TECHNOLOGY, COMPUTING, AND SIMULATION: Research Report
Perioperative core temperature perturbations are common and have been associated with numerous serious adverse outcomes (1–6). Consequently, core temperature monitoring is a critical aspect of anesthesia practice. New airway devices, including the laryngeal mask airway (LMA; Laryngeal Mask Company, Henley-on-Thames, United Kingdom) and the cuffed oropharyngeal airway (COPA), have been increasingly substituted for endotracheal intubation (7,8). A difficulty with this approach is that the most obvious core-temperature monitoring site, the distal esophagus, is no longer readily accessible. Therefore, we tested the hypothesis that body temperature measured with a thermocouple positioned on the LMA or COPA is sufficiently accurate and precise for clinical use.
With approval from the Committees on Human Research at Teikyo University and Tokyo Women’s Medical University and informed patient consent, we evaluated 36 patents (30 during orthopedic surgery lasting approximately 3 h and 6 receiving intentional hyperthermia therapy for malignant tumors). All were having procedures in which laryngeal or pharyngeal airways were appropriate. The type of airway was assigned randomly (n = 18 each). Randomization was based on computer-generated codes that were maintained in sequentially numbered opaque envelopes.
Patients were premedicated with IM midazolam (0.05 mg/kg) and were induced with IV propofol (1.5 mg/kg). Anesthesia was maintained with nitrous oxide (33%), oxygen, and sevoflurane (1.5%–2.5%); fentanyl was given per the judgment of the attending anesthesiologist. The patients breathed spontaneously but were assisted as required to maintain end-tidal PCO2 near 40 mm Hg. We measured LMA and COPA temperatures with Mon-a-therm® thermocouples (Mallinckrodt Anesthesiology Products, St Louis, MO) that were taped to the posterior mid-portion of the cuff.
Reference temperatures were measured in the nasopharynx and tympanic membrane with the Mon-a-Therm thermocouples. The probes were connected to Mallinckrodt Model 6510 two-channel electronic thermometers. These thermometers require no user calibration and have an accuracy near 0.1°C when used with Mon-a-Therm disposable thermocouples. Visual inspection with an otoscope confirmed that the ear canal was free of wax in each patient. The patients inserted the aural probes until they felt the thermocouple touch the tympanic membrane: appropriate placement was confirmed when patients easily detected a gentle rubbing of the attached wire. The aural canal was then occluded with cotton, the probe securely taped in place, and a gauze bandage positioned over the external ear. Ambient temperature was maintained at ≈23°C–24°C during surgery and ranged from 24°C to 42°C during hyperthermia therapy.
For the orthopedic patients, a full-length circulating-water mattress heated to 37°C was positioned under each patient, IV fluids were warmed to 37°C, and a single layer of standard surgical draping covered each patient. Hyperthermia in the cancer patients was achieved with an infrared warming chamber per clinical routine.
Temperatures were measured at 15-min intervals throughout surgery or hyperthermia therapy. Because temperatures are an objective measurement, the investigator who recorded temperatures was not blinded to the airway device. LMA, COPA, and the reference temperatures were compared using the square of the correlation coefficient of a linear regression (r2), and bias (the mean difference between two methods) (9). As in previous studies, temperatures differing <0.5°C were considered clinically acceptable (10,11). Data are presented as mean ± SD.
Morphometric and demographic characteristics of the patients having LMA or COPA airways were similar (Table 1).
Nasopharyngeal (Tnaso) and tympanic membrane (Ttym) temperatures ranged from 35°C to 37°C during surgery and from 35°C to 43°C during hyperthermia therapy. As might be expected, nasopharyngeal and tympanic membrane temperatures were nearly identical, with a slope of 1.01 (r2 = 0.98); the difference between these two sites averaged 0.10°C ± 0.15°C.
Temperatures measured on the LMA correlated well with both Tnaso and Ttym (Table 2; Fig. 1). Temperatures measured on the COPA also correlated well with the Tnaso and Ttym (Table 2, Fig. 2).
The fraction of temperatures that differed from Tnaso by more than ±0.5°C was 8% with LMA and 11% with COPA; the fraction of temperatures that differed from Ttym by more than ±0.5°C was 7% with LMA and 10% with COPA. However, most of the deviations exceeding 0.5°C were observed during therapeutic hyperthermia. Excluding these patients, only 4% of the temperatures differed from Tnaso by more than ±0.5°C with LMA and 5% with COPA; similarly, only 3% of the temperatures differed from Ttym by more than ±0.5°C with LMA and 4% with COPA (Table 2, Figs. 3 and 4).
Roughly, half of all patients having general anesthesia are no longer tracheally intubated. It would therefore be helpful to have a reliable temperature monitor incorporated into oral, laryngeal, and pharyngeal airways. Approximately 90% of the values recorded from the COPA or LMA were within 0.5°C of the reference core sites. When the patients having therapeutic hyperthermia were excluded, only approximately 4% of the values differed by more than 0.5°C. Our data thus suggest that thermocouples incorporated in the cuffs of COPAs and LMAs provide reasonably accurate estimates of core temperature.
A critical aspect of our experimental design was that the test thermocouples were positioned on the airway cuffs rather than on the airways themselves. This positioning assured that the thermocouples were in close proximity to highly perfused oropharyngeal tissues and were simultaneously insulated from the cooling effect of respiratory gases by the air in the cuffs. Although not tested in this study, it seems unlikely that comparable accuracy would be obtained from thermometers positioned directly on the airways.
We tested a relatively wide range of core temperatures (35°C–43°C); however, it remains possible that accuracy would be worse at lower temperatures. Accuracy would surely be worse during cardiopulmonary bypass, but of course, COPA or LMAs would not be used during bypass. The patients in our study all breathed spontaneously; however, these results would probably be just as accurate in mechanically ventilated patients because the effects of minute ventilation (and thus the cooling effect) would be similar.
In summary, we compared temperatures obtained from thermocouples positioned on the cuffs of LMA and COPA with core temperatures obtained from the nasopharyngeal and tympanic membrane temperatures. LMA and COPA temperatures correlated well with the temperatures at the two reference sites, rarely differing by more than 0.5°C. We thus conclude that temperatures monitored from the cuffs of COPA or LMAs are suitable for routine clinical use.
Dr. Sessler has a personal financial interest in Radiant Medical, Inc.
Supported, in part, by NIH Grant GM49670 (Bethesda, MD) and the Commonwealth of Kentucky Research Challenge Trust Fund (Louisville, KY).
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© 2003 International Anesthesia Research Society
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