Tympanic vs. Rectal Temperatures: Does it Matter?
Abstract & Commentary
Synopsis: The three infrared (tympanic) thermometers studied provided a closer estimate of core body temperature than rectal temperature. Clinical bias (difference between sites) was greater in febrile than in afebrile patients.
Source: Rotello LC, et al. Crit Care Med 1996;24: 1501-1506.
While estimation of core temperature is without a doubt an important component of routine care of critically ill patients, its most accurate representation remains controversial. Simultaneous readings of infrared ear, equilibrated rectal, and pulmonary artery (PA) temperatures were measured in 20 critically ill adult patients to assess clinical bias (mean difference between body sites) and variability (standard deviation of the between-site differences). Two models of clinically available infrared ear thermometers and one experimental model were evaluated for clinical repeatability. The rectal thermometers used were electronic predictive types, manufactured by one company.
Patients were excluded if they had any contraindication to temperature measurement at any site (e.g., hemotympanum, CSF otorrhea, external otitis, recent myocardial infarction, bowel or rectal surgery, or immunosuppression). Patients were also excluded if their baseline PA temperature was between 37.6°C and 37.9°C in order to separately study febrile and afebrile patients. The patients included had a variety of clinical conditions and interventions including vasoactive drugs and mechanical ventilation.
The mean rectal temperatures were significantly greater than PA temperatures by 0.3°C, while none of the infrared ear temperatures were greater than the PA temperature. One subject was excluded from analysis because his tympanic temperature was more than 2°C greater than his PA temperature; his cardiac output of 15 L/min and arterio-venous shunt for hemodialysis was felt to have variable effects on his core temperature measurement and peripheral blood flow.
Clinical variability was greater for all three infrared ear thermometers compared with the rectal. The temperature difference between body sites, or clinical bias, was greater in all of the thermometers tested for febrile compared to afebrile patients.
COMMENT BY DOREEN M. ANARDI, RN
Taking temperatures seems pretty basic, but unique patient characteristics, interventions, and equipment specifications can make getting reliable information tricky. Rotello et al point out the appealing features of infrared ear thermometry: It is quick, clean, saves personnel time, and is more comfortable for the patient. Rectal thermometry, however, through years of experimental studies, has been held up as the gold standard to represent core body temperature, even though the reduced blood flow characteristics of the rectum make it physiologically ill-suited to measure core body temperature. Studies have shown that rectal temperatures lag from 3 minutes to 1.5 hours behind changes in true core temperature. While the difference from PA temperature measured in this study is small, it is large enough to indicate an intervention such as culturing body fluids for sources of infection or the administration of medication.
There are clinical conditions and patient characteristics that make one temperature measurement site preferable to another. That site should be identified for each patient and temperatures consistently measured there so that trends can be sensibly evaluated. Contraindications to tympanic and rectal temperatures have been mentioned, but even the gold standard PA temperatures can be rendered inaccurate by the vigorous infusion of IV fluids.
There is still much that is not understood about body temperature fluctuations; it is unclear why fever increases the clinical bias (differences between sites) as noted in this study. The temperature profiles of hypothermic patients constitute another area of needed investigation.
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