Temporal Artery versus Bladder Thermometry during Adult Medical-Surgical Intensive Care Monitoring: An Observational Study

Accurate and precise temperature measurement is an important part of the clinical examination because an abnormal temperature reading can alter clinical management and patient prognosis[1]. The ideal system for measuring temperature should be minimally invasive and provide rapid results in a reliable, accurate and safe manner to support clinical decisions. However, questions still arise about the optimal method for temperature measurement in critically ill patients.

Most authorities including the American College of Critical Care Medicine and the Infectious Diseases Society of America consider temperature to be most accurately measured by intravascular, esophageal or bladder thermometry[2]. However, these devices are semi-invasive and may not be appropriate for use in all patients. Rectal thermometers are an alternative, but have limitations including patient perception that these are unpleasant and intrusive, access to the rectum may be limited by patient position, and patient movement may dislodge thermometers[2]. In addition, rectal thermometry has been associated with delays in recording temperature changes[3, 4], and has been implicated in the spread of enteric pathogens[5]. Oral temperature measurement is safe, convenient, and familiar for alert and cooperative patients. However, in critically ill patients, oral temperatures are often not practical due to intubation or inability of the patient to cooperate. Furthermore, hyperventilation or ingested substances may affect oral temperature measurement[6, 7]. The tympanic membrane temperature is believed to reflect the temperature of the hypothalamus and thus the core body temperature[8]. Infrared ear thermometry detects radiant energy from the tympanic membrane and ear canal through an otoscopic probe. However, multiple studies have shown consistently poor agreement between infrared tympanic membrane and rectal thermometers[9].

Infrared technology has been adapted to noninvasive temporal artery thermometry. Because the temporal artery has a high arterial perfusion rate that remains unchanged under most conditions, measurement of temperature via skin areas perfused by the temporal artery provides an easy, noninvasive estimate of core temperature[10]. This has resulted in wide adoption of the technology not only on general hospital wards, but also in intensive care units (ICU). However, professional society guidelines for measuring patient temperature in the ICU have recommended against using temporal artery thermometry because environmental temperature and sweating have been associated with unreliable temperature measurements[2, 11]. Given the widespread clinical application of this new technology, we performed a prospective evaluation of the agreement in temperature assessment between a temporal artery thermometer and an established method of core temperature measurement, bladder thermometry as performed in clinical practice[12].

Table 1 summarizes the 14 patients enrolled in the study, 5 of whom were males and 9 females. The patients' ages ranged from 19 to 74 years (mean 51 ± 18). The diagnoses included trauma (n = 5), sepsis (n = 3), neurological disorders (n = 3) and other medical problems (n = 3). Patients were intubated (n = 14) and severely ill with a mean APACHE II score of 24 (± 5). During the study period, the majority of patients (9/14) received at least one temperature modifying intervention such as acetaminophen.

A total of 760 temporal artery thermometer observations and 1,066 bladder thermometer observations were recorded from these 14 patients. Of these, 736 temporal artery and bladder temperature observations were recorded within one minute of each other and used for analysis. Table 1 summarizes the temperatures measured using the temporal artery and bladder thermometers. The mean difference between temporal artery and bladder temperatures was -0.44°C (95% confidence interval [CI], -0.47°C to -0.41°C), with temporal artery readings lower than bladder temperatures.

Figure 1 summarizes the difference in temperature readings between the temporal artery and bladder. Agreement between the two devices was greatest for normothermia (36.0°C to < 38.3°C) (mean difference -0.35°C [95% CI, -0.37°C to -0.33°C]). The temporal artery thermometer recorded higher temperatures during hypothermia (< 36°C) (mean difference 0.66°C [95% CI, 0.53°C to 0.79°C]) and lower temperatures during hyperthermia (≥38.3°C) (mean difference -0.90°C [95% CI, -0.99°C to -0.81°C]).

Table 2 summarizes the operating characteristics of the temporal artery thermometer compared to the bladder thermometer for diagnosing fever. The temporal artery thermometer demonstrated a low sensitivity, but good specificity for diagnosing fever. Figure 2 summarizes the receiver operating characteristic curve of the temporal artery thermometer for diagnosing fever (area under the curve 0.80; 95% CI, 0.76 to 0.83). The accuracy of the temporal artery thermometer for diagnosing hypothermia was not assessed due to insufficient measurements of hypothermia (25/736, 3.4%).

