The Systematic Bias of Ingestible Core Temperature Sensors Requires a Correction by Linear Regression

Wearable and ingestible technology to evaluate and prevent exertional heat illness: A narrative review

Exertional heat illness remains a constant threat to the athlete, military service member, and laborer. Recent increases in the number and intensity of environmental heat waves places these populations at an ever increasing risk and can be deadly if not recognized and treated rapidly. For this reason, it is extremely important for medical providers to guide athletes, service members, and laborers in the implementation of awareness, education, and measures to reduce or mitigate the risk of exertional heat illness. Within the past 2 decades, a variety of wearable technology options have become commercially available to track an estimation of core temperature, yet questions continue to emerge as to its use, effectiveness, and practicality in athletics, the military, and the workforce. There is a paucity of data on the accuracy of many of these newer devices in the setting of true heat stroke physiology, and it is important to avoid overreliance on new wearable technology. Further research and improvement of this technology are critical to identify accuracy in the diagnosis and prevention of EHI.

Changing iodine status and the incidence of thyroid disease in mainland China: A prospective 20-year follow-up study

BACKGROUND

We aimed to assess the long-term effects of the transition in iodine status on the incidence of thyroid disorders over 20 years of follow up.

METHODS

The original prospective cohort study, started in 1999 (n=3,761), classified three regions in north China based on iodine status (insufficient iodine, more than adequate iodine, and excessive iodine, respectively) for 5 years. Subsequently, participants were followed for up to another 15 years to assess the long-term effects of shifts to adequate iodine on the incidence of thyroid disorders. Panshan transitioned from insufficient to adequate iodine and Huanghua transitioned from excessive to more than adequate iodine. Both regions were compared to Zhangwu, which iodine status changed from more than adequate to adequate iodine (from 214μg/L to 167.2μg/L). A cluster sampling method was used to select participants in the three regions. Participants completed questionnaires and underwent thyroid ultrasonography. Urinary iodine concentrations (UIC), serum thyroid hormone concentration, and thyroid antibodies were measured.

RESULTS

When the iodine status changed from insufficient to adequate (with the median UIC increasing from 88μg/L to 141.9μg/L), the incidence density of subclinical hyperthyroidism, positive thyroperoxidase antibody (TPOAb), positive thyroglobulin antibody (TgAb), and goiter decreased significantly (p<0.05 for all). Additionally, the cumulative incidence of subclinical hypothyroidism was significantly lower compared to the region where the iodine status changed from being more than adequate to adequate (1.9% vs. 6.0%, p<0.001). When the iodine status changed from excessive to more than adequate (median UIC from 634μg/L to 266.7μg/L), a significant decrease in the incidence density of subclinical hyperthyroidism, positive thyroid antibodies, positive TgAb, and goiter (p<0.05 for all), were also found. However, an increase in thyroid nodule incidence density (17.26 vs. 28.25 per 1000 person-years, p<0.001) was seen.

CONCLUSIONS

The incidence of thyroid disorders (except for thyroid nodules) stabilized or decreased among adults in the three communities from year 5 to year 15 of follow-up. Appropriate iodine fortification is safe and effective over the long term. Restoring urinary iodine to appropriate levels reduces population risk for thyroid disorders.

Reliability of Biomarkers of Physiological Stress at Rest and Post-exertional Heat Stress

The purpose of this study was to assess the reliability of blood biomarkers that can signify exercise-induced heat stress in hot conditions. Fourteen males completed two heat stress tests separated by 5-7 days. Venous blood was drawn pre- and post- heat stress for the concentration of normetanephrine, metanephrine, serum osmolality, copeptin, kidney-injury molecule 1, and neutrophil gelatinase-associated lipocalin. No biomarker, except copeptin, displayed systematic trial order bias (p≥0.05). Normetanephrine, copeptin and neutrophil gelatinase-associated lipocalin presented acceptable reliability (CV range: 0.9-14.3%), while greater variability was present in metanephrine, osmolality and kidney-injury molecule 1 (CV range: 28.6-43.2%). Normetanephrine exhibited the largest increase (p<0.001) in response to heat stress (trial 1=1048±461 pmol. L^-1; trial 2=1067±408 pmol. L^-1), whilst kidney-injury molecule 1 presented trivial changes (trial 1=-4±20 ng. L^-1; trial 2=2 ± 16 ng. L^-1, p>0.05). Normetanephrine, copeptin, and neutrophil gelatinase-associated lipocalin demonstrated good reliability and sensitivity to an acute bout of heat stress. These biomarkers may be suitable for application in laboratory and field research to understand the efficacy of interventions that can attenuate the risk of thermal injury whilst exercising in the heat.

