Prediction of Heat Strain by Heart Rate for Firefighters in Protective Clothing

Abstract

The current study aimed to design simple physiological heat strain models and criteria which can be used in real-time monitoring system of firefighters health to prevent heat-related illness on the line of duty. Heart rate was chosen as the main predictor because of its simple instrumentation and popularity. The study consists of two parts: First, an investigation on the actual frequency of Korean firefighters heat-related illness and influential factors aggravating heat strain. Second, development of heat strain models mainly using well stabilized resting heart rate. In the second part, potent additional variables were searched along with a series of validity tests in various conditions. <br/> <br/> In the first part, nationwide survey on Korean firefighters heat strain was conducted (N=674 in total). It was revealed that firefighters routinely experienced mild heat-related illness (HRI) symptoms. Among firefighters, 74.8% of them experienced HRI symptoms, and 5% of firefighters experienced more than 30 times of muscle cramps, headache in a year. Also, these firefighters experience more than 40 times of dizziness and more than 50 times of nausea. Nevertheless, majority of firefighters (98.5%) did not report those experiences because they have always done that. Firefighters heat strain was aggravated by short duration of rest and inefficient cooling practices during the rest periods. However, the primary difficulties to meet the sufficient rest were: 1) Firefighters were pushed to re-start working in emergent situations. It can be contributed by lack of personnel and rehabilitation system, 2) Self-identified physiological restoration commonly used by firefighters (41%) cannot work well in detecting their heat strain especially for the heat-acclimatized group like firefighters. The following step was carried out to develop an easy physiological method to replace firefighters self-diagnosis of heat strain and to facilitate real-time heat strain monitoring system. <br/> <br/> The second part was prediction of rectal temperature by well stabilized resting heart rate (rHR). Stabilized heart rate was used because heart rate for initial 1-2min after exercise is drastically changing and is affected by physical exertion. Three models were presented: Model 1 for the prediction of rectal temperature (rTre), Model 2 for the prediction of increase in rectal temperature during exercise (∆T_re) for duration time of t, and Model 3 for the prediction of rectal temperature at the end of exercise (eTre) as follows: <br/> <br/> Model 1: rT_re=36.36+1.3×〖10〗^(-4)×rHR^2 (Marginal R2=0.8, 95% PI: 0.61oC) <br/> Model 2A: ∆T_re= =-0.71854+0.00535∙rHR+0.05173∙t (Marginal R2=0.865)<br/> Model 3A: eT_re=rT_re+ ∆T_re (Pseudo-R2=0.716, 95% PI: 0.54oC)<br/> <br/> From this model, resting heart rate criteria was suggested, which allowed firefighters to evaluate the current heat strain and it also provided precautionary criteria by predicting the future heat strain after working. For example, the resting heart rate of 110 bpm corresponds to 37.9oC (95% PI 0.6oC), but it can also indicate that rectal temperature can reach to 39.2oC after additional 20 min work. Thus, the firefighter need to rest more. On the other hand, resting heart rate of 90 bpm was acceptable level not to induce heat illness after 20 min firefighting. The models were verified under the following environmental settings: ambient temperatures within 25-32oC, wearing full firefighters clothing and gears (9-15 kg