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    Figure 1—

    Bland-Altman plots of the difference between temperature measured by a thermistor-tipped PA catheter and the temperature measured by a predictive rectal thermometer (A); an infrared auricular thermometer designed for veterinary use (B); and a subcutaneous temperature-sensing microchip in the interscapular region (C), lateral shoulder region (D), and sacral region (E) against the mean of the temperature as measured by the PA catheter and the temperature measured by that device or a device at that location in 8 dogs. The horizontal reference line corresponds to zero differences (ie, if the reading from the described device [or device at a specific location] matched the core temperature reading, then all points would fall on this line). Points located above the reference line represent an underestimation of temperature by the device in comparison to core body temperature (as measured by the PA catheter).

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Comparison of three methods of temperature measurement in hypothermic, euthermic, and hyperthermic dogs

Rebecca J. GreerDepartment of Veterinary Medicine and Surgery, College of Veterinary Medicine, University of Missouri, Columbia, MO 65211.

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Leah A. CohnDepartment of Veterinary Medicine and Surgery, College of Veterinary Medicine, University of Missouri, Columbia, MO 65211.

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John R. DodamDepartment of Veterinary Medicine and Surgery, College of Veterinary Medicine, University of Missouri, Columbia, MO 65211.

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Colette C. Wagner-MannDepartment of Veterinary Medicine and Surgery, College of Veterinary Medicine, University of Missouri, Columbia, MO 65211.

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F. A. MannDepartment of Veterinary Medicine and Surgery, College of Veterinary Medicine, University of Missouri, Columbia, MO 65211.

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Abstract

Objective—To assess the reliability and accuracy of a predictive rectal thermometer, an infrared auricular thermometer designed for veterinary use, and a subcutaneous temperature-sensing microchip for measurement of core body temperature over various temperature conditions in dogs.

Design—Prospective study.

Animals—8 purpose-bred dogs.

Procedures—A minimum of 7 days prior to study commencement, a subcutaneous temperature-sensing microchip was implanted in 1 of 3 locations (interscapular, lateral aspect of shoulder, or sacral region) in each dog. For comparison with temperatures measured via rectal thermometer, infrared auricular thermometer, and microchip, core body temperature was measured via a thermistor-tipped pulmonary artery (TTPA) catheter. Hypothermia was induced during anesthesia at the time of TTPA catheter placement; on 3 occasions after placement of the catheter, hyperthermia was induced via administration of a low dose of endotoxin. Near-simultaneous duplicate temperature measurements were recorded from the TTPA catheter, the rectal thermometer, auricular thermometer, and subcutaneous microchips during hypothermia, euthermia, and hyperthermia. Reliability (variability) of temperature measurement for each device and agreement between each device measurement and core body temperature were assessed.

Results—Variability between duplicate near-simultaneous temperature measurements was greatest for the auricular thermometer and least for the TTPA catheter. Measurements obtained by use of the rectal thermometer were in closest agreement with core body temperature; for all other devices, temperature readings typically underestimated core body temperature.

Conclusions and Clinical Relevance—Among the 3 methods of temperature measurement, rectal thermometry provided the most accurate estimation of core body temperature in dogs.

Abstract

Objective—To assess the reliability and accuracy of a predictive rectal thermometer, an infrared auricular thermometer designed for veterinary use, and a subcutaneous temperature-sensing microchip for measurement of core body temperature over various temperature conditions in dogs.

Design—Prospective study.

Animals—8 purpose-bred dogs.

Procedures—A minimum of 7 days prior to study commencement, a subcutaneous temperature-sensing microchip was implanted in 1 of 3 locations (interscapular, lateral aspect of shoulder, or sacral region) in each dog. For comparison with temperatures measured via rectal thermometer, infrared auricular thermometer, and microchip, core body temperature was measured via a thermistor-tipped pulmonary artery (TTPA) catheter. Hypothermia was induced during anesthesia at the time of TTPA catheter placement; on 3 occasions after placement of the catheter, hyperthermia was induced via administration of a low dose of endotoxin. Near-simultaneous duplicate temperature measurements were recorded from the TTPA catheter, the rectal thermometer, auricular thermometer, and subcutaneous microchips during hypothermia, euthermia, and hyperthermia. Reliability (variability) of temperature measurement for each device and agreement between each device measurement and core body temperature were assessed.

Results—Variability between duplicate near-simultaneous temperature measurements was greatest for the auricular thermometer and least for the TTPA catheter. Measurements obtained by use of the rectal thermometer were in closest agreement with core body temperature; for all other devices, temperature readings typically underestimated core body temperature.