We performed a prospective evaluation of the agreement in temperature assessment between a widely adopted new methodology, temporal artery thermometry and an established core measurement methodology, bladder thermometry as performed in clinical practice[12]. The temporal artery thermometer closely agreed with the bladder thermometer for normothermic (36°C to < 38.3°C) temperatures. However, there was less agreement for temperatures below 36°C and temperatures greater than or equal to 38.3°C. The temporal artery thermometer had low sensitivity, but good specificity for diagnosing fever.

Our data support previous studies that have noted a lack of agreement between temporal artery thermometry measurements and standard core temperature measurement sites: bladder, pulmonary artery, rectum, axillary and tympanic membrane[18‐23]. Temporal artery thermometers appear superior to axillary and tympanic membrane infrared thermometers, but inferior to bladder, pulmonary artery and rectal thermometers[18‐23]. Suleman et al. compared a temporal artery thermometer with pulmonary artery and bladder thermometers in patients post-cardiac surgery and with a temperature of at least 37.8°C[20]. The performance of the thermometer was poor with a sensitivity of 0%, but specificity of 100% for fever in their adult patients[20]. Conversely, Greenes and Fleisher determined that in a sample of infants evaluated in the emergency department, the temporal artery was accurate for those with a high rectal fever and recommended it as an alternative to tympanic membrane thermometry[21]. Schuh et al. found the temporal artery thermometer to be an effective screening instrument for fever in children under 24 months of age, but insufficiently accurate to replace rectal thermometry[22]. Hebbar et al. judged the temporal artery thermometer to demonstrate marked variability compared to rectal temperature measurements in critically ill pediatric patients and recommended against it replacing invasive core temperature measurements[19]. However, Myny et al. evaluated the temporal artery thermometer against a pulmonary artery thermometer in a sample of normothermic critically ill adult patients and judged it to have acceptable reliability and accuracy[24]. Similarly, Lawson et al. demonstrated reasonable agreement between pulmonary artery catheter thermometers and both temporal artery and oral thermometers, but poor agreement with axillary and tympanic thermometers[23]. Kimberger et al. evaluated a temporal artery thermometer peri-operatively in a neurosurgical operating room and post-operative ICU and concluded that it was not an adequate substitute for core temperature monitoring[18].

Our study adds to the existing literature by providing the single largest clinical evaluation (736 paired observations) of a temporal artery thermometer in critically ill adult patients with a wide temperature range (35.4 to 40.0°C) in a medical-surgical ICU. In the ICU, temperature measurement is an essential component of patient assessment and management decisions. Our data highlight that although the temporal artery thermometer is convenient to use, minimally invasive and provides rapid results it agrees insufficiently with an established core temperature measurement methodology to be clinically useful in patients where careful temperature monitoring is important (e.g. sepsis surveillance, neurological injury, active heating or cooling of patients). This is most clearly illustrated by the small mean difference between the temporal artery and bladder thermometers for normothermic temperatures, but much larger mean differences for hypothermic and hyperthermic temperatures. This would give the instrument low sensitivity and the inability to rule out either hypothermia or fever. Conversely, our data demonstrate that the high specificity of the instrument indicates that if a fever is identified with a temporal artery thermometer then it is likely a true positive. Our results are perhaps not surprising given that skin temperature measurement can be potentially affected by patient (regional perfusion, diaphoresis, vasoactive medications), instrument (dirt or oil on temporal artery thermometer lens) and environmental (external heating or cooling devices such as fans) factors[2, 11, 25].

The results of our study need to be interpreted within the context of its limitations. First, although a wide range of temperatures was recorded in our study, there were few recordings of hypothermia. Thus, statements about the performance of temporal artery thermometry under conditions of hypothermia are limited. Second, we employed a bladder thermometer as our reference standard. Although there is no universally accepted reference standard for measuring core temperature, some authorities consider a pulmonary artery thermometer to be the standard against which other devices should be compared[2]. However, pulmonary artery catheters are invasive and employed infrequently in clinical practice[26]. Conversely, bladder thermometers are less invasive and provide continuous readings that are essentially identical to intravascular thermometers over a wide range of temperatures regardless of urine flow[13‐15]. Third, we did not independently validate thermometer calibration or monitor nursing compliance with the recommended procedure for using the temporal artery thermometer which may have contributed to suboptimal performance. As such our study reflects the realities of a clinical as opposed to a laboratory evaluation of the temporal artery thermometer[12].

In summary temporal artery thermometer measurements agreed with bladder thermometer measurements over normothermic temperature ranges, but demonstrated limited agreement for temperatures below 36°C and temperatures greater than or equal to 38.3°C. Temporal artery thermometry produces somewhat surprising disagreement with an established method of core temperature measurement and should not be used in situations where body temperature needs to be measured with accuracy.