Working Soldiers to failure is a costly mistake, and until recently it’s been anybody’s guess at what temperature and exertion rate a given Soldier would max out. But now, wearable, chest-based sensors (far more accurate and informative than current wrist-worn models) can tell when a Soldier is nearing cardiac and temperature limits—protecting Soldiers, preventing heat casualties and generating data to help predict how Soldiers will perform under new environmental conditions.
by Dr. Reed W. Hoyt and Dr. Karl E. Friedl (COL, USA Ret.)
The Army uses wired and wireless monitoring systems to monitor real-time performance and safe operating limits of vehicles and aircraft, but no comparable systems exist for Soldiers.
The limiter to real-time physiological status monitoring (RT-PSM) has not been engineering technology. More than 50 years ago, John Glenn blasted off from Cape Canaveral while his medical team monitored a modest heart rate increase to 110 beats per minute. This ability to remotely monitor physiological signals has improved in terms of size, weight, comfort, cost and power. Today, for example, recreational athletes commonly monitor their heart rate with commercial, body-worn systems.
For at least as long as the space program has been around, the Army has conducted research on wearable monitoring technologies. When then-U.S. Sen. John Glenn flew on the shuttle Discovery in 1998, he swallowed a thermometer pill that monitored circadian body temperature rhythms in orbit, using a temperature monitoring system provided by the U.S. Army Research Institute of Environmental Medicine (USARIEM). A chest-worn system created through a successfully completed Army science and technology objective monitored the physiological responses of Austrian daredevil Felix Baumgartner during his 2012 jump from a helium balloon in the stratosphere. Soldier-monitoring capabilities have now matured to the point that they can be used to enhance safety and performance during work in hot environments. Thisfirst application of RT-PSM is being prepared for transition to the Program Executive Office for Soldier’s Integrated Soldier Sensor System program, through a technology transfer agreement with the U.S. Army Medical Research and Materiel Command.
Turning physiological data into actionable information needed by Soldiers makes the system worth the extra cost, weight, training and complexity. Raw data is interesting but not easily interpreted, even by a medic. For example, a high heart rate can variously indicate normal sympathetic activation needed to perform a task, cardiac compensation for hemorrhage or peripheral vasodilation in the heat, or response to an extreme psychological event. Similarly, measurement of core temperature is not, by itself, as useful as it might seem.
The wide range of normal core body temperature among warfighters was not well described until USARIEM field studies using wearable physiological monitors revealed how low core temperatures went in metabolically challenged Ranger School students, and how high they went during Marine patrolling activities in Iraq and Afghanistan. Laboratory studies would never have revealed these extremes of normal warfighter physiology. Clearly, contextual information and mathematical models that automatically interpret wearable sensor data streams are needed to interpret core temperature and provide actionable, individualized safety and performance information.
APPLYING THE TECHNOLOGY
Applications of RT-PSM technologies include dismounted route-planning decision support tools; performance and safety monitoring in high-risk chemical and biological threat environments requiring full protective gear; and performance and safety training for individuals and small-unit leaders. The use of RT-PSM to enhance Soldier performance and to avoid heat casualties is very different from medical management of casualties after they occur. Medics will bring their own U.S. Food and Drug Administration-certified medical devices to diagnose and treat casualties, upload secure medical data to central repositories and conduct remote telemedicine. In contrast, RT-PSM is not a medical system providing data for medical decisions, but rather a source of useful safety and performance information.
The Army National Guard (ARNG), an early adopter of thermal- or work-strain monitoring, is working with the USARIEM and Massachusetts Institute of Technology (MIT) Lincoln Laboratory to define requirements and concepts of operation. The ARNG’s Weapons of Mass Destruction Civil Support Teams (WMD-CSTs) train and respond to emergency events in full chemical, biological, radiological, nuclear and explosive protective gear. Chest-mounted physiological sensors provide work- and heat-strain data to downrange team members and to leaders at a command post (See Figure 1.) Other applications include the use of wearable sensors to quantify human thermal or work strain during field evaluations of new jungle uniforms performed by the Marine Expeditionary Rifle Squad “Gruntworks,” the human systems integration center at the Marine Corps Jungle Warfare Training Center, Camp Gonsalves, Okinawa, Japan.
In another Marine Corps research collaboration with USARIEM, RT-PSM technologies documented the physiological responses of Marines during foot patrols in Iraq and Afghanistan, resulting in knowledge that influenced Marine Corps field doctrine and tactics, techniques and procedures related to the conduct of patrols.
The current concept is to provide a read-out for the individual Soldier or squad leader that is comparable to a combination of engine temperature and tachometer—i.e., a thermal work-strain index based on heart rate and core temperature. According to extensive lab and field research by USARIEM, a high index indicates that someone is working close to his or her upper limit of cardiovascular performance and thermal tolerance and is likely to be stopped by the individual’s own physiological limits. This index has also been tested as a simplified, green-yellow-red stoplight risk designation for overall squad and individual squad member status.
Additional research by USARIEM, conducted in collaboration with the Gruntworks and with the Australian military’s Defence Science and Technology Group – Melbourne under a project arrangement, demonstrated that heart rate over time can be used to accurately estimate core body temperature, eliminating the need for temperature pills. This work has been validated as a patent submission and in peer-reviewed publications.
