"First responders and military personnel often carry out demanding work in hot environments, and in some cases wear thermally occlusive protective clothing," Mark Buller, from the US Army Research Institute of Environmental Medicine, told medicalphysicsweb. "These conditions are ripe for heat exhaustion and even heat stroke. The ability to monitor heat strain allows members of these professions to better manage their work routines and prevent heat illness."

Assessing core temperature

Although research is ongoing, non-invasive measurement of core temperature remains problematic as there is no clear way to measure deep body tissue temperature from the surface of the skin. As the body's thermoregulatory system controls blood flow to the skin to regulate core temperature, skin temperature will often vary independently from core temperature based on factors such as work rate, the environment and the clothing worn. Add in the need for a continuous ambulatory measurement and the number of challenges facing researchers increases again.

In developing their core temperature estimation method, Buller and his colleagues took what they call a "computational physiology" approach. "We started by examining the factors affecting core temperature and how these relate to each other," he commented. "By recognizing heart rate is central to both heat gain and heat loss, we were able to redefine non-invasive core temperature measurement in terms of a tracking problem. This allowed us to use the well characterized Kalman filter (KF) algorithm."

Development and validation

Data from 100 test volunteers collected from ten laboratory and field studies were used to both develop (17 data sets) and validate (83 data sets) the team's KF model. As Buller explains, the KF model optimally combines a core temperature dynamics model and a core temperature-to-heart rate observation model.

"By knowing how core temperature changes over time and the variance associated with the change; and knowing how an observation of heart rate relates to core temperature and the variance associated with this observation; then the KF will give a better estimate of core temperature than just using the observations alone," he notes. "The approach is computationally very simple."

With the team's KF model in hand, data from four laboratory studies were used for controlled comparisons of the effects of different environments, hydration states, clothing and acclimation state. In addition, five field physiological monitoring experiments were used to examine the performance in different climates with various levels of protective clothing. In all nine of these studies, core temperature was measured using conventional methods such as an ingested thermometer pill or a rectal probe. The researchers then used the available heart rate data to update their KF model every minute and compare their estimate of the core temperature with the actual reported value.

"It was important to demonstrate that our approach worked across a range of temperatures and with different work rates," commented Buller. "In addition to the environment, clothing plays an important role in how core temperature changes with work rate. We included studies in shorts and t-shirt, standard army uniforms, and fully encapsulated in chemical biological protection equipment. We also included two laboratory studies to address core temperature responses which increase with dehydration and decrease with heat acclimation."

Future plans

While the authors admitted that their approach is not a replacement for direct core temperature measurement, they concluded that it is an accurate and practical way to provide a real-time assessment of heat strain in an ambulatory environment.

According to Buller, the next steps are to implement the algorithm for use with real-time displays of heat strain, and examine a larger section of the first responder community. "We have currently implemented the algorithm in a research-grade physiological monitoring system that we have linked to Android tablet app," he said.

Disclaimer: The views expressed in this article are those of the authors and do not reflect the official policy of the Department of Army, Department of Defense, or the US Government.

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