Matt Brearley

Ice Slurry Ingestion Increases Core Temperature Capacity and Running Time in the Heat

PURPOSE To investigate the effect of ice slurry ingestion on thermoregulatory responses and submaximal running time in the heat. METHODS On two separate occasions, in a counterbalanced order, 10 males ingested 7.5 g·kg(-1) of either ice slurry (-1°C) or cold water (4°C) before running to exhaustion at their first ventilatory threshold in a hot environment (34.0°C ± 0.2°C, 54.9% ± 5.9% relative humidity). Rectal and skin temperatures, HR, sweating rate, and ratings of thermal sensation and perceived exertion were measured. RESULTS Running time was longer (P = 0.001) after ice slurry (50.2 ± 8.5 min) versus cold water (40.7 ± 7.2 min) ingestion. Before running, rectal temperature dropped 0.66°C ± 0.14°C after ice slurry ingestion compared with 0.25°C ± 0.09°C (P = 0.001) with cold water and remained lower for the first 30 min of exercise. At exhaustion, however, rectal temperature was higher (P = 0.001) with ice slurry (39.36°C ± 0.41°C) versus cold water ingestion (39.05°C ± 0.37°C). During exercise, mean skin temperature was similar between conditions (P = 0.992), as was HR (P = 0.122) and sweat rate (P = 0.242). After ice slurry ingestion, subjects stored more heat during exercise (100.10 ± 25.00 vs 78.93 ± 20.52 W·m(-2), P = 0.005), and mean ratings of thermal sensation (P = 0.001) and perceived exertion (P = 0.022) were lower. CONCLUSIONS Compared with cold water, ice slurry ingestion lowered preexercise rectal temperature, increased submaximal endurance running time in the heat (+19% ± 6%), and allowed rectal temperature to become higher at exhaustion. As such, ice slurry ingestion may be an effective and practical precooling maneuver for athletes competing in hot environments.

Systolic peak detection in acceleration photoplethysmograms measured from emergency responders in tropical conditions.

Photoplethysmogram (PPG) monitoring is not only essential for critically ill patients in hospitals or at home, but also for those undergoing exercise testing. However, processing PPG signals measured after exercise is challenging, especially if the environment is hot and humid. In this paper, we propose a novel algorithm that can detect systolic peaks under challenging conditions, as in the case of emergency responders in tropical conditions. Accurate systolic-peak detection is an important first step for the analysis of heart rate variability. Algorithms based on local maxima-minima, first-derivative, and slope sum are evaluated, and a new algorithm is introduced to improve the detection rate. With 40 healthy subjects, the new algorithm demonstrates the highest overall detection accuracy (99.84% sensitivity, 99.89% positive predictivity). Existing algorithms, such as Billauers, Lis and Zongs, have comparable although lower accuracy. However, the proposed algorithm presents an advantage for real-time applications by avoiding human intervention in threshold determination. For best performance, we show that a combination of two event-related moving averages with an offset threshold has an advantage in detecting systolic peaks, even in heat-stressed PPG signals.

Detection of a and b waves in the acceleration photoplethysmogram

BackgroundAnalyzing acceleration photoplethysmogram (APG) signals measured after exercise is challenging. In this paper, a novel algorithm that can detect a waves and consequently b waves under these conditions is proposed. Accurate a and b wave detection is an important first step for the assessment of arterial stiffness and other cardiovascular parameters.MethodsNine algorithms based on fixed thresholding are compared, and a new algorithm is introduced to improve the detection rate using a testing set of heat stressed APG signals containing a total of 1,540 heart beats.ResultsThe new a detection algorithm demonstrates the highest overall detection accuracy—99.78% sensitivity, 100% positive predictivity—over signals that suffer from 1) non-stationary effects, 2) irregular heartbeats, and 3) low amplitude waves. In addition, the proposed b detection algorithm achieved an overall sensitivity of 99.78% and a positive predictivity of 99.95%.ConclusionsThe proposed algorithm presents an advantage for real-time applications by avoiding human intervention in threshold determination.

Working in Hot Conditions—A Study of Electrical Utility Workers in the Northern Territory of Australia

Environmental conditions of Australias Northern Territory are seasonally conducive to excessive body heat storage by outdoor workers. For electrical utility workers who periodically work at height, in confined space, and in proximity to live power sources, the impact of the climate may be considered a hazardous condition. Therefore, this study examined the physiological and fluid balance responses of 20 power network workers (31.5 years; 86.0 kg; 1.71 m; BMI 29.5) throughout work shifts in the Northern and Southern regions of the Northern Territory, Australia. Twenty male heat-acclimatized power network workers provided written informed consent to be monitored during maintenance of electrical infrastructure that included replacing power pole components and transformer and substation repairs in the Northern (n = 13) and Southern regions (n = 7) of the Northern Territory (mean wet-bulb globe temperatures of 32.0°C and 28.7°C, respectively). An ingestible telemetry pill provided measurement of gastrointestinal temperature (Tgi), that when combined with heart rate values, provided physiological strain index (PSI). Urine specific gravity, sweat rate, and level of dehydration were also determined. The Tgi values of this study were within the ISO9886 limit for monitored, heat-acclimatized workers, with a peak of 38.4°C. Mean PSI was 2.6, which represents overall low strain, with periods of moderate strain. Urinary analysis indicated that workers were dehydrated prior to and following the work shift, however the mean sweat rate of 0.44 L.h−1 was matched by fluid consumption of 0.42 L.h−1 to limit body mass loss to 0.1% during the shift. This study demonstrates that heat acclimatized electrical utility workers adhere to ISO9886 requirements when undertaking self-paced activity in hot conditions.