Conclusions and Clinical Relevance—Among the 3 methods of temperature measurement, rectal thermometry provided the most accurate estimation of core body temperature in dogs.

Body temperature determination is an important component of the physical examination of an animal.1 Traditionally, veterinary practitioners have depended on equilibrium-type rectal thermometers to determine body temperature.1,2 Although usually well tolerated, rectal thermometry can be difficult in fractious animals or in animals with rectal (or perianal) disease and can be influenced by the presence of feces in the rectum.1–3 Predictive rectal thermometry measures the rate of temperature change in the first few seconds after thermometer placement to mathematically predict the final temperature; to our knowledge, this method of temperature measurement has not been evaluated in euthermic or hyperthermic dogs. Auricular thermometers were developed to provide temperature measurements less invasively.2,4,5 As with rectal thermometry, some animals resent the auricular procedure, and local pathologic changes may affect the reading obtained.2,3 A subcutaneous identification microchip with a temperature sensor has been developed for dogs and is currently marketed in Europe and Asia. To our knowledge, there are no published studies regarding accuracy or reliability of the subcutaneous microchip device for temperature determination. Although results of some studies6,7 have indicated that rectal and auricular temperatures correlate with core body temperatures in dogs, core body temperature is still considered the most accurate method of temperature assessment. Such core temperature measurements can be achieved by use of thermistors placed in the esophagus, urinary bladder, or central vascular compartment.8–12 Despite the tremendous importance of accurate temperature measurement for clinical management of dogs, there is a paucity of scientific comparison of any of the minimally invasive methods of temperature assessment with core body temperature. The purpose of the study reported here was to assess the reliability and accuracy of 3 temperature-measuring devices (a predictive rectal thermometer, an infrared auricular device designed for veterinary use, and a subcutaneous temperature-sensing microchip) for measurement of core body temperature (determined by use of a thermistor-tipped PA catheter) over a variety of temperature conditions in dogs. We hypothesized that, in dogs, rectal thermometry would provide the most reliable and accurate estimate of core body temperature among these temperature-measuring methods.

Materials and Methods

Eight sexually intact mixed-breed (purpose-bred) hounds aged 8 months to 5 years (mean age, 1.5 years) and weighing 19.1 to 26 kg (42.0 to 57.2 lb; mean weight, 22.5 kg [49.5 lb]) were used in the study. The group included 3 males and 5 females. Body condition scores were 4 or 5 of 9 for all dogs; coat length was short in 7 dogs and medium in 1 dog. All dogs were considered to be in good health on the basis of findings of physical examination, including an aural examination. Dogs were housed in a routine manner in animal facilities accredited by the Association for Assessment and Accreditation of Laboratory Animal Care International. All experimental procedures were reviewed and approved by the Animal Care and Use Committee of the University of Missouri, Columbia. Dogs were adopted to private homes after completion of the study.

For each dog, temperature was measured by use of 4 thermometry devices during 4 study periods over 3 days. To provide a range of body temperatures at which the thermometry devices could be assessed, conditions were manipulated during each study period so that dogs were euthermic or developed hypothermia or hyperthermia. Hypothermia was induced during the first study period without manipulation other than induction of anesthesia, whereas hyperthermia was induced during study periods 2, 3, and 4 via administration of a low dose of endotoxin. Temperature readings during euthermia were obtained prior to onset of either hypo- or hyperthermia during each study period.

Two temperature-sensing microchipsa were implanted in each dog a minimum of 7 days prior to temperature measurements to allow resolution of any inflammation that might result from device implantation. The microchips were placed according to the manufacturer's recommendations in either the suggested or an alternate location in the body. For 5 dogs, a microchip was placed in the deep subcutaneous tissues of the dorsum between the shoulder blades (interscapular location) as suggested by the manufacturer. Three of the 8 dogs had non–temperature-sensing microchips placed in the typical interscapular location prior to enrollment in the study. To avoid interference from that existing microchip and evaluate use of the temperature-sensing microchip when placed in an alternate location, the temperature-sensing microchip was placed in the subcutaneous tissues of the dorsal aspect of the sacral area in those 3 dogs. Also, all dogs had a microchip placed SC over the distal aspect of the scapula and proximal portion of the humerus (designated the lateral shoulder location) to mimic migration of a microchip from the suggested interscapular location.

When possible, accuracy of thermometer devices was evaluated in vitro. The microchips were evaluated at the site of manufacture before shipping and were accurate to within 0.3°C (0.54°F) of environmental temperature. Rectal thermometersb were tested just prior to use in 40°C (104°F) and 25°C (77°F) water baths; prior to testing the rectal thermometers, the water bath temperatures were determined with a digital thermometerc that was traceable to the bureau of standards. Rectal thermometer readings were within 0.1°C (0.18°F) of agreement with water bath temperatures in each instance. After removal of the PA catheterd at the end of data collection, the thermistor unit of each catheter was checked in an identical manner, and all readings were accurate to within 0.1°C of water bath temperatures. Accuracy of the auricular thermometere was not evaluated in vitro.

