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Tympanic temperature during therapeutic hypothermia
  1. D Hasper,
  2. J Nee,
  3. J C Schefold,
  4. A Krueger,
  5. C Storm
  1. Department of Nephrology and Medical Intensive Care Medicine, Campus Virchow-Klinikum, Charité Universitätsmedizin Berlin, Berlin, Germany
  1. Correspondence to Dr D Hasper, Department of Nephrology and Medical Intensive Care Medicine, Campus Virchow-Klinikum, Charité Universitätsmedizin Berlin, Augustenburger Platz 1, 13353 Berlin, Germany; dietrich.hasper{at}charite.de

Abstract

Objective Prehospital induction of therapeutic hypothermia after cardiac arrest may require temperature monitoring in the field. Tympanic temperature is non-invasive and frequently used in clinical practice. Nevertheless, it has not yet been evaluated in patients undergoing mild therapeutic hypothermia (MTH). Therefore, a prospective observational study was conducted comparing three different sites of temperature monitoring during therapeutic hypothermia.

Methods Ten consecutive patients admitted to our medical intensive care unit after out-of-hospital cardiac arrest were included in this study. During MTH, tympanic temperature was measured using a digital thermometer. Simultaneously, oesophageal and bladder temperatures were recorded in a total of 558 single measurements.

Results Compared with oesophageal temperature, bladder temperature had a bias of 0.019°C (limits of agreement ±0.61°C (2SD)), and tympanic measurement had a bias of 0.021°C (±0.80°C). Correlation analysis revealed a high relationship for tympanic versus oesophageal temperature (r=0.95, p<0.0001) and also for tympanic versus bladder temperature (r=0.96, p<0.0001).

Conclusions That tympanic temperature accurately indicates both oesophageal and bladder temperatures with a very small discrepancy in patients undergoing MTH after cardiac arrest is demonstrated in this study. Although our results were obtained in the hospital setting, these findings may be relevant for the prehospital application of therapeutic hypothermia as well. In this case, tympanic temperature may provide an easy and non-invasive method for temperature monitoring.

  • Cardiac arrest
  • therapeutic hypothermia
  • temperature monitoring
  • tympanic temperature
  • cardiac care
  • emergency ambulance systems
  • environmental medicine
  • hypothermia
  • nursing
  • prehospital

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Introduction

Following the current guidelines, a narrow temperature range of 32–34°C is recommended for mild therapeutic hypothermia (MTH) after cardiac arrest.1 Although theoretically, temperatures > 34°C may be less effective, on the other hand, adverse effects of MTH may increase when temperature falls <32°C.2 As a result, temperature monitoring is an important but unattended issue when applying MTH. Although brain temperature is the primary target, it is almost never available in patients after cardiac arrest. Although a criterion standard has still not been defined, temperature monitoring using probes placed either in the bladder or in the oesophagus is probably the most widely used method during MTH.

As demonstrated in animal studies, there is a clear rationale to induce therapeutic hypothermia as soon as possible after return of spontaneous circulation.3 Therefore, the induction of MTH in the field will increase in the years to come.4 With regard to the practical aspects, there is a need of additional equipment for the emergency medical service staff. Nevertheless, methods of temperature monitoring used in the intensive care unit seem to be not very suitable for the prehospital setting.

Measurement of tympanic temperature is non-invasive and frequently used in clinical practice. Nevertheless, its reliability is still under debate.5–7 Furthermore, to the best of our knowledge, tympanic temperature has not been evaluated in patients undergoing MTH. Therefore, we have conducted a study comparing three different temperature monitoring sites in patients after cardiac arrest.

Material and methods

The study protocol was approved by the local ethics committee on human research. All data were collected within the normal daily intensive care routine in an anonymous fashion. The institutional review board therefore waived the need for informed patient consent. A total number of ten consecutive patients after out-of-hospital cardiac arrest were included in this study. Hypothermia was initiated in all patients with infusion of cold saline and maintained with the Arctic Sun Temperature Management System (Artic Sun 2000; Medivance, Louisville, Colorado, USA). This system consists of hydrogel-coated water circulating energy transfer pads applied to the torso and the thighs.8 The target temperature of 33°C was maintained for 24 h.

