Measurement Of Core Body Temperature Research Articles

A comparison of technologies used for estimation of body temperature

Body temperature measurement is an important clinical parameter. The performance of a number of non-invasive thermometers was measured by comparing intra- and inter-operator variability (n = 100) and clinical accuracy (n = 61). Variability was elevated in febrile compared to normothermic subjects for axillary and oral electronic contact thermometer measures and a temporal artery thermometer (p < 0.001 for both). Temporal artery thermometry and one mode of an infrared tympanic thermometer demonstrated significant clinical inaccuracy (p < 0.001 for both). Electronic contact thermometer repeatability and reproducibility are highly variable in febrile adults both in the axilla and oral cavity. Infrared thermometry of the skin over the superficial temporal artery is unreliable for measuring core body temperature, particularly in febrile subjects and patients in theatre. The infrared tympanic thermometers tested are acceptable for clinical practice; however, care should be exercised with the different modes of operation offered.

Technique for Continuously Monitoring Core Body Temperatures to Prevent Heat Stress Disorders in Workers Engaged in Physical Labor

Measuring core body temperature is crucial for preventing heat stress disorders in workers. We developed a method for measuring auditory canal temperatures based on a thermocouple inserted into a sponge-type earplug. We verified that the tip of this thermocouple is positioned safely, allowing the wearer to engage in normal physical tasks; this position averaged 6.6 mm from the tympanic membrane. To assess this technique, we had six healthy male students repeat three cycles of exercise and rest (20 min of exercise and 15 min of rest) in a temperature-controlled chamber with temperatures set at 25, 30, or 35 degrees C, while monitoring the auditory canal, esophageal, rectal, and skin temperatures. We observed differences of a mere 0.30-0.45 degrees C between rectal temperatures and auditory canal temperatures measured with the thermocouple, the smallest such difference reported to date in studies involving auditory canal temperature measurement. We conclude that monitoring temperatures based on a technique involving an auditory canal plug can be used to estimate rectal temperatures accurately, and thereby to avoid conditions leading to heat stress disorders.

Reusable biotelemetric capsules: A convenient and reliable method for measuring core body temperature in large mammals during gut passage

It is still not fully understood how megaherbivores regulate their body temperature ( T b), particularly with respect to their unfavourable surface to volume ratio. The paucity of information is probably owing to the difficulty obtaining physiological parameters from such animals. We developed a precise and reliable non-invasive method for determining the T b of large-bodied mammals. We used this method on African and Asian elephants. Small capsules (30 g) containing a temperature-sensitive transmitter and a memory for onboard data storage were hand-fed 71 times to elephants ( N=21) and T b was measured during gut passage. In 64 cases, sensors were successfully retrieved. The operation and reliability of our data loggers was sufficient and compared favourably with any other published method.

The Effect of Physical Exertion in Chemical and Biological Personal Protective Equipment on Physiological Function and Reaction Time

Objectives. The primary objective of this study was to describe and compare changes in heart rate, venous pH, venous partial pressure of carbon dioxide (pCO2), venous bicarbonate level, lactate level, oxygen saturation (SpO2), and tympanic membrane (TM) temperature occurring in a group of healthy volunteers during 20 minutes of physical exertion, both with and without chemical and biological personal protective equipment (PPE). A further aim was to establish whether any significant prolongation of reaction time occurred after physical exertion in chemical and biological PPE, compared to baseline values without the protective equipment. Methods. Nineteen highly fit volunteers were subjected to a baseline reaction time test and measurement of physiological variables. They were then subjected to physical exertion on a treadmill: once while wearing a short-sleeved t-shirt, shorts, and running shoes and once while wearing chemical and biological PPE. Repeat measurements of the physiological variables were made after 10 and 20 minutes of physical exertion in both groups, after which repeat reaction time tests were conducted. Results. Results showed that physical exertion of 20 minutes undertaken by highly fit volunteers wearing PPE resulted in a higher heart rate response and TM temperature compared with control measurements. Decreased venous pH and increased venous pCO2 were also observed during exertion in the PPE group. Although differences in these variables between the control and PPE groups were statistically significant, they were not of clinical relevance in the sample of volunteers studied. No significant difference in reaction time before and after exertion in PPE was identified. Conclusions. This study did not identify any effect of 20 minutes of heavy exercise in highly fit volunteers wearing level C chemical and biological PPE on reaction time. Heart rate response and TM temperature were higher during exertion in PPE. These differences, along with other physiological alterations observed, were not of clinical relevance. Further studies using arterial blood gas analysis and a more accurate measure of core body temperature are needed to better assess the physiological effect of this level and duration of exercise on subjects wearing similar PPE. Other aspects of cognition also require investigation under these conditions, in order to assess their effect on patient and rescuer safety.