Thermometers were used according to the method for which they were designed. The rectal thermometers were of an electronic predictive type. Prior to temperature measurements, the temperature probe was covered with an unused plastic sheath made for the device. The thermometer was inserted approximately 3 cm (1.5 inches) into the rectum and held there until a digital reading was obtained on the thermometer. For use of the auricular thermometer, a plastic sheath made for the device was fitted onto the probe prior to temperature measurement. As suggested by the manufacturer, the ear was manipulated to straighten the ear canal and allow a more direct alignment of the probe with the tympanic membrane. The temperature-sensing microchips (2/dog) were placed via SC injection in the same fashion as traditional identification microchips. Temperature readings from the microchips were accomplished by slowly moving an electronic handheld scanning device designed for use with the microchips over the area of microchip implantation. Ease of use for each device was assessed subjectively.

At least 7 days after microchip placements, each dog was anesthetized for placement of a thermistortipped PA catheter. All dogs were administered butorphanol (0.5 mg/kg [0.23 mg/lb]), xylazine (0.25 mg/kg [0.114 mg/lb]), and glycopyrrolate (0.01 mg/kg [0.005 mg/lb]) IM. Once the dogs were sedated, a cephalic IV catheter was placed and an IV infusion of physiologic saline (0.9% NaCl) solution was begun at 10 mL/kg/h (4.5 mL/lb/h). Anesthesia was induced with thiopental to effect (approx 10 mg/kg, IV) to allow endotracheal intubation. Anesthesia was maintained via inhalation of isoflurane in oxygen. During anesthesia, periodic monitoring included subjective assessment of the depth of anesthesia, respiration rate, heart rate, hemoglobin oxygen saturation, and exhaled carbon dioxide concentration. The coat over a jugular vein was clipped, and the area was prepared by use of standard aseptic technique. With fluoroscopic guidance, a percutaneous sheath introducer was used to place the thermistor-tipped catheter in the PA. The catheter was sutured in place and the neck was bandaged.

Once instrumentation was complete, study period 1 was initiated. Temperature readings from the PA catheter, rectal and auricular thermometers, and both microchips were obtained in duplicate as near simultaneously as possible (all 10 readings were accomplished within 90 seconds in each dog). Because the PA catheter-derived temperature was provided as a continuous digital reading, it was recorded first and then again last among the temperature assessments; otherwise, the order of measurements was random. A single scribe recorded temperature measurements from the PA catheter as well as measurements obtained for the other devices by 2 or 3 other investigators. Auricular temperatures were measured by a single investigator (RJG). Readings from the rectal, auricular, and subcutaneous thermometry devices in each dog were obtained every 10 minutes during a 40-minute period. Hypothermia, defined as a core body temperature < 37.8°C (100°F), was achieved in 7 of 8 dogs during the first study period. Following completion of study period 1, dogs were allowed to recover from anesthesia in the University of Missouri Veterinary Medical Teaching Hospital's intensive care unit and remained there until the PA catheters were removed at completion of study period 4.

Study period 2 began later the same day after the dogs were fully recovered from anesthesia and alert and their core body temperature (assessed via the PA catheter) was at least 38.3°C (101.0°F). A low dose of endotoxinf (2 μg/kg [0.9 μg/lb]) was administered to the dogs to induce fever.13,14 Hyperthermia was defined as core body temperature > 39.17°C (102.5°F). Temperature readings from the PA catheter, rectal and auricular thermometers, and both microchips were obtained in duplicate as near simultaneously as possible immediately prior to endotoxin administration and every 10 minutes thereafter until core body temperature reached a plateau for 3 consecutive readings or began to decrease. At the conclusion of study period 2, physiologic saline solution (20 mL/kg [9.1 mL/lb]) was administered IV to each dog over 4 hours. Arterial blood pressure and clinical evidence of illness were monitored until the dogs appeared clinically normal after endotoxin administration.

Study period 3 was conducted the following day, and study period 4 was conducted the day after that. On the basis of increases in body temperature and subjective assessment of degree of illness (vomiting, diarrhea, marked lethargy, or signs of depression) during study period 2, subsequent doses of endotoxin were reduced for several dogs (lowest dose administered was 1 μg/kg [0.45 μg/lb]). As during study period 2, temperature readings from the PA catheter, rectal and auricular thermometers, and both microchips were obtained in duplicate as near simultaneously as possible immediately prior to endotoxin administration and every 10 minutes thereafter until core body temperature reached a plateau for 3 consecutive readings or began to decrease. The PA catheters were removed at the end of study period 4, and cephalic catheters were removed following infusion of physiologic saline solution.