Most recently, a task force composed of USARIEM, MIT Lincoln Laboratory and the Marine Expeditionary Rifle Squad conducted a first test of thermal work-strain monitoring during Marine Corps training at Camp Geiger, NC, using a chest-mounted RT-PSM system that communicated status information to instructors via an operationally acceptable wireless data link. An important lesson learned: the training cadre, realizing that the trainees were at relatively low thermal work strains and could be further challenged, wanted to use the information to push trainees harder than they would have without the monitoring. This demonstrates the kind of innovation that comes from the end users themselves during iterative field testing, suggesting, in this case, a “precision medicine” application of RT-PSM to more effectively train each individual according to his or her current level of fitness and acclimatization.
A MORE CAPABLE SOLDIER
Information from RT-PSM can also be used in mission planning and route finding. In collaboration with the U.S. Department of Agriculture’s Beltsville, MD, Human Nutrition Research Center, USARIEM’s Dr. Mark Buller tested the use of RT-PSM and novel algorithms to provide continuous pacing guidance to individuals who were tasked with completing a five-mile run within one hour under thermally challenging conditions. They were to pace themselves to complete the mission on time but not overheat, and to arrive as cool and physically capable as possible.
The pacing feedback supplied by Buller significantly improved their performance in comparison to a separate trial in which each individual used his or her own pacing strategy.
This approach could be used to train optimal movement strategies and guide redistribution of the workload among individuals in a team. The route planning and pacing algorithms are also excellent candidates for integration into the U.S. Army Geospatial Center route-planning tools under development.
Robert A. Heinlein captured the imagination of many when he described a wearable physiological status monitor on every soldier in his sci-fi novel “Starship Troopers.” Today, that is as quaint as the old Tom Swift books that imagined a day when spaceships might land on the moon. Although the current, affordable wrist-worn technologies have already far surpassed Heinlein’s vision, the Army’s interest centers on chest-based sensors. Current wrist- and arm-based sensors are power-hungry, largely proprietary and prone to motion artifacts; use militarily unacceptable modes of wireless communication; and cannot provide information obtained from the chest, such as respiration rates and body position.
An alternate approach to directly monitoring individual physiological responses is to use rational mathematical models to predict Soldier limits. Well-validated USARIEM thermal models can provide mission planning guidance and generalized predictions but are not intended for real-time use and will not precisely predict individual responses. A special exception is the USARIEM Probability of Survival Decision Aid used by U.S. Coast Guard Search and Rescue (https://www.uscg.mil/announcements/ALCOAST/325-10_alcoast.txt).
Key targets beyond thermal-work strain include assessments of hydration state, readiness and alertness, and musculoskeletal fatigue and strain. These may include unobtrusive sensors to monitor water consumption and loss; goggles that monitor eye responses; communication systems that also measure voice changes, speech content and breath chemistry; skin sensors that assess stress and alertness; and sensors to monitor extremity temperatures to protect and sustain performance in cold weather. Simple helmet or boot sensors could detect ground impact forces and lower extremity patterns of movement that can provide useful information about impending injury, fatigue and even behavioral changes.
USARIEM and Lincoln Laboratory have a longer-range plan to combine physiological monitoring with outward-looking detectors to trigger threat alarms that allow Soldiers to don protective ensembles. Even military working dogs may benefit from current work to develop thermal work-strain monitors based on collar-worn acoustic sensor systems analyzing panting patterns. Miniature, possibly implantable, body-heat powered sensors will be even better and are around the corner, leveraged in part by Small Business Innovation Research contracts and by an earlier Army program called Technologies for Metabolic Monitoring. The Army is also leveraging the National Science Foundation’s Center for Advanced Self-Powered Systems of Integrated Sensors and Technologies program that is developing the next-generation, ultra-low-power RT-PSM “system on a chip.”
Wearable physiological monitoring technologies are essential tools needed to understand Soldier physiology in training and operational field environments. Knowledge gained will lead to new insights into individual and small-unit leader readiness and help guide changes in field doctrine, materiel development targets and strategies, and trade-space analyses. Physiological models can embody knowledge gained from field and lab studies, and enable predictions for conditions not yet experienced. This capability goes beyond simply duplicating the roles of good leadership and training; it is an important part of what makes real-time monitoring useful.
Working Soldiers until failure is costly—there are the long-term costs of musculoskeletal injury and health management, lost expertise and time and expense of training replacements. RT-PSM systems can identify markers of compromised performance or safety, enabling early intervention when a known need exists.
For more information go to http://www.usariem.army.mil/index.cfm/about/divisions/bbmd.
Dr. REED W. HOYT is the chief of the Biophysics and Biomedical Modeling Division at USARIEM, Natick, MA. He holds a Ph.D. in physiology from the University of New Mexico School of Medicine. He has published more than 150 papers and technical reports and holds 10 patents. He is Level III certified in systems engineering.
Dr. KARL E. FRIEDL (COL, USA Ret.) is a fellow in the Oak Ridge Institute for Research and Education Knowledge Preservation Program, supporting the Biophysics and Biomedical Modeling Division at USARIEM. He holds a Ph.D. in physiology from the University of California at Santa Barbara. He retired after 30 years of Army active-duty service, during which he served as director of the Army Operational Medicine Research Program; commander, USARIEM; and director, U.S. Army Telemedicine and Advanced Technology Research Center.
This article was originally published in the January – March 2016 issue of Army AL&T magazine.
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