Temperature values by all 3 probes were collected simultaneously immediately after insertion and then every 5 minutes thereafter for a total of 30 minutes. The MRI-compatible temperature probes were not used for clinical decision making during each case. Data collected included the age, weight, gender, diagnoses, ambient operating room temperature, anesthetic technique (general, or combined general/regional technique), presence or absence of a warming blanket, type of surgical procedure, and its duration.
The relationship between each method (MRI-skin, MRI-core, and STRD) was determined by Spearman correlation analysis at each time point. The agreement between each method was then visually assessed by Bland–Altman plot at each time point. By averaging the temperature measurements at 7 points in time, the overall limits of agreements (1.96 times the SD of the mean difference) were calculated and compared. To consider the correlation between the repeated measures within each subject, repeated measures of analysis of variance (ANOVA) were performed. For this analysis, we considered the temperature difference between each of the alternative methods and the standard method as an outcome and tested effects of warming and time as covariates. All analyses were conducted by SAS 9.2 (SAS Institute, Inc., Cary, NC). The data are reported as estimated means with 95% confidence intervals (CI) with significance at P < 0.05. According to the Bland–Altman methods,11 60 subjects would allow a CI as narrow as 0.45° SD around the limits of agreement (where SD is the SD of the differences).
We enrolled 60 children with a mean age of 7.8 ± 6 years (mean ± SD) weighing of 32.4 (± 26.4) kg. Descriptive statistics are presented in Table 1. The estimated temperatures with 95% CI for all patients over time were 36.30°C (36.1, 36.4) for the STRD, 35.1°C (34.8, 35.3) for the unmodified (MRI-skin), and 36.3°C (36.1, 36.4) for the modified MRI (MRI-core) probe.
The estimated temperature difference between the STRD monitor and the MRI-core probe was 0.06°C with CI (−0.02, 0.15) and between the STRD and the MRI-skin was 1.19°C (0.97, 1.41), which is significant. The difference between the MRI-skin and MRI-core probes was 1.21 (0.94, 1.47), which is also significant.
The MRI-core probe measurements correlate with those of the STRD probe at all times with a correlation coefficient >0.7 (P < 0.05). The MRI-skin probe correlates significantly with the STRD probe starting at the 10-minute time point. However the correlation coefficient remains <0.5 at all time points (maximum 0.44) (Fig. 3).
The Bland–Altman analysis indicated that the 95% limits of agreement ranged from −0.9 to 3.4 between the STRD and the MRI-skin probe and from −1.3 to 1.2 between the STRD and the MRI-core probe. The interval width of agreement for the MRI-core probe is narrower and its estimated temperature is closer to the STRD measurements in comparison with the MRI-skin values, indicating a good agreement between standard esophageal and the MRI-core monitor that increases with time (Fig. 4).
Use of a warming blanket does not affect the difference between the temperatures measured with STRD and MRI-core over time (Fig. 5). However, the skin temperature measurement tended to converge with both core temperature measurements when a warming blanket is used as noted by the decrease of the measurement differences over time between STRD and the MRI-skin probes (P < 0.05) as well as between the MRI-core and the MRI-skin probes (P < 0.01, Fig. 5). After adjusting for room temperature, the results remained unchanged; the room temperature did not affect temperature measurements.
Our study shows a much closer correlation between standard esophageal core temperature measurements and those obtained by the MRI-core in comparison with the MRI-skin probe. The difference between the MRI-skin probe and both core probes decreased over time (Fig. 5). This could have been due to redistribution or warming blanket use that might have caused a decrease in measured temperature differences between the MRI-skin probe and both the standard (P = 0.02) and the MRI-core probe (P = 0.01) over time, mostly by an increase in the MRI-skin temperature reading. It is possible that the approximation between MRI-skin and the core temperatures measured is solely related to the close proximity of the MRI-skin probe to the warming blanket and not an actual increase in skin temperature as a result of body warming. However, it is likely that both phenomena have contributed to this result. In fact, the warming blanket did not affect the measurement differences between the standard probe and the MRI-core probe, suggesting a steady state comparison between the 2. The correlation between both these probes reached a stable maximum within 15 minutes after insertion, with a correlation coefficient >0.8 (Fig. 3). The MRI-skin probe never achieved a correlation coefficient >0.5, irrespective of a time factor. In our study we controlled for the effect of room temperature, which could theoretically have affected our readings.7
A limitation of our study is the non-MRI environment. Although the MRI suite would have been an ideal setting for this performance validation, the MRI-incompatible standard esophageal temperature probe prohibited this location.
This study confirms the possibility of real-time core body temperature assessment during MRI in children under general anesthesia by using a simple modification of a current MRI-compatible skin temperature probe. Benefits of this option include the ease of application, close correlation with the current “gold standard” in core temperature assessment, and subsequently the possibility to dismiss the limitations of calculated allowable scan times in favor of physiologically determined durations. With the current concern for possible neuromodulatory effects from anesthetic exposure in childhood and other risks, ultimately we expect decreased exposure of children to repeat anesthetics for scan completions.12 Procedural safety and quality of care would improve with the widespread application of routine temperature monitoring during MRIs, allowing early intervention for hypo- or hyperthermia.
Name: Viviane G. Nasr, MD.
Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.
Attestation: Viviane G. Nasr has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.
Name: Roman Schumann, MD.
Contribution: This author helped design the study, analyze the data, and write the manuscript.
Attestation: Roman Schumann has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.
Name: Iwona Bonney, PhD.
Contribution: This author helped conduct the study, analyze the data, and write the manuscript.
Attestation: Iwona Bonney has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.
Name: Lina Diaz, MD.
Contribution: This author helped design the study, conduct the study, and write the manuscript.
Attestation: Lina Diaz has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.
Name: Iqbal Ahmed, MD, FRCA.
Contribution: This author helped design the study, conduct the study, and write the manuscript.
Attestation: Iqbal Ahmed has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.
This manuscript was handled by: Dwayne R. Westenskow, PhD.
The authors wish to thank Yoojin Lee, MS, MPH, Statistical Associate, Biostatistics Research Center, Institute for Clinical Research and Health Policy Studies at Tufts Medical Center, for expert assistance with statistical analysis of the data.
a International Electrotechnical Commission. Medical electrical equipment. Part 2–33. Particular requirements for the safety of magnetic resonance equipment for medical diagnosis. Geneva, Switzerland: International Electrotechnical Commission, 2010.
b The American Society of Anesthesiologists. Standards for basic anesthetic monitoring. Effective July 2011. Available at: http://www.asahq.org/For-Members/Clinical-Information/Standards-Guidelines-and-Statements.aspx. Accessed June 2011.
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© 2012 International Anesthesia Research Society
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