The nasopharyngeal temperature probe should be placed in the upper nasopharynx to reflect accurate core temperature. However, there have been no studies conducted to predict parameters for the optimal depth of the nasopharyngeal temperature probe. The purpose of this study was to examine the correlation between the optimal depth to the upper nasopharynx and the distance from the philtrum to the tragus and height.

Two hundred patients (100 females and 100 males) were enrolled in the study. The distance from the philtrum to the tragus along the facial curvature was measured, and the optimal depth from the nostril to the upper nasopharynx was evaluated using nasendoscopy. The relationships between the optimal depth to the upper nasopharynx and the distance from the philtrum to the tragus and height were examined.

The distances from the philtrum to the tragus were 14.4 ± 0.5 cm in females and 15.1 ± 0.6 cm in males (P < 0.01). The depths from the nostril to the upper nasopharynx were 9.4 ± 0.6 cm in females and 10.0 ± 0.5 cm in males (P < 0.01). The correlation coefficients between the depth from the nostril to the upper nasopharynx and the distance to the tragus from the philtrum were 0.43 in females and 0.41 in males (P < 0.01). However, there were very weak correlations and no correlations between height and the depth from the nostril to the upper nasopharynx in females and males, respectively.

The depth from the nostril to the upper nasopharynx is correlated weakly with the distance from the philtrum to the tragus. Although the distance from the philtrum to the tragus is not a good predicting parameter for the optimal depth of nasopharyngeal temperature probe placement, subtraction of 5 cm from the distance is helpful to estimate the optimal depth of the nasopharyngeal temperature probe.