To the Editor:-I would like to offer an observation and to suggest a caveat, if I may, to the "where to monitor" guidelines, [1] especially as related to skin temperature.

I recently noted a strikingly rapid increase in indicated temperature, early in an anesthetic, derived from a probe located over an axillary artery beneath an upper arm blood pressure cuff. In verification, I measured a normal nasopharyngeal temperature and coincidentally noted that the cutaneous temperature, compared with the nasopharyngeal temperature, increased to a positive difference of 1.5 [degree sign]C over 5 min as the temperature of the upper body forced air warming blanket increased toward the set temperature of 43 [degree sign]C. Remarkably, the initial response (0.1 [degree sign]C) to the warming blanket temperature change seemed, in retrospect, to begin within less than 1 min. When the blanket set temperature was subsequently reduced to 38 [degree sign]C, the positive difference very rapidly decreased to +0.2 [degree sign]C and then, over the next 20 min, to -0.5 [degree sign]C. I have noted a similar, but smaller, positive difference effect from a cutaneous probe on an arm receiving warmed fluids at a rapid rate.

The increasing use of the laryngeal mask airway makes the esophageal site less attractive for core temperature measurement; skin temperature is an easy alternative. Rectal sites will never be frequently used outside of cardiac surgery, accuracy notwithstanding. Given the concurrent increasing application of forced air warming blankets and effective fluid warmers, I would like to suggest an alternative. An esophageal temperature probe, placed inside the finger of a disposable glove to minimize trauma, or a well-lubricated small bore esophageal stethoscope/temperature probe combination, inserted into the nasopharynx, are just as easy and provide a much more reliable measure of core temperature. Furthermore, in my experience, nasopharyngeal temperature is rarely spuriously overelevated or depressed, whereas skin temperature seems to be trustworthy only so long as it behaves as predicted.

C. F. Ward, M.D.

Anesthesia Service Medical Group; Green Hospital of Scripps Clinic; San Diego, California 92138–2807;

(Accepted for publication March 4, 1999.)

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