The disposable liquid crystal thermometer Clinitrend measures temperature at least as well as the non-disposable temporal artery infra-red thermometer Exergen by David Southern, Andrew Campbell, Stella Wright and Jim Turner
The UK National Institute for Health and Care Excellence (NICE) has recently reported that perioperative hypothermia increases morbidity and mortality of surgical patients and mandates regular temperature measurement. The ideal thermometer would be able to estimate core temperature measurement without invading the body core, would be cheap and disposable. The Clinitrend strip approaches this ideal if its accuracy is at least as good as other thermometers currently in use. We therefore evaluated the accuracy of Clinitrend in estimating core temperature as measured by a nasopharyngeal temperature probe and compared the accuracy to a peripheral temperature thermometer in current clinical use (Exergen).
Introduction
NICE has prioritised peri-operative normothermia when it issued Clinical Guideline 65 in 2008 [1]. This guideline acknowledged that most thermometers inferred core temperature from a reading taken peripherally and that there were discrepancies between different thermometer designs. The Guideline promoted the use of thermometers that were maintained according to manufacturer’s guidelines and cleaned according to local infection control policy. A disposable design simplifies these requirements, provided that its performance is equivalent to a non-disposable thermometer.
In this study, we designated nasopharyngeal temperature as the means of measuring core temperature. We then assessed concordance between this technique and two techniques that measure temperature in the frontal region of the head and then deduce core temperature. One of the thermometers (Clinitrend [2]) is disposable.
Methods
Following local research ethics committee approval and written informed consent from the individuals, we recruited 30 patients who were scheduled for elective surgery on a body region below the diaphragm. Operations were chosen that were anticipated to be of at least two hours duration; they included gynaecological, orthopaedic and abdominal procedures.
General anaesthesia was induced intravenously and maintained with a volatile agent. Following endotracheal intubation, a nasopharyngeal temperature probe [3] (temperature probe, Part no. 21090A, Philips Medical Systems) was inserted, a Clinitrend liquid crystal thermometer was applied to the central forehead and the Exergen [4] thermometer was passed over the skin of the temporal region. Measurements were taken according to the manufacturer’s instructions and were designated as the temperature at time zero.
Patients were then transferred to the operating theatre where surgery was performed. Patients were warmed using a forced air convection heater device applied to an appropriate body region below the clavicles. Temperature measurements were taken every 30 minutes using all three thermometers – nasopharyngeal (NAPOP), Clinitrend (Clin) and Exergen (Ex) devices. When the operation was finished, the nasopharyngeal temperature probe was removed and no further measurements were taken for this study.
Temperature was measured by the three thermometers at time 0, 30, 60, 90 and 120 minutes. Mean and standard deviation were calculated and a One-Sample Kolmogorov-Smirnov test performed to assess for normal distribution.
Results
Thirty patients were enrolled. Temperatures were measured at 30-minute intervals for two hours in 23 patients. In seven patients, surgery finished prematurely and patients were studied for only 90 minutes. Temperatures were recorded with three thermometers at all timepoints within the study periods producing a data set of 429 measurements. Data were similar (mean and standard deviation) at 90 and 120 minutes despite the reduction in the size of the study sample (Table 1). A One-Sample Kolmogorov-Smirnov test confirmed normal distribution of data. The mean and standard deviation of the data are shown in Table 1.
Tempature_liquid_crystal_thermometer
A Kernel density estimation (Figure 1) was calculated for temperature measured by NAPOP, Clinitrend and Exergen. This estimation of probability density function provides a visual impression of the ‘goodness of fit’ between two distribution curves. It does not allow calculation of statistical differences. The goodness of fit of Clinitrend appears to be closer to NAPOP than that of Exergen.
Figure 1. Kernel density plot of temperatures as measured by NAPOP, Clinitrend and Exergen.
For each patient at each timepoint, we calculated the difference between the NAPOP and the temperature measured by each test thermometer (temp-diff). The mean and standard deviation of temp-diff are shown for both test thermometers at 30-minute intervals (Figure 2). The mean Clinitrend reading was found to underestimate NAP