Glass Thermometer Injuries

Glass mercury thermometers were once used as the criterion standard for measuring core body temperature. Once broken, however, there is the dual hazard of broken glass and exposure to mercury. Previous studies have focused only on the mercury hazard rather than injuries related to thermometer glass. To identify injury patterns from glass thermometers, including glass-related injury and mercury exposure. We performed a retrospective cohort study of glass thermometer-related injuries in children evaluated in a pediatric emergency department between October 1995 and October 2007. Case identification was performed using a computer-assisted screening tool followed by a manual chart review. Collected data included age, sex, injury characteristics, physical examination findings, radiologic imaging, interventions, and disposition. To analyze injury rates during these years, we used a multiplicative Poisson model allowing for varying exposures. Thirty-three patients were identified among 627,592 who presented to the emergency department during the study period. Approximately 1 to 6 patients presented every year, including 3 patients in 2007. Decline in injury incidence is less than 9% per year (P = 0.041). Median patient age was 2.4 years (interquartile range, 0.4-3.8 years); 12 patients (36%) were female. Twenty-two patients (66%) underwent radiologic imaging to identify potential foreign body (21 underwent radiography and 1 underwent computed tomographic scanning). There were 15 mouth injuries (45%), 13 anal injuries (39%), and 1 ear injury. Glass was identified by imaging in 5 patients (15%) and by sigmoidoscopic evaluation in 1 patient (3%). Mercury exposure was identified in 14 patients (42%). Persistent use of glass mercury thermometers has resulted in pediatric injury especially in children younger than 4 years. We reported the different mechanisms of injury with the hope of eliminating its use and reinforcing the use of alternative thermometers.

Sleep-related sweating in obstructive sleep apnoea: association with sleep stages and blood pressure

The aim of this study was to investigate sleep-related sweating as a symptom of obstructive sleep apnoea (OSA). Fifteen otherwise healthy male non-smoking patients with untreated moderate-to-severe OSA underwent polysomnography, including measurements of skin and core body temperature and electrodermal activity (EDA) as an objective indicator of sweating. Evening and morning blood pressure was measured as well as catecholamines in nocturnal urine. All measurements were repeated after 3 months on successful continuous positive airway pressure (CPAP) treatment. The untreated OSA subjects had a mean (+/-SD) apnoea-hypopnoea index of 45.3 +/- 3.9 and a mean EDA index during sleep of 131.9 +/- 22.4 events per hour. Patients with higher EDA indices had higher systolic blood pressure in the evening and morning (P = 0.001 and 0.006) and lower rapid eye movement (REM) sleep percentage (P = 0.003). The EDA index decreased significantly to 78.5 +/- 17.7 in the patients on CPAP treatment (P = 0.04). The decrease correlated with lower evening systolic and diastolic blood pressure (P = 0.05 and 0.006) and an increase in REM% (P = 0.02). No relationship was observed between EDA and skin or core body temperature, or to catecholamine levels in urine. OSA patients who experience sleep-related sweating may have increased blood pressure and decreased REM sleep compared with other OSA patients. CPAP treatment appears to lower blood pressure and increase REM sleep to a higher extent in these patients compared with other OSA patients.

Antinociceptive activity of Paederosidic Acid Methyl Ester (PAME) from the n-butanol fraction of Paederia scandens in mice

Antinociceptive activity of Paederosidic Acid Methyl Ester (PAME), a chemical compound isolated from the n-butanol fraction of Paederia scandens, was evaluated in mice using chemical and thermal models of nociception. PAME given by intraperitoneal injection at doses of 20, 40 and 60 mg/kg produced significant inhibitions on chemical nociception induced by intraperitoneal acetic acid, subplantar formalin or capsaicin injections and on thermal nociception in the tail-flick test and the hot plate test. In the pentobarbital sodium-induced sleep time test and the open-field test, PAME neither significantly enhanced the pentobarbital sodium-induced sleep time nor impaired the motor performance, indicating that the observed antinociceptive activity of PAME was unlikely due to sedation or motor abnormality. Core body temperature measurement showed that PAME did not affect temperature within a 2-h period. Moreover, PAME-induced antinociception in the hot plate test was insensitive to naloxone or nimodipine but significantly antagonized by L-NAME (N (G)-nitro- l-arginine methyl ester), methylene blue and glibenclamide. These results suggested that PAME-produced antinociception was possibly related to the pathway of NO-cGMP-ATP sensitive K + channels, which merited further studies regarding the precise site and mechanisms of action.