Estimating core body temperature using electrocardiogram signals

Suppressing the elevation in core body temperature is an important factor in preventing heatstroke. However, there is still no non-invasive method to sense core body temperature. This study proposed an algorithm that estimates core body temperature based on electrocardiogram signals. A total of 12 healthy men (mean age ± SD = 39.6 ± 13.4) performed an ergometric exercise load test under two conditions of exercise load in an environmental chamber adjusted to a temperature of 35°C and humidity of 50%. Vital sensing data such as electrocardiograms, core body temperatures, and body surface temperatures were continuously measured, and physical data such as body weight were obtained from participants pre- and post-experiment. According to basic physiological knowledge, heart rate and body temperature are closely related. We analyzed the relationship between core body temperature and several indexes obtained from electrocardiograms and found that the amount of change in core body temperature had a strong relationship with analyzed data from electrocardiograms. Based on these findings, we developed the amount of change in core body temperature estimation model using multiple regression analysis including the Poincaré plot index of the ECG R-R interval. The estimation model showed an average estimation error of -0.007°C (average error rate = -0.02%) and an error range of 0.457–0.445°C. It is suggested that continuous core body temperature change can be estimated using electrocardiogram signals regardless of individual characteristics such as age and physique. Based on this applicable estimation model, we plan to enhance estimation accuracy and further verify efficacy by considering clothing and environmental conditions.

Modified Stroop Task Performance When Wearing Protective Clothing in the Heat: An Evaluation of the Maximum Adaptability Model

INTRODUCTION

This exploratory study investigated whether performance in a behavioural inhibition task followed the shape proposed by the Maximum Adaptability Model during progressive exertional heat stress-that is, an initial improvement in cognitive performance is followed by a plateau, and subsequent decline once body temperature continues to rise unabated.

METHODS

Seventeen adult males walked on a treadmill at 4 km•h^-1 (1% grade) for up to 120 min, in three protective clothing ensembles, across three simulated environments. The simulated environments were equivalent to wet bulb globe temperatures 21, 30 and 37°C. Cognitive function was assessed using a modified colour-word Stroop Task, with performance expressed as inverse efficiency scores in the simple (congruent) and more complex (incongruent) task conditions. The Stroop Task was completed before a trial, at termination, and every 30 min during walking, and core body temperature was continuously measured. Data were modelled using Bayesian penalised regression, with core body temperature included as a non-linear term (i.e., second degree polynomial).

RESULTS

We did not find any evidence that core body temperature had an effect on congruent or incongruent inverse efficiency scores, and no evidence that the relationship between these variables followed the shaped described by the Maximum Adaptability Model. There was, however, evidence that higher pre-exercise serum osmolality values were associated with slower congruent (β = 9.19) and incongruent (β = 8.67) inverse efficiency scores. The posterior probability that these effects were greater than zero was 0.971 and 0.952, respectively.

CONCLUSIONS

In young, fit men, performance in the behavioural inhibition task was unaffected by increases in body temperature up to 39°C and did not follow the shape proposed by the Maximum Adaptability Model. A secondary finding of the study was that pre-exercise hydration status affected performance in the inhibition task. Future studies are needed to confirm this result.

Ingestible devices for studying the gastrointestinal physiology and their application in oral biopharmaceutics

Ingestible sensor systems are unique tools for obtaining physiological data from an undisturbed gastrointestinal tract. Since their dimensions correspond to monolithic oral dosage forms, such as enteric coated tablets or hydrogel matrix tablets, they also allow insights into the physiological conditions experienced by non-disintegrating dosage forms on their way through the gastrointestinal tract. In this work, the different ingestible sensor systems which can be used for this purpose are described and their potential applications as well as difficulties and pitfalls with respect to their use are presented. It is also highlighted how the data on transit times, pH, temperature and pressure as well as the data from different animal models commonly used in drug product development such as dogs and pigs have contributed to a deeper mechanistic understanding of oral drug delivery.