In 1 dog, the interscapular microchip device was surgically removed after completion of the described studies. The device itself was reevaluated by the manufacturer, and the tissues surrounding the device were evaluated via histologic examination; tissue samples underwent aerobic and anaerobic microbiologic cultures by use of standard techniques at the Veterinary Medical Diagnostic Laboratory of the University of Missouri.

Statistical analysis—Data were analyzed by use of a statistical software package.g Repeatability (variability) for each device or device location was described as the mean difference in near simultaneous (duplicate) measures obtained by use of the same device and SD of the difference in duplicate measures. The range of temperature difference for duplicate measurements by each device was also calculated. A formal test to compare the variances of measurements made close together in time was conducted by use of a mixed-model approach. The response variable used was the sample variance of the duplicate measures. Because measurements were obtained via different devices from the same dog under the same conditions (or study period), the dog was included as a random effect. The device and the study period were included as fixed effects. Measurements taken at 10-minute intervals were treated as repeated measures, and the correlation structure for these measurements was modeled as autoregressive of order 1. Heterogeneous variances for the response variable across different devices were included in the model. Pairwise comparisons of mean responses for different devices were made by use of least squares means. For other comparisons, the first reading from each device was compared with the core body temperature at the first PA catheter reading at the same time. The agreement between each of the other devices and the core body temperature (determined via the PA catheter) was assessed in 2 ways. First, simple descriptive statistics were used to determine how frequently the measured temperature agreed with the core body temperature within 0.5°C (0.9°F), 0.75°C (1.35°F), 1.0°C (1.8°F), and so forth at 0.25°C (0.5°F) increments to the level of 3.5°C (6.3°F). Second, Bland-Altman scatterplots were used to provide visual assessment of agreement between readings for the thermistor-tipped PA catheter and each of the other devices or device locations. Although commonly reported in similar studies, correlation and regression are not appropriate analytic tools with which to determine accuracy of a method of measure, compared with a gold standard technique.15 For all analyses, a value of P ≤ 0.01 was considered significant.

Results

Placement of the indwelling thermometry devices (ie, microchips and PA catheters) was achieved without complication in all dogs. No adverse reactions were detected; there was no evidence of swelling, redness, or signs of pain at the site of microchip insertion. Temperature readings were obtained in duplicate from all 8 dogs on 297 separate occasions (594 recorded temperature readings). Temperature readings obtained from the PA catheter (core temperature), rectal and auricular thermometers, and microchips from the lateral shoulder location were each recorded in duplicate on all 297 occasions. Because only 5 dogs had the thermistor microchip implanted in the interscapular region, temperature readings were recorded in duplicate on 180 occasions. Because 3 dogs had the thermistor microchip implanted in the subcutaneous tissues in the dorsal aspect of the sacral region, sacral temperature readings were recorded in duplicate on 117 occasions.

Core temperatures from 35.3°C (95.5°F) to 41.4°C (106.5°F) were recorded. As determined from PA catheter readings, hypothermia was accomplished in 7 of 8 dogs during study period 1; core body temperature readings were in the hypothermic range on 36 occasions. During all 4 study periods combined, temperatures were within reference limits on 110 occasions. As expected, administration of endotoxin to the dogs during periods 2, 3, and 4 resulted in increases in core body temperature with only mild to moderate systemic signs of illness. Core body temperature in the hyperthermic range was achieved on 151 occasions during the entire study. During study period 2, 6 dogs became hyperthermic (24 readings in the hyperthermic range); during periods 3 and 4, all 8 dogs became hyperthermic (61 and 66 readings in the hyperthermic range, respectively). All dogs became lethargic within an hour of endotoxin administration, and vomiting and diarrhea developed in several dogs. All dogs appeared to recover fully from endotoxin administration and returned to apparently normal activity within 4 hours.

