Optimal Depth for Nasopharyngeal Temperature Probe Positioning

BACKGROUND: The nasopharynx is considered 1 of the 4 generally reliable core temperature measurement sites. But curiously, there is no consensus on how far past the nares to insert the probe. Insertion depth is likely to influence the accuracy of nasopharyngeal temperature measurements because probes near the nares will be cooled by ambient air; similarly, probes inserted too far may approach the airway and be cooled by ventilation gases. We thus determined the range of nasopharyngeal probe insertion depths that best approximate reference core temperature measured in the distal esophagus. METHODS: In 36 adults undergoing noncardiac surgery with endotracheal intubation, we inserted a nasopharyngeal thermometer 20 cm past the nares and an esophageal temperature probe 40 cm from the incisors. The nasopharyngeal probe was withdrawn sequentially 2 cm at a time at 5-minute intervals. Pairs of nasopharyngeal and reference distal esophageal temperatures were then compared and summarized by Bland and Altman methods. RESULTS: All nasopharyngeal probe insertion depths between 10 and 20 cm past the nares provided temperatures similar to reference distal esophageal temperatures. At those depths, the bias was typically approximately -0.1 degrees C, with SD of approximately +/- 0.15 degrees C; the limits of agreement thus were easily within our a priori specified clinically acceptable range of -0.5 degrees C and 0.5 degrees C. CONCLUSIONS: Any nasopharyngeal probe insertion depth between 10 and 20 cm well represents core temperature in adults having noncardiac surgery.