The Double Sensor—A non-invasive device to continuously monitor core temperature in humans on earth and in space

The objective of our study was to establish whether rectal temperature recordings in humans could be replaced by a non-invasive skin temperature sensor combined with a heat flux sensor (Double Sensor) located at the forehead to monitor core body temperature changes due to circadian rhythms. Rectal and Double Sensor data were collected continuously for 24h in seven men undertaking strict head-down tilt bed-rest. Individual differences between the two techniques varied between -0.72 and +0.55 degrees C. Nonetheless, when temperature data were approximated by cosinor analysis in order to compare circadian rhythm profiles between methods, it was observed that there were no significant differences between mesor, amplitude, and acrophase (P>0.310). It was therefore concluded that the Double Sensor technology is presently not accurate enough for performing single individual core body temperature measurements under resting conditions at normal ambient room temperature. Yet, it seems to be a valid, non-invasive alternative for monitoring circadian rhythm profiles.

Influence of Sensor Ingestion Timing on Consistency of Temperature Measures

The validity and the reliability of using intestinal temperature (T int) via ingestible temperature sensors (ITS) to measure core body temperature have been demonstrated. However, the effect of elapsed time between ITS ingestion and T int measurement has not been thoroughly studied. Eight volunteers (six men and two women) swallowed ITS 5 h (ITS-5) and 29 h (ITS-29) before 4 h of varying intensity activity. T int was measured simultaneously from both ITS, and T int differences between the ITS-5 and the ITS-29 over the 4 h of activity were plotted and compared relative to a meaningful threshold of acceptance (+/-0.25 degrees C). The percentage of time in which the differences between paired ITS (ITS-5 vs ITS-29) were greater than or less than the threshold of acceptance was calculated. T int values showed no systematic bias, were normally distributed, and ranged from 36.94 degrees C to 39.24 degrees C. The maximum T int difference between paired ITS was 0.83 degrees C with a minimum difference of 0.00 degrees C. The typical magnitude of the differences (SE of the estimate) was 0.24 degrees C, and these differences were uniform across the entire range of observed temperatures. Paired T int measures fell outside of the threshold of acceptance 43.8% of the time during the 4 h of activity. The differences between ITS-5 and ITS-29 were larger than the threshold of acceptance during a substantial portion of the observed 4-h activity period. Ingesting an ITS more than 5 h before activity will not completely eliminate confounding factors but may improve accuracy and consistency of core body temperature.

Characterization of the sickness response in young and aging rats following E. coli infection

To more fully characterize the sickness response in young (3 mo) and older (24 mo) rats, we measured core body temperature (CBT), activity level, and body weight changes for 7 days following a peripheral immune challenge with Escherichia coli. CBT increases were delayed and blunted during the 12 h following infection in older rats. Indeed, in aging subjects the initial response was hypothermia, but this was followed by a significant and prolonged elevation in CBT lasting 3 days. Young rats, in contrast, generated a rapid and robust CBT elevation lasting just over a day. Activity level was significantly reduced only on the day of E. coli administration in both young and older rats. Body weight loss was equivalent in both age groups one day after E. coli administration, although there was a trend for older rats to continue losing more weight across the next 6 days than in young rats. This is the first study to examine CBTs in young and older rats for a protracted amount of time, thereby revealing that aging rats do have an exaggerated, albeit delayed, fever which is in keeping with other exaggerated sickness behavioral responses observed in aging rodents.

