were on, the ambient temperature in the bore of the magnet was 21°C–22°C, and the temperature-associated interlocks were enabled. Thus, if one assumes that the temperature increase in this patient was caused by scanner deposition of RF power to a well-insulated baby, then SAR values calculated by the manufacturer to prevent excessive heating for infants and young children may need to be revised. Although decreased cardiac output caused by ventricular dysfunction, sevoflurane, or positive-pressure ventilation, dehydration caused by prolonged fasting, or anticholinergics could have contributed to an increase in body temperature, we do not believe that these were significant causative factors in this patient.

Cardiac MRI typically has long imaging times (52 min [range, 3–141 min] for infants at our institution), several breath-hold sequences, and requires immobility to obtain satisfactory images. Because infants and young children are unable or unlikely to cooperate, sedation or anesthesia is required. Although larger body surface areas of infants compared with adults may be anticipated to lead to higher cooling rates, other factors, including insulating sedated infants and the use of coils that enclose a larger fraction of the body, may counter this effect and lead to increased RF heating. Intraoperative thermoregulatory responses during the thermal steady-state may actually increase central temperature in children despite constant ambient temperatures (7). In patients with severe acyanotic CHD, minimum oxygen consumption and heat production is increased, and low cardiac output and peripheral vasoconstriction reduce the redistribution of heat from the core to the periphery (8).

Conventional thermocouple or thermistor-based techniques may cause artifacts or erroneous measurements because of direct heating of the temperature probes (9). Lack of MRI-compatible temperature probes has limited routine body temperature monitoring during anesthesia for MRI (10,11). Fluoroptic thermometric systems are unaffected by high magnetic field strength and RF pulses and have been shown to be safe and reliable for recording temperatures during MR procedures (9).

In conclusion, this case report highlights an under-appreciated risk of MRI. Because cardiac MRI can require high SAR sequences and long scan times, anesthesia compromises the thermoregulatory system, and CHD can alter thermal homeostasis, children with cardiac disease are at a disadvantage in terms of their ability to regulate body temperature during RF exposure, and significant hyperthermia may result.

	Footnotes

 
¹ Back calculated SAR to produce 2.4°C rise in body temperature in 90 min assuming patient is a 10-kg mass of water with specific heat of 4.2 J · g^–1 · °C^–1.

	References

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5. Sessler DI. Mild perioperative hypothermia. N Engl J Med 1997; 336: 1730–7.[Free Full Text]
6. International Electrotechnical Commission. Medical electrical equipment. II. Particular requirements for the safety of magnetic resonance equipment for medical diagnosis. Geneva, Switzerland: International Electrotechnical Commission, 1995.
7. Gurtner C, Paut O, Bissonnette B. Temperature regulation: physiology and pharmacology. In: Bissonnette B, Dalens BJ, eds. Pediatric anesthesia. New York: McGraw-Hill, 2002: 173–86.
8. Kennaird DL. Oxygen consumption and evaporative water loss in infants with congenital heart disease. Arch Dis Child 1976; 51: 34–41.[Abstract/Free Full Text]
9. Shellock FG. Patient monitoring in the MR environment. In: Kanal E, ed. 87th Scientific Assembly and Annual Meeting of the Radiological Society of North America. Chicago, IL: Radiological Society of North America, Inc, 2001: 49–58.
10. Young AE, Brown PN, Zorab JS. Anaesthesia for children and infants undergoing magnetic resonance imaging: a prospective study. Eur J Anaesthesiol 1996; 13: 400–3.[Medline]
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