Ease of use and rapidity of results differed with each device. Placement of the PA catheter required anesthesia and was an invasive procedure. The PA catheter provided continual temperature readings. Rectal thermometry was easily accomplished in the dogs, but during much of the study period, the dogs were either anesthetized or lethargic. Unlike equilibrium rectal thermometry, results were obtained rapidly via predictive rectal thermometry (a period of only 10 to 15 seconds was required to obtain each reading). Although the response time of the auricular thermometer was extremely rapid (approx 1 second), correct positioning of the probe required a period of some seconds and, frequently, an interval of ≥ 15 seconds elapsed before the thermometer provided a second reading. If the foldable arm of the auricular thermometer was not closed completely between temperature readings, the device would display an error message during attempts to obtain a second reading and required another 15-second interval for recalibration. This often resulted in a 30- to 45-second time lapse before a second auricular temperature reading could be obtained. It was our subjective impression that several dogs resented positioning of the auricular probe more than they resented positioning of the rectal probe. Although initial placement of the microchip required SC injection, actual temperature readings were completely noninvasive and required no restraint. Once the scanner device was passed over the site of the implant, the temperature reading was displayed instantaneously.

Variability between successive readings from the same thermometry device or device location at the same time period was assessed (Table 1). Variability was least for the thermistor-tipped PA catheter and greatest for the auricular thermometer. A formal test of the null hypothesis of equal variances across devices was done via the mixed-model approach. Initial results based on all observations indicated that the variance for the auricular thermometer was significantly greater than that of any other device (P < 0.001 for all paired comparisons). Examination of residuals revealed some extreme values for 1 dog at 1 site. When the analysis was rerun with those extremes excluded, the results were the same. The mixed-model analysis was done again with the auricular thermometer excluded. There was a significant (P < 0.001) device effect. Pairwise comparisons revealed that the PA device had a significantly smaller variance than all other devices (P < 0.001 for all comparisons). No other differences were significant at the P ≤ 0.01 level, suggesting that the reliabilities of the rectal thermometer and microchip device were similar. Both the rectal thermometer and microchip device were more reliable than auricular thermometry in these dogs.

Table 1—

Variability between duplicate temperature measurements obtained from 8 dogs by use of a thermistor-tipped PA catheter, a predictive rectal thermometer, an infrared auricular thermometer designed for veterinary use, and subcutaneous temperature-sensing microchips (in 3 body locations).

Device or locationMean difference between readingsSD of the differenceRange of differences
PA catheter0.0040.059−0.30–0.30
Rectal thermometer0.0090.235−0.90–0.80
Auricular thermometer0.3000.776−2.45–5.34
Microchip
 Interscapular region0.0160.189−0.50–0.80
 Lateral shoulder region0.0110.249−1.30–1.90
 Sacral region0.0180.236−0.50–0.90

Variability is measured by the SD of the differences, which was calculated from 297 duplicate measurements for the PA catheter, rectal thermometer, auricular thermometer, and microchip in the lateral shoulder region. Variability was calculated from 180 duplicate measurements from the microchip in the interscapular location and 117 duplicate measurements from the microchip in the sacral location. Variability was least for the PA catheter and greatest for the auricular thermometer; other temperature-measuring devices or device locations did not significantly differ with regard to variability.

Agreement between the core body temperature and temperatures determined by use of the rectal or auricular thermometers or thermistor microchip in any location was also assessed (Table 2). Of the 3 thermometry devices and microchip locations evaluated in the study dogs, the smallest difference between core body temperature and temperature readings was achieved by use of the rectal thermometer; 94.28% (280/297) of rectal temperature readings were within 0.5°C of the PA catheter measurement of core body temperature. Agreement between core body temperature and the temperature derived from the thermistor microchip placed in the interscapular location was similar to that between core body temperature and the temperature derived from the auricular thermometer; 50% (90/180) of temperature readings obtained from the interscapular microchip and 45.5% (135/297) of readings obtained by use of the auricular thermometer were within 0.5°C of the PA catheter measurement of core body temperature. Differences of approximately 3.5°C between core body temperature and some temperature readings from the microchip (in any location) and from the auricular thermometer were detected. The worst agreement between core body temperature and temperature determined by any other device or device location was associated with the thermistor microchip in the sacral location.

Table 2—

Agreement between temperature readings obtained from 8 dogs by use of rectal and auricular thermometers and thermistor microchips (in 3 body locations) with core body temperature (determined by use of a thermistor-tipped PA catheter). Agreement is presented both as percentage of all readings for each device or location that agreed within the given range of temperature intervals and, parenthetically, as the actual number of readings that agreed within the given range.