Thoracic Epidural Analgesia with Low Concentration of Bupivacaine Induces Thoracic and Lumbar Sympathetic Block

Clinical benefits of thoracic epidural anesthesia (TEA) are partly ascribed to thoracic sympathetic block. However, data regarding sympathetic activity during TEA are scarce and contradictory. This prospective, randomized, double-blind study evaluated the segmental propagation of sympathetic block after low-concentration, high-volume TEA using digital thermography. Twenty-four patients were included in the study. Thoracic epidural catheters were placed at a median insertion level of T8-T9. Patients were accommodated for 20 min to the room temperature of 23 degrees +/- 0.3 degrees C. Skin temperature was recorded by digital thermography. After baseline measurement of heart rate, arterial pressure, and core body and skin temperature, 10 ml saline (control group) or 10 ml bupivacaine, 0.25% (TEA group), respectively, was administered epidurally. Five minutes (t5) and 20 min (t20) after baseline measurements, hemodynamic parameters and core body temperature were again measured, and sensory block was identified by loss of cold-warm discrimination. In the thumb, the toe, and each thoracic dermatome, difference from baseline temperature was calculated at t5 and t20. Data were analyzed by Mann-Whitney U test. Baseline characteristics did not differ among groups. Median spread of sensory block at t20 was T5-L5. At both t5 and t20, skin temperature decreased more in the control group than in the TEA group in all thoracic dermatomes (P < 0.05). Toe temperature increased in the TEA group compared with the control group (P < 0.05), whereas thumb temperature remained unchanged. TEA with 10 ml bupivacaine, 0.25%, induced thoracic and lumbar sympathetic block that precedes and exceeds sensory block. Caudal limit of sympathetic block could not be demonstrated in this study.

Body temperature, thermoregulatory behaviour and pelt characteristics of three colour morphs of springbok ( Antidorcas marsupialis)

Using intra-abdominal miniature data loggers, we measured core body temperature in female springbok ( Antidorcas marsupialis) of three colour morphs (black, normal and white), free-living in the Karoo, South Africa, for one year. During winter, white springbok displayed lower daily minimum body temperatures (37.4 ± 0.5 °C), than both black (38.1 ± 0.3 °C) and normal (38.0 ± 0.6 °C) springbok. During spring, black springbok displayed higher daily maximum body temperatures (40.7 ± 0.1 °C) than both white (40.2 ± 0.2 °C) and normal (40.2 ± 0.2 °C) springbok. These high maximum body temperatures were associated with larger daily amplitudes of nychthemeral rhythm of body temperature (2.0 ± 0.2 °C), than that of white (1.6 ± 0.1 °C) and normal (1.7 ± 0.2 °C) springbok. Biophysical properties of sample springbok pelts were consistent with these patterns, as the black springbok pelt showed lower reflectance in the visible spectral range, and higher heat load from simulated solar radiation, than did the pelts of the other two springbok. Black springbok had lower diurnal activity in winter, consistent with them having to forage less because their metabolic cost of homeothermy was lower, but were disadvantaged in hot periods. White springbok, by contrast, were more protected from solar heat load, but potentially less able to meet the energy cost of homeothermy in winter. Thus energy considerations may underlie the rarity of the springbok colour morphs.

Calibration of an ingestible temperature sensor

An ingestible telemetric sensor for measuring core body temperature is increasingly being utilized in occupational and athletic studies of heat strain. There is a need for a uniform method of calibrating these sensors in the scientific community in order to effectively compare the results of different researchers. The purpose of the present investigation was to determine and present such a calibration procedure. Sensors were placed in a water bath heated to nine discrete temperatures, and the recorded values were compared to that of a traceable thermometer. It was observed that sensor 2 recorded temperatures higher than sensors 1 and 3, and that all sensors were higher than the traceable thermometer, highlighting the need for a calibration procedure. The findings of this study suggest a number of recommendations for a calibration procedure including: (1) four water bath temperatures in the range of 33–41 °C should be utilized; (2) sensors should be immersed for a minimum of 4 min prior to taking a measurement; (3) a linear regression relating sensor temperature to a traceable thermometer is an appropriate method to adjust raw data. Switching the sensor off after calibration and reactivating it prior to ingestion will not influence the accuracy of temperature measurement.

