Research progress in core body temperature measurement during target temperature

Guiying LIU, Yanyan ZHANG, Ling TANG

1Department of Neurosurgery, Dongfang Hospital of Beijing University of Chinese Medicine, Beijing, China; 2Department of Neurosurgery, Dongfang Hospital of Beijing University of Chinese Medicine, Beijing, China; 3Department of Nursing,Dongfang Hospital of Beijing University of Chinese Medicine,Beijing, China

ABSTRACT

Core body temperature (CBT) is increasingly attracting attention as crucial data during target temperature management (TTM).Accurate and continuous measurement of human CBT can effectively identify and monitor central high fever, and provide a basis for the effective implementation during TTM, which is therefore of great significance for human health care and disease monitoring.The reliable core measurement sites are nasopharynx, esophagus, bladder, rectum, pulmonary artery, etc., but the measurement methods in these sites are all invasive.At present, the medical field is more inclined to noninvasive data collection methods through monitoring an appropriate site (such as forehead, mouth, or axilla)depending on clinical circumstances, so as to ensure the comfort and security of patients to the greatest extent.This review will provide reference choosing more safe and accurate temperature measurement methods for patients during TTM by reviewing the sites and accuracy of invasive and noninvasive CBT measurements.

Keywords:Core body temperature, neurocritical care, target temperature management

INTRODUCTION

Core body temperature (CBT) is increasingly attracting attention as crucial data during target temperature management (TTM).CBT is the best parameter for expressing the heat load of the body and is often considered a determining factor in the diagnosis and treatment of infection.[1,2]Accurate and continuous measurement of CBT by some devices are of great significance for human health care and disease monitoring.[3]

Through the TTM which can be used to achieve low body temperature or maintain normal body temperature, the patient’s CBT is intended to be strictly controlled within a predetermined range (32°C-36°C).[4,5]TTM reduces brain damage by lowering brain temperature, decreasing cerebral oxygen and glucose consumption and reducing the need for adenosine-triphosphate, aiming to improve outcomes in a variety of clinical situations including cardiac arrest, traumatic brain injury, stroke, and myocardial infarction.[6]The best TTM requires direct monitoring of brain temperature which is the temperature of the blood perfusing the hypothalamus;however, brain temperature measurement techniques are invasive and impractical in most cases.Guidelines from a French expert panel recommended indirect temperature measurement adopted during TTM--to measure CBT to represent brain temperature.[7]The potential measurement sites are the pulmonary artery, esophagus, and bladder, etc.[8,9]This review will provide reference choosing more safe and accurate temperature measurement methods for patients during TTM by reviewing the sites and accuracy of invasive and noninvasive body temperature monitoring methods.

SIGNIFICANCE OF CORE BODY TEMPERATURE DURING TARGET TEMPERATURE MANAGEMENT

TTM is recommended by the American Heart Association and the International Liaison Committee on Resuscitation as a class 1 intervention for postarrest neuroprotection in patients remaining unresponsive after cardiac arrest.[10]The TTM standard process includes three periods, low-temperature induction phase, low-temperature maintenance phase, and rewarming period.[11]During the induction phase, CBT is rapidly lowered to the target temperature which is maintained at least 24-48 h during the maintenance phase and finally was recovered at a rate of ≤0.25°C/h during the rewarming phase.As CBT decreases, these mechanisms for regulating brain blood flow and oxygen metabolism will provide a certain degree of neuro-protection and regulation of brain edema and intracranial pressure (ICP),[12]but the risk of side effects such as hypokalemia, infection, arrhythmias may also increase.The rewarming period emphasizes the importance of slow rewarming since rapid rewarming increases the possibility of vasodilation, hypotension, activation of the systemic inflammatory response, and inelastic brain edema following elevated ICP.[13]Therefore, precisely monitoring CBT during rewarming phase is required.[6]