Physiological responses of medical team members to a simulated emergency in tropical field conditions.

INTRODUCTION Responses to physical activity while wearing personal protective equipment in hot laboratory conditions are well documented. However less is known of medical professionals responding to an emergency in hot field conditions in standard attire. Therefore, the purpose of this study was to assess the physiological responses of medical responders to a simulated field emergency in tropical conditions. METHODS Ten subjects, all of whom were chronically heat-acclimatized health care workers, volunteered to participate in this investigation. Participants were the medical response team of a simulated field emergency conducted at the Northern Territory Emergency Services training grounds, Yarrawonga, NT, Australia. The exercise consisted of setting up a field hospital, transporting patients by stretcher to the hospital, triaging and treating the patients while dressed in standard medical response uniforms in field conditions (mean ambient temperature of 29.3°C and relative humidity of 50.3%, apparent temperature of 27.9°C) for a duration of 150 minutes. Gastrointestinal temperature was transmitted from an ingestible sensor and used as the index of core temperature. An integrated physiological monitoring device worn by each participant measured and logged heart rate, chest temperature and gastrointestinal temperature throughout the exercise. Hydration status was assessed by monitoring the change between pre- and post-exercise body mass and urine specific gravity (USG). RESULTS Mean core body temperature rose from 37.5°C at the commencement of the exercise to peak at 37.8°C after 75 minutes. The individual peak core body temperature was 38.5°C, with three subjects exceeding 38.0°C. Subjects sweated 0.54 L per hour and consumed 0.36 L of fluid per hour, resulting in overall dehydration of 0.7% of body mass at the cessation of exercise. Physiological strain index was indicative of little to low strain. CONCLUSIONS The combination of the unseasonably mild environmental conditions and moderate work rates resulted in minimal heat storage during the simulated exercise. As a result, low sweat rates manifested in minimal dehydration. When provided with access to fluids in mild environmental conditions, chronically heat-acclimatized medical responders can meet their hydration requirements through ad libitum fluid consumption. Whether such an observation is replicated under a harsher thermal load remains to be investigated.

Cold-water immersion and iced-slush ingestion are effective at cooling firefighters following a simulated search and rescue task in a hot environment

Firefighters are exposed to hot environments, which results in elevated core temperatures. Rapidly reducing core temperatures will likely increase safety as firefighters are redeployed to subsequent operational tasks. This study investigated the effectiveness of cold-water immersion (CWI) and iced-slush ingestion (SLUSH) to cool firefighters post-incident. Seventy-four Australian firefighters (mean ± SD age: 38.9 ± 9.0 years) undertook a simulated search and rescue task in a heat chamber (105 ± 5 °C). Testing involved two 20-min work cycles separated by a 10-min rest period. Ambient temperature during recovery periods was 19.3 ± 2.7 °C. Participants were randomly assigned one of three 15-min cooling protocols: (i) CWI, 15 °C to umbilicus; (ii) SLUSH, 7 g·kg(-1) body weight; or (iii) seated rest (CONT). Core temperature and strength were measured pre- and postsimulation and directly after cooling. Mean temperatures for all groups reached 38.9 ± 0.9 °C at the conclusion of the second work task. Both CWI and SLUSH delivered cooling rates in excess of CONT (0.093 and 0.092 compared with 0.058 °C·min(-1)) and reduced temperatures to baseline measurements within the 15-min cooling period. Grip strength was not negatively impacted by either SLUSH or CONT. CWI and SLUSH provide evidence-based alternatives to passive recovery and forearm immersion protocols currently adopted by many fire services. To maximise the likelihood of adoption, we recommend SLUSH ingestion as a practical and effective cooling strategy for post-incident cooling of firefighters in temperate regions.

Frequency analysis of photoplethysmogram and its derivatives

There are a limited number of studies on heat stress dynamics during exercise using the photoplethysmogram (PPG). We investigate the PPG signal and its derivatives for heat stress assessment using Welch (non-parametric) and autoregressive (parametric) spectral estimation methods. The preliminary results of this study indicate that applying the first and second derivatives to PPG waveforms is useful for determining heat stress level using 20-s recordings. Interestingly, Welchs and Yule-Walkers methods in agreement that the second derivative is an improved detector for heat stress. In fact, both spectral estimation methods showed a clear separation in the frequency domain between measurements before and after simulated heat-stress induction when the second derivative is applied. Moreover, the results demonstrate superior performance of the Welchs method over the Yule-Walkers method in separating before and after the three simulated heat-stress inductions.