Tympanic temperature was measured using a digital thermometer (Braun Thermo Scan pro 4000; Welch Allyn, Jungingen, Germany). This device measures the infrared heat generated by the eardrum. Simultaneously to the measurement at the tympanum, the results of oesophageal (Mon-A-Therm 12 SrCH; Tyco, Neustadt, Germany) and bladder (Brillant Silicone; Rüsch, Kernen i.R., Germany) temperature probes were recorded (figure 1). Temperature measurement was performed during the induction, the maintenance and the rewarming phase of MTH.

Figure 1

Chart of study design. Temperatures were recorded simultaneously at three different locations during therapeutic hypothermia at 186 different time points (overall 186×3=558 measurements).

MedCalc (Version 11.0) was used for statistical analysis. Descriptive parameters are given as median and 95% confidence interval (CI). Univariate analysis of pairs of temperature measurements between the groups was performed using the Wilcoxon test. The method of Bland and Altman was used to analyse the agreement between the different points of temperature measurement, and correlation analysis was used to analyse the relation of the datasets.

Results

Ten patients after out-of-hospital cardiac arrest were included in this study (eight men, median age 71.5 years, nine with ventricular fibrillation as initial rhythm). In these patients, 186 datasets (overall 558 values) were collected (figure 1). Median temperature was 33.30°C (95% CI 33.10 to 33.68) in the bladder, 33.40°C (95% CI 33.20 to 33.68) in the oesophagus and 33.40°C (95% CI 33.30 to 33.60) at the tympanum. Compared with oesophageal temperature, bladder temperature had a bias of 0.019°C (limits of agreement ±0.61°C (2×SD)), and tympanic measurement had a bias of 0.021°C (±0.80°C; table 1, figure 2). Correlation analysis revealed a high relationship for tympanic versus oesophageal temperature (r=0.95, p<0.0001, 95% CI 0.93 to 0.96) and also for tympanic versus bladder temperature (r=0.96, p<0.0001, 95% CI 0.95 to 0.97).

Table 1

Comparison of different locations of measurement

Figure 2

Bland–Altman plot and correlation scatter plot. Left row: Bland–Altman plots; temperatures are given in degrees Celsius; vertical axis shows the temperature difference between the locations compared; mean temperature on the horizontal axis refers to the average of the temperature, SD. Right row: Scatter plot; correlation represents the relationship between dataset; correlation coefficient is given as r.

Discussion

In summary, our study demonstrates that tympanic temperature accurately indicates both oesophageal and bladder temperatures with a very small bias in patients undergoing MTH after cardiac arrest. These data are in accordance with results obtained from patients during cardiac surgery.9

At present, tympanic temperature is only available for intermittent measurements. Therefore, there is little basis for comparison when relating it to continuous temperature monitoring with probes either in the bladder or in the oesophagus. Nevertheless, there may be situations in which our results may have high clinical relevance: First, our data may encourage hospitals not frequently treating patients after cardiac arrest to apply MTH to these patients. MTH does not necessarily require sophisticated technical equipment but is also feasible with simple procedures available in every hospital. Second, the application of MTH even in the field may increase in the future.10 In this prehospital setting, a safe, simple and easy to use method for temperature monitoring should be available to the emergency medical service staff. Tympanic temperature fulfils most of these requirements. This method is non-invasive and applicable in almost any situation. With regard to the limited resources in the field, it should be outlined that tympanic thermometers are inexpensive and reusable. Furthermore, there are no known adverse effects.

Nevertheless, one should keep in mind that our results cannot be automatically translated in the prehospital setting. Measurement of tympanic temperature in the field might be influenced by specific prehospital factors. For example, different cooling methods as the infusion of ice-cold saline have been established in the prehospital setting, whereas the application of some technical devices used in the intensive care unit seems not practicable in this setting. Therefore, further studies evaluating the feasibility of tympanic temperature measurements in the field are necessary.

In summary, we demonstrated that temperature monitoring during MTH after cardiac arrest seems feasible with tympanic temperature. Although we have generated our results in the intensive care unit, we believe that tympanic temperature monitoring may be of interest for the prehospital induction of MTH.

Acknowledgments

We thank Astrid Caemmerer for assistance and support throughout the study. Temperature probes were provided by Tyco GmbH, Neustadt, Germany.

References

Footnotes

  • Dietrich Hasper and Jens Nee contributed equally.

  • Competing interests None.

  • Ethics approval This study was conducted with the approval of the local ethics committee on human research.

  • Provenance and peer review Not commissioned; externally peer reviewed.

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