Insertion Depth of an Esophageal Stethoscope with a Thermistor: Agreement with Tympanic Temperature and an Acceptable Range Referenced to the Maximal Heart-Sound Position

Background and Objectives: The distal esophagus is a preferred site for intraoperative core-temperature monitoring, but there is no consensus on the insertion depth of an esophageal stethoscope with a thermistor (EST). We aimed to identify the depth of closest agreement with tympanic membrane (TM) temperature and the acceptable range around it. Materials and Methods: Forty adults (ASA I–II, 20–65 years) undergoing elective supine surgery under general anesthesia were studied prospectively (ClinicalTrials.gov NCT05552092). After induction, an 18-Fr EST was inserted orally, and the maximal heart-sound (MHS) depth was recorded by auscultation. From 30 min after induction, it was withdrawn in 2-cm steps every 5 min through ten positions (45 to 27 cm), with paired esophageal and TM measurements at each step. Results: The mean TM−esophageal difference rose from −0.27 °C at 45 cm to +0.77 °C at 27 cm, with the population mean crossing zero at 38.0 cm. At each patient’s best-agreement step, selected post hoc as the closest of ten, bias was −0.02 °C (95% limits of agreement −0.14 to +0.11 °C), with all patients within ±0.5 °C. The per-patient interpolated zero-crossing depth lay 2.1 cm deeper than the MHS position (p < 0.001). Exploratory analysis showed that placement from MHS −1 cm to at least +4 cm kept ≥90% of measurements within ±0.5 °C. Only the MHS position was associated with the best-agreement step (r = 0.39, p = 0.014), accounting for 15% of its variance. Conclusions: The EST agreed closely with TM temperature over a wide range of depths centered just distal to the MHS position, and shallow placement degraded accuracy faster than deep placement. The MHS position is a bedside landmark rather than a predictor of an individual patient’s depth, and the range derived here requires prospective validation.