Pre-deployment heat acclimatization guidelines for disaster responders

INTRODUCTION Minimal preparation time is a feature of responding to sudden onset disasters. While equipment and supplies are prepared for deployment at short notice, less is known of the physical preparation of medical responders. With many disaster-prone areas classified as tropical regions, there is potential for responders to endure a combination of high ambient temperatures and relative humidity during deployment. Heat acclimatization, defined as the physiological and perceptual adaptations to frequent elevations of core body temperature (Tc), is a key strategy to improve tolerance of hot conditions by medical responders. METHODS Pre-deployment heat acclimatization guidelines were developed based upon the duration of physical training and the subjective rate of perceived exertion (session RPE). An objective of individual training sessions was the perception of body temperature as warm to hot. The guidelines were implemented for Team Bravo (2nd rotation) of the Australian Medical Assistance Team (AusMAT) deployed to Tacloban, Philippines following Typhoon Haiyan in November 2013. The guidelines were distributed electronically five to seven days prior to deployment and were followed by a consultation. A group training session in hot conditions was undertaken prior to departure. RESULTS The AusMAT responders to utilize the guidelines were based in cool or temperate climates that required extra layers of clothing, training during warmer parts of the days, or warm indoor conditions to achieve session objectives. Responders reported the guidelines were simple to use, applicable to their varied training regimens, and had improved their confidence to work in the heat despite not completing the entire 14 day period. CONCLUSION The pre-deployment heat acclimatization guidelines provided AusMAT responders the ability to quantify their physical training and promoted physiological adaptations to maximize health, safety, and performance during deployment. While maintaining year-round heat acclimatization is considered essential for medical responders, these guidelines may facilitate beneficial adaptations once notified of deployment.

Influence of Chronic Heat Acclimatization on Occupational Thermal Strain in Tropical Field Conditions.

Objective: To examine whether non-heat acclimatized (NHA) emergency responders endure greater physiological and perceptual strain than heat acclimatized (HA) counterparts in tropical field settings. Methods: Eight HA and eight NHA men urban search and rescue personnel had physiological and perceptual responses compared during the initial 4 hours shift of a simulated disaster in tropical conditions (ambient temperature 34.0 °C, 48% relative humidity, wet bulb globe temperature [WBGT] 31.4 °C). Results: From the 90th minute through to end of shift, HA (38.5 °C) sustained a significantly higher gastrointestinal temperature than NHA (38.1 °C) (mean difference 0.4 ± 0.2 °C, 95% confidence interval [CI] 0.2 to 0.7 °C, P = 0.005) despite comparable heart rate (P = 0.30), respiratory rate (P = 0.88), and axilla skin temperature (P = 0.47). Overall, perception of body temperature was similar between cohorts (P = 0.87). Conclusions: The apparent tolerance of greater physiological strain by HA responders occurred in the absence of perceptual differences.

Accuracy of Tympanic Temperature Measurement in Firefighters Completing a Simulated Structural Firefighting Task

INTRODUCTION In the course of their duties, firefighters risk heat stroke and other medical conditions due to exertion in high-temperature environments. Infrared tympanic temperature measurement (TTym) is often used by Emergency Medical Services (EMS) to assess the core body temperature of firefighters. The accuracy of TTym in this setting has been called into question. Hypothesis/Problem This study aimed to examine the accuracy of TTym for core body temperature assessment at emergency firefighting events compared with gastrointestinal temperature measurement (TGI) as measured by ingestible thermometers. METHODS Forty-five (42 male, three female) professional urban firefighters from an Australian fire service completed two 20-minute work periods in a 100°C (± 5°C) heat chamber while wearing personal protective clothing (PPC) and breathing apparatus (weighing approximately 22 kg). Measurements were taken immediately before entering, and on exiting, the heat chamber. Tympanic temperature was assessed by an infrared tympanic thermometer and TGI was measured by ingestible sensor and radio receiver. RESULTS Complete data were available for 37 participants. Participant temperatures were higher on exiting the heat chamber than at baseline (TTym: 35.9°C (SD=0.7) vs 37.5°C (SD=0.8); TGI: 37.2°C (SD=0.4) vs 38.6°C (SD=0.5)). Tympanic temperature underestimated TGI on average by 1.3°C (SD=0.5) before entering the chamber and by 1.0°C (SD=0.8) following the exercise. Using pooled data, the average underestimation was 1.2°C (SD=0.7). CONCLUSION Tympanic thermometers cause an unreliable measure of core body temperature for firefighters engaged in fire suppression activities. Accurate and practical measures of core body temperature are required urgently.