LOCATION OF CORE BODY TEMPERATURE MONITORING

CBT refers to internal body temperatures, generally the temperatures of the thoracic cavity, abdominal cavity and central nerve of the human body, set by hypothalamic thermoregulatory center, usually maintained within 36°C-38°C, with an average of about 37°C, up and down 1°C-1.3°C.[14]High fever can increase the metabolic needs of the brain, aggravate the ischemic injury, cause cerebral vasodilation, ultimately aggravate ICP, increase mortality or lead to poor prognosis,[15]and therefore should be prevented or treated.Clinically, continuous CBT monitoring during TTM is usually used in the perioperative period and intensive care units.The bladder, rectum and esophagus, and pulmonary arteries are relatively reliable core measurement sites, but there is no definite agreement on the best measurement site.[16]According to the recent guidelines of the Neurocritical Care Society, temperature measurements in the esophagus or bladder are recommended during TTM.[17]

TIME OF THE CORE BODY TEMPERATURE MONITORING

Ideally, all patients receiving TTM treatment should undergo continuous CBT measurements which should be considered as the primary source of temperature monitoring for the patients.Studies suggest that patients with intracerebral hemorrhage, subarachnoid hemorrhage, or acute ischemic stroke should have continuous or at least hourly CBT measured, and achieve continuous or near-continuous monitoring regardless of the location selected.[18-20]However,the specific clinical situation is that continuous monitoring is sometimes interrupted, in which case the measurement is recommended at least once an hour.

CORE BODY TEMPERATURE MEASUREMENT METHODS

Invasive measurement methods

CBT monitoring sites include the nasopharynx, tympanic membrane, hypogloeeis, temporal muscle, bladder,rectum, esophagus, pulmonary artery, or jugular vein.[21,22]Among them, four monitoring sites are considered to be the most preferable sites for CBT in clinical practice,including the tympanic membrane, nasopharynx, esophagus,and pulmonary artery.[7,23]However, they have different characteristics of the correlation with brain temperature,the convenience of operation, and accuracy of measurement.Depending on the method used, the obtained values may vary and differ from the actual CBT.[24]

Pulmonary artery catheter measurement is considered to be the golden standard for CBT monitoring.It is a useful tool for hemodynamic monitoring in high-risk patients during surgery and in intensive care.However, pulmonary artery temperature measurement is too invasive, risky, and expensive for most patients,[25,26]meanwhile, it can lead to some complications such as pneumothorax, cardiac arrhythmias, pulmonary infarction, and infection,[24]so with the development of modern technology, its routine use has been decreased.

Nasopharyngeal temperature measurement is considered a reliable method of CBT monitoring, and the insertion of the probe into the nasopharynx is quite simple and safe.Therefore, the nasopharynx is a frequently-used temperature measurement site during surgical procedures.However, the method is greatly affected by room temperature as well as exhalation and inhalation process; at the same time, too long continuous monitoring and stimulation to patient’s nasal mucosa using temperature measurement probe can cause discomfort to the patient, even bleeding, which is therefore not suitable for patients with consciousness or head injury.[27]Clinically, the nasal temperature is also rarely used to represent patient’s brain temperature.

Esophageal temperature probe usually requires to intubate patients, and the best insertion site is 32-38 cm away the distal esophagus which is under the tracheal bifurcation and closest to the left atrium.The probe placement method of esophageal temperature measurement is complex,so the success of the catheterzation also requires X-line examination.Esophageal temperature is closely related to brain temperature at low-temperature induction phase during TTM.

The rectum is unaffected by room temperature and has rich blood supply from the hemorrhoid artery, so the rectal temperature measurement is often considered the standard for clinical temperature measurement, but it has its limitation--slow response in the dynamic state mainly due to buffer effect and thermal diffusion effect of rectal tissue and feces.[28]

The tympanic vascular system shares with the brain to regulate the thermal balance between the two parts,[29]which indicates that the tympanic temperature has the potential to reflect brain temperature.[30]The tympanic membrane temperature measurement is particularly suitable for prehospital and emergency use because of its operability,minimally invasive, and quick response characteristics, but less used in patients with acute neurological disorders.Although the tympanum is the only anatomy close to the brain and is the most noninvasive method commonly used for brain temperature estimation,[31]the accuracy of the measurement remains to be determined, because of measurement error,measurement equipment, and/or real temperature difference between ears.[32]

