Infrared tympanic temperature and ear canal morphology.

Several publications indicate that the infrared tympanic temperature (IRTT) underestimates the core temperature of the body when the ear canal is long, curvy and narrow. In order to quantify these observations, a study was performed in 10 subjects. The IRTT was determined and compared to the oesophageal temperature (Tes), taken as the reference for core temperature. Also, the oral and rectal temperatures were monitored. A three-dimensional print of the ear canal was made to determine the ear canal morphology. The core temperature of the subjects was increased by at least 1 degrees C during the experiment in order to investigate the dynamics of the core temperature assessment. Two devices were used to determine the IRTT: the Braun Thermoscan PRO 1 and the predecessor of the Braun IRT3020 (code name IRT3000P). Both IRTT-devices underestimated the core temperature, as measured by Tes, by 0.38 degrees C on average. The difference DeltaT between IRTT and Tes was related to ear canal morphology. The circumference of the ear canal at the distal bend in the ear canal and the visibility of the tympanum were the most important parameters. About 22% of the variance in DeltaT was explained by ear canal morphology for the steady state resting period. Wide ear canals and good visibility of the tympanic membrane were related to a smaller DeltaT. A good visibility of the tympanic membrane was generally found in the absence of cerumen. The IRT3000P showed better results than the PRO 1 (DeltaT: -0.31 +/- 0.27 degrees C and -0.44 +/- 0.30 degrees C respectively). Also, the IRT3000P was less dependent on ear canal morphology. The dynamic response of the measured core temperatures was determined by the decrease or rise in core temperature after the heating period was ended. The oesophageal temperature dropped by 0.22 degrees C. The IRTT and oral temperature showed an identical increase of 0.19 degrees C. The slow reacting rectal temperature had an after rise of 0.49 degrees C.