Smart Patch for Skin Temperature: Preliminary Study to Evaluate Psychometrics and Feasibility

There is a need for continuous, non-invasive monitoring of biological data to assess health and wellbeing. Currently, many types of smart patches have been developed to continuously monitor body temperature, but few trials have been completed to evaluate psychometrics and feasibility for human subjects in real-life scenarios. The aim of this feasibility study was to evaluate the reliability, validity and usability of a smart patch measuring body temperature in healthy adults. The smart patch consisted of a fully integrated wearable wireless sensor with a multichannel temperature sensor, signal processing integrated circuit, wireless communication feature and a flexible battery. Thirty-five healthy adults were recruited for this test, carried out by wearing the patches on their upper chests for 24 h and checking their body temperature six times a day using infrared forehead thermometers as a gold standard for testing validity. Descriptive statistics, one-sampled and independent t-tests, Pearson’s correlation coefficients and Bland-Altman plot were examined for body temperatures between two measures. In addition, multiple linear regression, receiver operating characteristic (ROC) and qualitative content analysis were conducted. Among the 35 participants, 29 of them wore the patch for over 19 h (dropout rate: 17.14%). Mean body temperature measured by infrared forehead thermometers and smart patch ranged between 32.53 and 38.2 °C per person and were moderately correlated (r = 0.23–0.43) overall. Based on a Bland-Altman plot, approximately 94% of the measurements were located within one standard deviation (upper limit = 4.52, lower limit = −5.82). Most outliers were identified on the first measurement and were located below the lower limit. It is appropriate to use 37.5 °C in infrared forehead temperature as a cutoff to define febrile conditions. Users’ position while checking and ambient temperature and humidity are not affected to the smart patch body temperature. Overall, the participants showed high usability and satisfaction on the survey. Few participants reported discomfort due to limited daily activity, itchy skin or detaching concerns. In conclusion, epidermal electronic sensor technologies provide a promising method for continuously monitoring individuals’ body temperatures, even in real-life situations. Our study findings show the potential for smart patches to monitoring non-febrile condition in the community.

The Effects of Heat Exposure During Intermittent Exercise on Physical and Cognitive Performance Among Team Sport Athletes

This study investigated the effects of heat exposure on physical and cognitive performance during an intermittent exercise protocol so as to reflect the incremental fatigue experienced during team sports. Twelve well-trained male team sport players completed an 80-minute cycling intermittent sprint protocol (CISP), alongside computerized vigilance and congruent (i.e., simple) and incongruent (i.e., complex) Stroop tasks of cognitive functioning, in two counterbalanced temperature conditions; hot (32°C[50%rh]) and control (18°C[50%rh]). Incongruent Stroop accuracy declined over time (p = .002), specifically in the second (M^diff = –3.75, SD = 0.90%, p = .009) and third (M^diff = –4.58, SD = 1.22%, p = .019) quarters compared to the first quarter of the CISP; but there were no differences between temperature conditions. Congruent Stroop reaction time (RT) was quicker in the second quarter of exercise in the hot condition (M = 561.99, SD = 112.93 ms) compared to the control condition (M=617.80, SD = 139.71 ms; p = .022), but no differences were found for congruent Stroop accuracy nor vigilance measures. Additionally, peak power output was lower during the third quarter of the CISP in the hot condition (M = 861.31, SD = 105.20 W) compared to the control condition (M = 900.68, SD = 114.84 W; p < .001). Plasma normetanephrine and metanephrine concentrations increased from pre- to post-CISP (M^diff = +616.90, SD = 306.99, p < .001; and M^diff = +151.23, SD = 130.32, p = .002, respectively), with a marginal interaction suggesting a higher normetanephrine increase from pre- to post-CISP in the hot versus the control condition (p = .070). Our findings suggest that accuracy for more complex decisions suffered during prolonged high-intensity intermittent exercise, perhaps due to exercise-induced catecholamine increases. Athletes may have also reduced physical effort under increased heat exposure, indicating how cognitive performance may be sustained in physically demanding environments.

Independent or simultaneous lowering of core and skin temperature has no impact on self-paced intermittent running performance in hot conditions

Purpose

To investigate the effects of lowering core (T_gi) and mean skin temperature (T_sk) concomitantly and independently on self-paced intermittent running in the heat.