Bladder temperature measurement is the method of continuous monitoring using catheter with temperature wire, with small trauma and wide range of applications.The thermometric catheter is disposable to reduce cross-infection and is fixed with sac fixation, not easy to slip.[33]However,because the bladder temperature measurement is invasive operation, it should be done in aseptic operation during nursing work, and the catheter should be maintained and replaced regularly to prevent the occurrence of catheter-associated urinary tract infections.[34]

In short, temperature measurement in the pulmonary artery,nasopharynx, rectum, tympanic membrane, esophagus, and bladder are all invasive methods, so their measurement accuracy depends on the accuracy of the temperature sensor,with the error of 0.2°C-0.3°C.[35]Due to the disadvantage of invasiveness, those methods require patients to be in the state of anesthesia and to remain still for a long time but are unable to measure body temperature in the moving state.Those methods are also not applicable for sober patients as they cause discomfort to the patients.In addition, disinfection must be done for the thermometer probe to prevent cross-infection.

Noninvasive monitoring methods

CBT is increasingly valued as vital data during TTM.The ideal core temperature monitoring system should be noninvasive, continuous, accurate (even in conditions where large and rapid temperature changes occur), regardless of the technique used and the operator, and easy to use.[36,37]Under the premise of current measurement equipment and techniques, it is recommended that noninvasive temperature measurements on the forehead, mouth, and axilla are not done during TTM because these measurements are not well associated with brain temperature.[13]Therefore, the key is to select an appropriate monitoring system depending on clinical circumstances.And how to use the noninvasive methods to calculate CBT of the human body has become a research hotspot.[37]

3M ™ Bair Hugger ™ temperature monitoring system

The 3M ™ Bair Hugger ™ temperature monitoring system is a disposable sensor with a disposable skin temperature probe placed on the side forehead above the eyebrow,providing isothermal channel through a single-use sensor located on the forehead.When the sensor forms a heat insulation zone, it becomes an isothermal channel,eliminating the heat loss, thus bringing the CBT to the skin surface, then connecting the Te probe to the monitoring device (control unit, 120V), and finally connecting to the bedside monitor on which the data are continuously recorded.[38]

The surface of the temporal artery is located on the lateral forehead above the eyebrow and is the ideal location for noninvasive continuous CBT monitoring because it, powered by the internal carotid artery and close to the aorta, is one of the closest positions to the core temperature.[39]The accuracy of the 3M ™ Bair Hugger ™ temperature monitoring system has been evaluated in a variety of intraoperative environmental and intensive care unit settings, initially studied in nonemergency cardiac surgery populations,demonstrating consistency with CBT readings of the golden standard pulmonary artery catheter.[40]The meta-analysis conducted by Conwayetal.[39]compared the noninvasive temperature monitoring system (3M ™ Bair Hugger ™temperature monitoring system, Saint Paul, Minnesota)versus esophageal, bladder, or rectum measurements, and contained 22 comparisons of 16 individual studies including data from 952 participants in 314137 paired measurements.The comprehensive estimate of the average deviation is 0.03°C.The results show that the temperature measured using the 3M ™ Bair Hugger ™ temperature monitoring system may differ from CBT by 1°C.These results may have an important impact on practice.

The temple touch pro sensor

Temple Touch Pro (TTP) sensor consists of two units:A sensor insert and an external monitor connection unit with an optional data recording system.The sensor unit consists of an array of thermistors located near the skin surface, another set of thermistors above the insulator,and a second insulator between the upper unit and the environment.The readings are transmitted to the monitor connection unit, where CBT is estimated by a proprietary algorithm and can be displayed on a conventional bedside monitoring system.The communication between the two units may be wired or wireless.Evronetal.[41]used this sensor attached to the temporal artery to continuously monitor CBT for 50 adults and pediatric surgical patients,and compared with the esophageal or nasopharyngeal temperature measurements, indicating a high degree of consistency.In that study, the wired version was used, and the components are connected through the sensor unit cable.The temperature from the TTP system is recorded by a monitor connector connected to the vital signs displayer to display the patient’s temperature.