A practical approach to circadian rhythm sleep disorders

Circadian rhythm sleep disorders are common in clinical practice. The disorders covered in this review are delayed sleep phase disorder, advanced sleep phase disorder, free-running, irregular sleep-wake rhythm, jet lag disorder and shift work disorder. Bright light treatment and exogenous melatonin administration are considered to be the treatments of choice for these circadian rhythm sleep disorders. Circadian phase needs to be estimated in order to time the treatments appropriately. Inappropriately timed bright light and melatonin will likely worsen the condition. Measurements of core body temperature or endogenous melatonin rhythms will objectively assess circadian phase; however, such measurements are seldom or never used in a busy clinical practice. This review will focus on how to estimate circadian phase based on a careful patient history. Based on such estimations of circadian phase, we will recommend appropriate timing of bright light and/or melatonin in the different circadian rhythm sleep disorders. We hope this practical approach and simple recommendations will stimulate clinicians to treat patients with circadian rhythm sleep disorders.

Human Tau in an Ultradian Light-Dark Cycle

Multiple experimental approaches have been used to assess the free-running period, or “tau,” of humans. One approach has been to study circadian rhythms in free-running blind humans. For example, Sack et al. (1992) measured the plasma melatonin rhythm in 11 free-running blind people and reported an average tau of 24.55 h.

The effect of changes in core body temperature on the QT interval in beagle dogs: a previously ignored phenomenon, with a method for correction

Background and purpose:Body core temperature (Tc) changes affect the QT interval, but correction for this has not been systematically investigated. It may be important to correct QT intervals for drug-induced changes in Tc.Experimental approach:Anaesthetized beagle dogs were artificially cooled (34.2 °C) or warmed (42.1 °C). The relationship between corrected QT intervals (QTcV; QT interval corrected according to the Van de Water formula) and Tc was analysed. This relationship was also examined in conscious dogs where Tc was increased by exercise.Key results:When QTcV intervals were plotted against changes in Tc, linear correlations were observed in all individual dogs. The slopes did not significantly differ between cooling (−14.85±2.08) or heating (−13.12±3.46) protocols. We propose a correction formula to compensate for the influence of Tc changes and standardize the QTcV duration to 37.5 °C: QTcVcT (QTcV corrected for changes in core temperature)=QTcV–14 (37.5 – Tc). Furthermore, cooled dogs were re-warmed (from 34.2 to 40.0 °C) and marked QTcV shortening (−29%) was induced. After Tc correction, using the above formula, this decrease was abolished. In these re-warmed dogs, we observed significant increases in T-wave amplitude and in serum [K+] levels. No arrhythmias or increase in pro-arrhythmic biomarkers were observed. In exercising dogs, the above formula completely compensated QTcV for the temperature increase.Conclusions and implications:This study shows the importance of correcting QTcV intervals for changes in Tc, to avoid misleading interpretations of apparent QTcV interval changes. We recommend that all ICH S7A, conscious animal safety studies should routinely measure core body temperature and correct QTcV appropriately, if body temperature and heart rate changes are observed.

The Effect of Cool Water Ingestion on Gastrointestinal Pill Temperature

Telemetric gastrointestinal (GI) temperature pills are now commonly used to measure core body temperature and could minimize the risk of heat illness while maximizing operational effectiveness in workers subject to high levels of thermal strain. To quantify the effect of repeated cool water ingestion on the accuracy of GI pill temperature. Ten operational firefighters ingested a pill to measure GI temperature (T1int) before overnight sleep. Two hours following breakfast and 11.5 h after ingesting T1int, the firefighters ingested a second pill (T2int) before performing 8.5 h of intermittent activity (repetitive cycles of 30 min of seated rest followed by 30 min of general firefighter duties). During the first 2 min of each 30-min rest period, the firefighters consumed 250 mL of chilled water (5-8 degrees C). Water ingestion had a highly variable effect both within and between subjects in transiently (32 +/- 10 min) reducing the temperature of T2int in comparison with T1int. In general, this transient reduction in T2int became progressively smaller as time following ingestion increased. In some firefighters, the difference between T1int and T2int became negligible (+/- 0.1 degrees C) after 3 h, whereas in two others, large differences (peaking at 2.0 degrees C and 6.3 degrees C) were still observed when water was consumed 8 h after pill ingestion. These results show that a GI pill ingested immediately prior to physical activity cannot be used to measure core body temperature accurately in all individuals during the following 8 h when cool fluids are regularly ingested. This makes GI temperature measurement unsuitable for workers who respond to emergency deployments when regular fluid consumption is recommended operational practice.