Range of agreement (°C)Rectal thermometer (n = 297)Auricular thermometer (n = 297)Thermistor microchip
Interscapular region (n = 180)Lateral shoulder region (n = 297)Sacral region (n = 117)
≤0.594.28 (280)45.45 (135)50.00 (90)38.38 (114)9.40 (11)
≤0.7598.32 (292)63.97 (190)57.78 (104)52.19 (155)12.82 (15)
≤1.099.66 (296)78.11 (232)65.00 (117)68.35 (203)27.35 (32)
≤1.25100.00 (297)87.21 (259)81.11 (146)76.43 (227)35.04 (41)
≤1.5100.00 (297)93.60 (278)88.33 (159)82.83 (246)48.72 (57)
≤1.75100.00 (297)95.96 (285)92.22 (166)85.52 (254)56.41 (66)
≤2.0100.00 (297)97.31 (289)96.11 (173)88.89 (264)64.10 (75)
≤2.25100.00 (297)97.64 (290)96.67 (174)91.25 (271)69.23 (81)
≤2.5100.00 (297)98.32 (292)97.78 (176)95.29 (283)79.49 (93)
≤2.75100.00 (297)98.32 (292)98.89 (178)96.97 (288)86.32 (101)
≤3.0100.00 (297)98.99 (294)98.89 (178)98.65 (293)88.89 (104)
≤3.25100.00 (297)99.33 (295)98.89 (178)98.99 (294)90.60 (106)
≤3.5100.00 (297)99.33 (295)100.00 (180)99.33 (295)97.44 (114)
Figure 1—
Figure 1—

Bland-Altman plots of the difference between temperature measured by a thermistor-tipped PA catheter and the temperature measured by a predictive rectal thermometer (A); an infrared auricular thermometer designed for veterinary use (B); and a subcutaneous temperature-sensing microchip in the interscapular region (C), lateral shoulder region (D), and sacral region (E) against the mean of the temperature as measured by the PA catheter and the temperature measured by that device or a device at that location in 8 dogs. The horizontal reference line corresponds to zero differences (ie, if the reading from the described device [or device at a specific location] matched the core temperature reading, then all points would fall on this line). Points located above the reference line represent an underestimation of temperature by the device in comparison to core body temperature (as measured by the PA catheter).

Citation: Journal of the American Veterinary Medical Association 230, 12; 10.2460/javma.230.12.1841

Agreement between core body temperature (assessed by use of the PA catheter) and the other thermometry devices or device locations was also examined graphically (Figure 1). Temperature assessments derived from the rectal thermometer were in best agreement with core body temperature. Auricular thermometer readings and interscapular microchip temperature readings also provided a reasonable assessment of core temperature in most dogs, but were less accurate than the rectal thermometer temperature readings. For all devices other than the rectal thermometer, temperature readings from the device generally underestimated core body temperature.

Differences between core temperature and interscapular microchip temperature were relatively consistent from time to time in each dog, but varied greatly among dogs. The mean absolute difference between core temperature and interscapular temperature was calculated for each dog. The readings that differed most greatly from the core temperature at hypothermic, euthermic, and hyperthermic temperatures were associated with 1 dog; each occasion in which the interscapular microchip temperature reading and core body temperature differed by 1.5°C (3.2°F) or more was attributed to data obtained from this single dog. The dog was not dissimilar to the other dogs with regard to coat length, body condition score, weight, and breed. When this dog was removed from consideration, agreement between interscapular microchip temperature readings and core body temperatures improved greatly (Table 3).

Table 3—

Comparison of agreement between temperature readings from the microchip in the interscapular location with core body temperature (determined by use of a thermistor-tipped PA catheter) with and without the inclusion of a single dog responsible for the greatest discrepancy in agreement. Agreement presented both as percentage of readings that agreed within the given range of temperature intervals and, parenthetically, as the actual number of readings that agreed within the given range.

Range of agreement (°C)Single dog (n = 5)All dogs excluded (n = 4)
≤0.550.0 (90/180)62.0 (90/146)
≤0.7557.8 (104/180)71.2 (104/146)
≤1.070.0 (117/180)85.0 (123/146)
≤1.2581.1 (146/180)94.5 (138/146)
≤1.588.3 (159/180)100.0 (146/146)

For the single dog with the greatest discrepancy between interscapular microchip temperature readings and core temperature readings, the interscapular microchip was removed. Radiographically, appropriate placement of the microchip in the subcutaneous tissues between the scapulae was confirmed. The microchip was palpable in the subcutaneous tissue, but the implantation site was not discolored, swollen, excessively warm, or in any other way grossly abnormal. The microchip itself was returned to the manufacturer for further evaluation and was reported to be undamaged and provided temperature readings in vitro that were within 0.3°C of ambient temperature.h Specimens of the tissue surrounding the microchip were examined histologically and findings were unremarkable. Microbial culture of tissue samples yielded no growth.