Methods

10 males (30.5 ± 5.8 years, 73.2 ± 14.5 kg, 176.9 ± 8.0 cm, 56.2 ± 6.6 ml/kg/min) completed four randomised 46-min self-paced intermittent protocols on a non-motorised treadmill in 34.4 ± 1.4 °C, 36.3 ± 4.6% relative humidity. 30-min prior to exercise, participants were cooled via either ice slurry ingestion (INT); a cooling garment (EXT); mixed-cooling (ice slurry and cooling garment concurrently) (MIX); or no-cooling (CON).

Results

At the end of pre-cooling and the start of exercise T_gi were lower during MIX (36.11 ± 1.3 °C) compared to CON (37.6 ± 0.5 °C) and EXT (36.9 ± 0.5 °C, p < 0.05). Throughout pre-cooling T_sk and thermal sensation were lower in MIX compared to CON and INT, but not EXT (p < 0.05). The reductions in thermophysiological responses diminished within 10–20 min of exercise. Despite lowering T_gi, T_sk, body temperature (T_b), and thermal sensation prior to exercise, the distances covered were similar (CON: 6.69 ± 1.08 km, INT: 6.96 ± 0.81 km, EXT: 6.76 ± 0.65 km, MIX 6.87 ± 0.70 km) (p > 0.05). Peak sprint speeds were also similar between conditions (CON: 25.6 ± 4.48 km/h, INT: 25.4 ± 3.6 km/h, EXT: 26.0 ± 4.94 km/h, MIX: 25.6 ± 3.58 km/h) (p > 0.05). Blood lactate, heart rate and RPE were similar between conditions (p > 0.05).

Conclusion

Lowering T_gi and T_sk prior to self-paced intermittent exercise did not improve sprint, or submaximal running performance.

Electronic supplementary material

The online version of this article (10.1007/s00421-019-04173-y) contains supplementary material, which is available to authorized users.

Validity of a noninvasive estimation of deep body temperature when wearing personal protective equipment during exercise and recovery

BACKGROUND

Deep body temperature is a critical indicator of heat strain. However, direct measures are often invasive, costly, and difficult to implement in the field. This study assessed the agreement between deep body temperature estimated from heart rate and that measured directly during repeated work bouts while wearing explosive ordnance disposal (EOD) protective clothing and during recovery.

METHODS

Eight males completed three work and recovery periods across two separate days. Work consisted of treadmill walking on a 1% incline at 2.5, 4.0, or 5.5 km/h, in a random order, wearing EOD protective clothing. Ambient temperature and relative humidity were maintained at 24 °C and 50% [Wet bulb globe temperature (WBGT) (20.9 ± 1.2) °C] or 32 °C and 60% [WBGT (29.0 ± 0.2) °C] on the separate days, respectively. Heart rate and gastrointestinal temperature (T_GI) were monitored continuously, and deep body temperature was also estimated from heart rate (ECTemp).

RESULTS

The overall systematic bias between T_GI and ECTemp was 0.01 °C with 95% limits of agreement (LoA) of ±0.64 °C and a root mean square error of 0.32 °C. The average error statistics among participants showed no significant differences in error between the exercise and recovery periods or the environmental conditions. At T_GI levels of (37.0-37.5) °C, (37.5-38.0) °C, (38.0-38.5) °C, and > 38.5 °C, the systematic bias and ± 95% LoA were (0.08 ± 0.58) °C, (- 0.02 ± 0.69) °C, (- 0.07 ± 0.63) °C, and (- 0.32 ± 0.56) °C, respectively.

CONCLUSIONS

The findings demonstrate acceptable validity of the ECTemp up to 38.5 °C. Conducting work within an ECTemp limit of 38.4 °C, in conditions similar to the present study, would protect the majority of personnel from an excessive elevation in deep body temperature (> 39.0 °C).

On the use of wearable physiological monitors to assess heat strain during occupational heat stress

Workers in many industries are required to perform arduous work in high heat-stress conditions, which can lead to rapid increases in body temperature that elevate the risk of heat-related illness and even death. Traditionally, effort to mitigate work-related heat injury has been directed toward the assessment of environmental heat stress (e.g., wet-bulb globe temperature), rather than toward the associated physiological strain responses (e.g., heart rate and skin and core temperatures). However, because a worker’s physiological response to a given heat stress is modified independently by inter-individual factors (e.g., age, sex, chronic disease, others) and intra-individual factors both within (e.g., medication use, fitness, acclimation and hydration state, others) and beyond (e.g., shift duration, illness, others) the worker’s control, it becomes challenging to protect workers on an individual basis from heat-related injury without assessing those physiological responses. Recent advancements in wearable technology have made it possible to monitor one or more physiological indices of heat strain. Nonetheless, information on the utility of the wearable systems available for assessing occupational heat strain is unavailable. This communication is therefore directed toward identifying the physiological indices of heat strain that may be quantified in the workplace and evaluating the wearable monitoring systems available for assessing those responses. Finally, emphasis is placed on the barriers associated with implementing these devices to assist in mitigating work-related heat injury. This information is fundamental for protecting worker health and could also be utilized to prevent heat illnesses in vulnerable people during leisure or athletic activities.