Full recovery of an avalanche victim with profound hypothermia and prolonged cardiac arrest treated by extracorporeal re-warming

Survival of hypothermic avalanche victims with cardiac arrest is rare. This report describes full recovery of a 29-year-old backcountry skier completely buried for 100 min at 3.0m (9.8 ft) depth. On extrication he was unconscious, but breathing spontaneously into an air pocket; core body temperature measured 22.0 degrees C (71.6 degrees F). He was intubated and ventilated on site. Ventricular fibrillation commenced during helicopter transportation, whereby chest compression was lacking for 15 min. At the nearest hospital continuous cardiopulmonary resuscitation was initiated, but defibrillation failed. Tympanic core body temperature measurement confirmed life-threatening hypothermia of 21.7 degrees C (71.1 degrees F) and serum K(+) was 4.3 mmol/l, necessitating transferral to a hospital with cardiopulmonary bypass facilities. Defibrillation finally succeeded following re-warming, by femoral veno-arterial bypass, to 34.5 degrees C (94.1 degrees F). Total duration of cardiac arrest was 150 min. The patient developed pulmonary oedema, treated by extracorporeal membrane oxygenation, but progressed well and was discharged from hospital on day 17, fit to resume professional and social activities. Follow-up cerebral magnetic resonance imaging 2 years after avalanche burial demonstrated only minimal changes attributable to unrelated, prior cranial trauma. Extensive neurological and psychological investigations gave excellent results. This report confirms previous literature that an air pocket with patent airways is essential for survival of a completely buried avalanche victim after 35 min and endorses the recommended management strategies of the International Commission for Mountain Emergency Medicine ICAR MEDCOM. In particular, all hypothermic victims extricated with an air pocket and free airways must be treated optimistically, even despite prolonged cardiac arrest. This remarkable case documents the fastest drop in core temperature ever recorded during snow burial, namely 9.0 degrees C (16.2 degrees F)/h, and the second-lowest reversible core temperature in avalanche literature.

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

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. Prospective study. 8 purpose-bred dogs. 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. 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. Among the 3 methods of temperature measurement, rectal thermometry provided the most accurate estimation of core body temperature in dogs.

Relationship of Thermal Status to Productivity in Heat-Stressed Dairy Cows Given Recombinant Bovine Somatotropin

The responses of lactating Holstein cows to daily administration of bovine somatotropin (bST) were measured at thermoneutrality (Tn) and under both constant and cycled heat-stress conditions to determine the relationship between thermal status and bST-induced shifts in milk production. All tests included a 5-d acclimation period at Tn (18°C), followed by a 2-d increase in ambient temperature to 28.5°C. After d 3, ambient temperature was cycled between 28.5 (day) and 25.5°C (night) for 4 d. Daily injections with either 31mg of bST or saline began on d 1 of the experiment. Milk production, feed intake, and respiratory rate (RR) were measured daily. Intraperitoneal, telemetric temperature transmitters were used for a continuous measure of core body temperature (Tcore). Blood samples were collected during each phase to evaluate the changes in serum chemistry in response to bST and heat stress. Following a 15-d recovery, cows were switched across injection treatments and the study was repeated. Milk production decreased by ∼18.4% below the initial yield at Tn by the end of 7 d of heat challenge. Although a reduction in milk production occurred during heat stress in both groups, milk production was higher in bST-treated cows compared with control cows during periods of constant and cyclic heat. Likewise, bST treatment during the entire period increased the milk-to-feed ratio over the control level by ∼11.3%. Plasma insulin-like growth factor 1 and serum nonesterified fatty acids accompanied the increased growth hormone level with bST treatment (∼122.0 and 88.8%, respectively), whereas plasma urea nitrogen was reduced by ∼13.3% to reflect the shift to lipid metabolism. There was no difference in Tcore of the treatment and control groups at Tn. Both bST and control cows increased RR and Tcore above the Tn level by ∼94.8 and 2.9%, respectively, during constant heat, with a greater increase in Tcore of bST-treated compared with control cows (∼0.6%). The increase in RR during heat stress preceded Tcore by 1 d for both groups. During cyclic heat, Tcore decreased by ∼0.4% compared with constant heat in both the control and bST-treated groups. Bovine somatotropin treatment increased milk production similarly during the Tn and heat-stress periods, ∼8.3% over the control; however, the bST-induced increase in milk-to-feed ratio was greatest during the continuous and cyclic heat-stress phases, ∼16.2%. This increase occurred together with the elevation in Tcore.