Tcore ™ Monitoring system

The Tcore ™ Monitoring System (Daimler AG, Munich,Germany) is a single device required to be connected to the forehead, consisting of two temperature probes.One is near the forehead skin and the other faces the environment.They are separated by a known heat, and CBT is calculated:CBT = Tskin+ K (Tskin- Tenvironment), where Tskindenotes the skin temperature underneath the sensor, Tenvironmentrefers to the temperature of the environment above the sensor,and K is a given coefficient determined by the quotient of the heat conduction coefficient of the insulator and the heat transfer coefficient of human tissue.Soehleetal.[42]collected 57,302 data pairs of CBT and Tbloodin 22 patients and calculated a deviation of - 0.02°C and 95% confidence interval of -0.48°C-0.44°C after repeated measurements by Brand and Altman test.Compared with the iliac artery blood temperature, studies have shown that the iliac artery blood temperature is a very accurate response to the gold standard pulmonary artery temperature.[42,43]Although the Tcore ™ sensor is connected to the forehead skin, it provides accurate CBT measurements during the intraoperative period.

Other wireless body temperature sensor

Wenetal.[27]used wireless body temperature sensor iTher-monitor® to measure CBT of elective surgery via the axilla and explored the feasibility and effect of body surface monitoring of CBT using the monitoring system.CorTemp,a capsule telemetry temperature sensor, is used to the intestinal temperature for subjects.[44]The subjects swallowed the CorTemp sensor which can read the gastrointestinal temperature measurements and then deliver the data to a handheld wireless receiver and display temperature information in real time in a digital format.Ultra-broadband radiation measurement is a novel radiometry technique that takes advantage of bio-matched antennas, broadband measurements, and forward modeling of layered tissues for noninvasive and accurate CBT monitoring.[45]

CONCLUSION

TTM is a dynamic process with dynamic monitoring and intervention strategies to ensure patient safety and effectiveness of treatment.The CBT during TTM is vital data,and guidelines and good clinical practice recommend to maintain CBT within 36°C-38°C in all surgical patients.[46,47]Consequently, it is mandatory to adequately monitor CBT as this allows prompt diagnosis and management of thermal disturbances and may prevent complications.[48]Pulmonary artery, nasopharyngeal, hypogloeeis, rectum,eardrum, esophagus, bladder, and other temperature measurements belong to the invasive temperature measurement methods, so the measurement accuracy depends on the accuracy of the temperature sensor.Those methods bring uncomfortable feelings to patients.Meanwhile, to prevent cross-infection, the temperature probes need to be strictly disinfected.The noninvasive continuous temperature monitoring system or sensor is to reflect CBT as accurately as possible to the skin that the probe touches, and display it on the bedside monitor,providing convenience for clinicians to monitor continuous and noninvasive CBT.These devices are increasingly used to replace more invasive CBT measurements in surgery and intensive care.

At present, more and more researches make a comparison of the accuracy of the noninvasive monitoring system or sensor versus invasive CBT measurements, because the CBT measurement sites are different among those researches, the accuracy of the temperature monitoring systems remains to be further researched.We can focus on the development of continuous noninvasive temperature monitoring methods,which will be a suitable alternative to approximate CBT during TTM.Especially, Since December 2019, new infectious diseases caused by coronavirus disease 2019 have emerged in China, the disease is predominantly attacking the lungs.[49]Body temperature control is a frequently used screening test for infectious diseases,[50]so the research of noninvasive temperature monitoring sensors is of importance to CBT monitoring.

Financial support and sponsorship

This study is supported by the Young Teacher Project of Beijing University of Chinese Medicine (No.:2018-JYB-JS155).

Conflicts of interest

There are no conflicts of interest.