Estimating Resting Core Temperature Using Heart Rate

ECTemp™ is a heart rate (HR)-based core temperature (CT) estimation algorithm mainly used as a real-time thermal-work strain indicator in military populations. ECTemp™ may also be valuable for resting CT estimation, which is critical for circadian rhythm research. This investigation developed and incorporated a sigmoid equation into ECTemp™ to better estimate resting CT. HR and CT data were collected over two calorimeter test trials from 16 volunteers (age, 23 ± 3 yrs; height, 1.72 ± 0.07 m; body mass, 68.5 ± 8.1 kg) during periods of sleep and inactivity. Half of the test trials were combined with ECTemp™’s original development dataset to train the new sigmoid model while the other was used for model validation. Models were compared by their estimation accuracy and precision. While both models produced accurate CT estimates, the sigmoid model had a smaller bias (−0.04 ± 0.26°C vs. −0.19 ± 0.29°C) and root mean square error (RMSE; 0.26°C vs. 0.35°C). ECTemp™ is a validated HR-based resting CT estimation algorithm. The new sigmoid equation corrects lower CT estimates while producing nearly identical estimates to the original quadratic equation at higher CT. The demonstrated accuracy of ECTemp™ encourages future research to explore the algorithm’s potential as a non-invasive means of tracking CT circadian rhythms.

An Overt Chemical Protective Garment Reduces Thermal Strain Compared with a Covert Garment in Warm-Wet but Not Hot-Dry Environments

Objectives: A commercial chemical, biological, radiological and nuclear (CBRN) protective covert garment has recently been developed with the aim of reducing thermal strain. A covert CBRN protective layer can be worn under other clothing, with equipment added for full chemical protection when needed. However, it is unknown whether the covert garment offers any alleviation to thermal strain during work compared with a traditional overt ensemble. Therefore, the aim of this study was to compare thermal strain and work tolerance times during work in an overt and covert ensemble offering the same level of CBRN protection.

Methods: Eleven male participants wore an overt (OVERT) or covert (COVERT) CBRN ensemble and walked (4 km·h⁻¹, 1% grade) for a maximum of 120 min in either a wet bulb globe temperature [WBGT] of 21, 30, or 37°C (Neutral, WarmWet and HotDry, respectively). The trials were ceased if the participants' gastrointestinal temperature reached 39°C, heart rate reached 90% of maximum, walking time reached 120 min or due to self-termination.

Results: All participants completed 120 min of walking in Neutral. Work tolerance time was greater in OVERT compared with COVERT in WarmWet (P < 0.001, 116.5[9.9] vs. 88.9[12.2] min, respectively), though this order was reversed in HotDry (P = 0.003, 37.3[5.3] vs. 48.4[4.6] min, respectively). The rate of change in mean body temperature and mean skin temperature was greater in COVERT (0.025[0.004] and 0.045[0.010]°C·min⁻¹, respectively) compared with OVERT (0.014[0.004] and 0.027[0.007]°C·min⁻¹, respectively) in WarmWet (P < 0.001 and P = 0.028, respectively). However, the rate of change in mean body temperature and mean skin temperature was greater in OVERT (0.068[0.010] and 0.170[0.026]°C·min⁻¹, respectively) compared with COVERT (0.059[0.004] and 0.120[0.017]°C·min⁻¹, respectively) in HotDry (P = 0.002 and P < 0.001, respectively). Thermal sensation, thermal comfort, and ratings of perceived exertion did not differ between garments at trial cessation (P > 0.05).

Conclusion: Those dressed in OVERT experienced lower thermal strain and longer work tolerance times compared with COVERT in a warm-wet environment. However, COVERT may be an optimal choice in a hot-dry environment. These findings have practical implications for those making decisions on the choice of CBRN ensemble to be used during work.