Discussion

In the present study, 3 clinical temperature-measurement devices were compared with core body temperature in dogs during periods of hyperthermia, hypothermia, and euthermia. Because high body temperature is a much more common clinical problem than low body temperature, greatest emphasis was given to evaluation of these thermometry devices during hyperthermia rather than hypothermia. Core body temperature represents the standard to which other measures of body temperature are compared, but measurement of core temperature is invasive and therefore not suitable for daily clinical application. Rectal temperature is assessed most often clinically as a substitute for core body temperature. Recently, digital rectal thermometers have largely supplanted the use of glass mercury thermometers. Digital thermometers are of either the equilibrium or predictive type. In our study, a predictive thermometer, in which the rate of temperature change during the first few seconds after thermometer placement is measured and used to mathematically predict the final temperature, was evaluated. We are aware of only 1 study6 involving comparison of core body temperature with temperature readings derived from predictive rectal thermometers in dogs, and that study was conducted only under conditions of hypothermia. Auricular thermometry was developed as a rapid and less invasive alternative to rectal thermometry for use in humans. Auricular thermometers use infrared technology to sense heat emanating from the tympanic membrane; because the tympanic membrane shares blood flow with the hypothalamus via the carotid artery, these temperature readings are purported to approximate core temperature. Because the anatomic features of humans and other animals differ, thermometry devices have been developed specifically for use in the ear canal of domestic pet animals. Although multiple studies1,2,16-19,i have been performed to evaluate auricular and rectal temperature measurements in several species, data regarding comparisons of auricular temperature readings with core body temperature are sparse.6 To our knowledge, comparison of temperature assessments by use of a subcutaneously placed temperature-sensing microchip with either rectal temperature readings or core body temperature in dogs has not been reported. This microchip device is already marketed for use in dogs in Europe and is being considered for market in the United States. Further assessment of reliability and accuracy of the device is warranted before its routine use in dogs can be advocated.

Reliability of a measuring device refers to the degree of stability among measurements when those measurements are repeated under identical conditions and does not refer to the dependability of the measuring device.20 In the present study, reliability of each device was evaluated as repeatability of device-derived temperature readings via calculation of the SD of the difference in duplicate measures and description of the range of differences for duplicate measures from each device. As expected, reliability was greatest for the thermistortipped PA catheter device that was used to assay core body temperature in the study dogs. Repeatability was markedly worse for the auricular thermometer than for any other temperature-measuring device. The positioning of the measuring device critically influences results of auricular thermometry, whereas appropriate positioning of a rectal thermometer is simple and easy to achieve; after initial placement, appropriate positioning of the thermistor-tipped PA catheter or microchip for temperature measurements is established for subsequent occasions. Therefore, user variability in positioning of the auricular thermometer might have contributed to the poor repeatability of temperature measurements for this device, compared with the others.19 However, to minimize such user variability, auricular temperature measurements were obtained by a single investigator.

Compared with core body temperature determined via the thermistor-tipped PA catheter, the accuracy of temperature readings obtained by use of the predictive rectal thermometer was highest among the thermometry devices. Rectal temperature change often lags behind changes in core body temperature in humans.11,21 Because the conditions applied to the dogs in the present study resulted in a rapidly changing core temperature, a similar lag effect may have biased the results and caused the rectal thermometer to appear less accurate than it actually is. Despite the lag effect, rectal thermometry provided an accurate estimation of core body temperature in the dogs of the present study. There are conditions in which this may not be the case. Fecal material could interfere with the juxtaposition of the temperature probe with the rectal mucosal wall, thereby affecting the temperature reading.5 Rectal inflammation might result in local increases in temperature, whereas thrombotic conditions that interfere with local blood flow could result in decreased local temperature, compared with core body temperature. In individuals with these conditions, an alternative means of temperature measurement would be required. As reported elsewhere,4,21 the rectal thermometer used in the present study was the only device that frequently (189/297 [63.6%] readings) gave results that were higher than the true core body temperature.

Previous studies1,2,6,16-19 of auricular thermometry in animals have revealed varied estimates of accuracy. Because of the anatomic differences in the ear canal of dogs and humans, infrared auricular thermometers made for use in people are likely to detect the temperature of the skin of the external ear canal rather than of the tympanic membrane when used in dogs, thereby underestimating core body temperature among canids. Auricular thermometers designed for use in veterinary patients, such as the one used in our study, supposedly provide more accurate results.19 Although auricular temperature has been found to correlate closely with rectal temperature in dogs with or without otitis externa,7 correlation does not imply accuracy. Other studies2,17 have failed to demonstrate accuracy of auricular as compared to rectal temperatures, and comparisons of auricular temperature with core body temperature in euthermic or hyperthermic dogs have not been conducted, to our knowledge. Similarly conflicting results regarding accuracy of auricular thermometry were obtained in studies1,16-19 of humans, cats, and other species, all of which leaves clinicians unsure as to the usefulness of the auricular technique. Typically, auricular temperature readings are lower than the core body temperatures.19 This could be related to improper device positioning, but might also be related to actual temperature differences at different sites within the body.19 Interestingly, auricular thermometry may be more accurate at hypothermic than hyperthermic temperatures in dogs, cats, and children.6,19,22 Although the number of readings obtained during each temperature condition prevented meaningful statistical analysis of each different temperature subset in the present study, our data would appear to be in agreement with that finding.

Instrumentation placed within the subcutaneous space for measurement of body temperature has been used in research settings for many years.23,24 The subcutaneously placed device evaluated in the present study is the only identification microchip combined with temperature-sensing technology that is currently marketed for purchase by owners of pet dogs in Europe. Although the product has been available in Europe since 2004, it is not yet marketed in the United States. To our knowledge, there are no published studies regarding either the reliability or accuracy of the device. Once the microchip is in place, the temperature measurement is obtained simply by moving an electronic scanning device over the area of microchip implantation. In the present study, microchip temperature readings were obtained rapidly and with no objection from any dog. When placed in the suggested interscapular location, the microchip provided a reasonable approximation of core temperature in most dogs but was not as accurate as rectal thermometry. Although agreement between core body temperature and temperature readings obtained from the thermistor microchip in the interscapular region was good for most dogs, temperatures recorded from the PA catheter and microchip were widely different in 1 dog. For this dog, there were no obvious differences in conformation or coat, compared with the other dogs, and no evidence of pathologic changes or infection within the tissue surrounding the microchip. The microchip itself was returned to the manufacturer for evaluation and was reportedly accurate in vitro to within 0.3°C. We do not have an explanation for why the microchip provided far less accurate results in this particular dog. Although not part of the present study, the authors placed an identical microchip in the inter-scapular location of another dog taking part in a different research study; in that dog, rectal temperature and microchip temperature readings were also widely discrepant, again without obvious cause.

The temperature-sensing microchip provided the most accurate results when placed in the recommended interscapular location. In studies23–25 involving other types of subcutaneous temperature transponders in humans, differences in temperature readings were associated with location of microchip implantation and amount of subcutaneous adipose tissue. There may be inherent differences in subcutaneous temperatures in different body locations. Likewise, there may be an inherent difference between the subcutaneous temperature and the core or rectal temperature. In the present study, environmental conditions, body condition, and hair coat, or possibly lag time between changes in core and subcutaneous temperatures, might account for some of the disagreement between subcutaneous temperature readings and core body temperature. All of our experiments were performed in the climate-controlled environment of a veterinary teaching hospital and in dogs of similar body condition and coat type.

During hypothermia, hyperthermia, and euthermia in the study dogs, temperature readings obtained from all thermometry devices in all locations followed the changes in core body temperature (determined via the thermistor-tipped PA catheter). However, predictive rectal thermometry provided the most accurate approximation of core temperature in dogs. Although rapidity of results is an advantage of auricular or microchip thermometry over equilibrium thermometry, use of predictive digital thermometers provides results nearly as quickly. We suggest that neither the thermistor microchip nor auricular thermometer used in the present study should be relied on as the sole means of temperature measurement in dogs when hypothermia or hyperthermia is suspected. For routine use in most dogs, predictive rectal thermometry is reliable and appears to provide the most accurate estimation of core body temperature of any of the devices evaluated.

ABBREVIATIONS

PA

Pulmonary artery

a.

Temperature-sensing microchips, Digital Angel Corp, Saint Paul, Minn.

b.

Welch Allyn predictive digital thermometer, Welch Allyn Inc, San Diego, Calif.

c.

Traceable digital thermometer, Fisher Scientific, Pittsburgh, Pa.

d.

Argon flow directed thermodilution catheter, Argon Medical, Athens, Tex.

e.

Vet-Temp model VT-110, Advanced Monitors Corp, San Diego, Calif.

f.

Purified LPS, Sigma, St Louis, Mo.

g.

SAS, version 9.1, SAS Institute Inc, Cary, NC.

h.

Skoog D, Digital Angel Corp, Saint Paul, Minn: Personal communication, 2005.

i.

Sharp PE, Sanow CT, Oteham CP, et al. Use of the Thermoscan® thermometer in determining body temperature in laboratory rabbits, rodents, dogs, and cats (abstr). Contemp Top Lab Anim Sci 1993;32:28.

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Contributor Notes

Supported by Schering-Plough Animal Health Corporation.

The authors thank Drs. Emily Southward, Michael Karagiannis, and Efrat Kelmer for technical assistance and Dr. Richard Madsen for statistical assistance.

Address correspondence to Dr. Cohn.