Effects of live fire training on recruits

BY D.L. SMITH, Ph.D.; T.S. MANNING, M.S.; AND S.J. PETRUZZELLO, Ph.D.

Firefighting is a physically and psychologically demanding profession and a potentially hazardous occupation. During the five-year period from 1995 to 1999, an average of 99 firefighters per year lost their lives in the line of duty.¹ The leading cause of death among on-duty firefighters in the United States during the past five years has been heart attack, accounting for 40 percent to 49 percent of the deaths each year.^2-6 (1)

Firefighting places considerable strain on the cardiovascular system because of the heavy work firefighters perform (including carrying the weight of the personal protective equipment) and thermal strain. Thermal strain results from several factors, including (a) the metabolic heat produced by working muscles; (b) the heavy, insulative, nonpermeable protective gear, which adds to the metabolic work that must be done and traps the metabolic heat next to the body; and, in some cases, (c) the radiant heat associated with the fire.

The cardiovascular demands of heavy muscular work and increased body temperature result in competing demands for blood flow to (a) the metabolically active muscles to support heavy muscular work, (b) the skin in response to the thermoregulatory demands resulting from muscular work in a hot environment, and (c) vital organs (including the brain and heart) to support life. The cardiovascular demands are exacerbated by excessive fluid loss through profuse sweating and vasodilation of vessels in the skin and muscle.

Despite National Fire Protection Association (NFPA) statistics, which report that heart attack is the leading cause of death among U.S. firefighters, very few studies have documented the magnitude of the cardiovascular strain of firefighting beyond recording heart rate (HR). Several studies have reported that HR reaches maximal or near maximal values very quickly and remains elevated during fire suppression activities.^7-11 However, little data are available regarding changes in the pumping capacity of the heart [measured as stroke volume (SV-the amount of blood ejected from the heart with each beat)] as a result of strenuous, firefighting activities. (11)

Documenting changes in stroke volume is important because hyperthermia and profuse sweating have the potential to decrease SV during firefighting activities. Furthermore, the magnitude of associated psychological strain has not been carefully studied despite the fact that the firefighter’s perception of exertion, thermal sensations, and respiratory distress are critical factors in determining work time during a live fire situation. Therefore, this study was undertaken to describe cardiovascular (HR, SV) and psychological responses during strenuous firefighting activities in a training structure that contained live fires.

STUDY PROTOCOL

We studied seven apparently healthy men who were recruited from a University of Illinois Fire Service Institute Fire Academy course, a six-week residential training program for recruit firefighters. Prior to testing, all participants signed written informed consent documents indicating that they understood all procedures, risks, and benefits of the study and indicated that their participation in the project was voluntary. The testing protocol was approved by the Human Subjects Institutional Review Boards at the University of Illinois and Skidmore College.

Prior to testing, all participants underwent an echocardiography examination at a local medical center to measure cardiac dimensions and the cross-sectional area of the aorta. A trained ultrasound technician obtained all measurements with the subject in the supine position.

Cardiac and psychological data were collected during live fire training drills during the fifth week of a six-week fire academy course. By the fifth week of this course, subjects were familiar with the firefighting tasks they would perform and had been exposed to live fire drills. For this study, subjects completed three trials of a standardized set of firefighting tasks in a training structure that contained live fires. Subjects wore turnout gear issued by their respective departments, including bunker coat and pants, helmet, hood, gloves, boots, and self-contained breathing apparatus (SCBA). In all cases, the gear met National Fire Protection Association (NFPA) standards. The total weight of the gear and SCBA averaged 26.2 (±1.8) kg.

Every subject completed three trials of a standardized set of tasks. Each of the three trials consisted of the same four tasks: dragging a hose dummy, carrying a five-gallon extinguisher up two flights of stairs and discharging it, hoisting a hose, and chopping on a block of wood. Between the first and the second trial, SCBAs were changed, data were collected, and subjects were returned to the course as soon as possible (approximately three minutes). Following the second trial, subjects rested for approximately 10 minutes before returning to the course to complete the third trial. Data were collected prior to initiating the firefighting activities and at the end of each trial.

All measurements were made on the third floor, in a measurement station that was maintained at near-ambient temperatures. There were live fires on the second and fourth floors. After initial measurements were obtained, subjects descended an interior stairwell (15 steps) and performed the dummy drag on the second floor. The dummy drag entailed dragging on hands and knees a 24.1-kg hose dummy around the perimeter of the room two times (room dimensions were 4.3 2 5.5 m).

Following completion of the dummy drag, the firefighters carried a 23.6-kg bucket up two flights of stairs (30 steps) and discharged a 21/2-gallon hand-pump can on the fourth floor. The subjects then went out on the fourth-floor balcony and hoisted a 19.5-kg hose from the second-floor balcony to the fourth floor, a distance of 8.4 m, and then lowered it in a controlled manner.

Finally, subjects descended the interior stairwell from the fourth to the third floor (15 steps) and performed the chopping task. The chopping task involved moving a 109.1-kg wooden log 1.5 m along a sled using a 7.3-kg sledge. At the end of the trial, subjects returned to the testing station immediately adjacent to the wood-chopping area. Measurements were initiated within 30 seconds of leaving the training course and were typically completed within three minutes.

HR was assessed by an HR watch. A portable Doppler ultrasound unit was used to measure velocity of blood in the ascending aorta. The acquisition of the Doppler data began as soon as possible on the subject’s entering the measurement station (initiated within one minute). SV was calculated as the product of the cross-sectional area of aorta (CSA-measured by echocardiography prior to testing), and the velocity of blood in the aorta (Time Velocity Integral-TVI), measured by Doppler ultrasound, according to the following formula¹² :

SV (cc³) = CSA (cm²) 2 TVI (cm)

Ratings of perceived exertion (RPE), perceptions of respiratory distress and thermal sensations, and state anxiety were measured immediately following each trial. Ratings of perceived exertion were recorded using the 15-point Borg scale.¹³, where perceptions of exertion ranged from 7 = “very, very light” to 19 = “very, very hard.” Perceptions of respiratory distress were assessed using a seven-point psychophysical category scale developed by Morgan and Raven with anchors that ranged from “My breathing is okay right now” to “I can’t breathe.”¹⁴ Perceptions of thermal sensations were assessed using the eight-point rating scale developed by Young et al with verbal anchors ranging from unbearably cold to unbearably hot.¹⁵

TEST RESULTS

Subjects performed three consecutive trials of firefighter drills; each trial consisted of the same four tasks performed in the same order. A rest period of approximately 10 minutes separated the second and third trials (to simulate on-scene rehabilitation). The time to complete the trials averaged approximately seven minutes (Trial 1 = 7:05; Trial 2 = 6:47; Trial 3 = 7:14) and was not significantly different among the trials. Temperatures for the testing areas on the second, third, and fourth floors averaged 46.67C, 49.37C, and 60.57C, respectively.

Descriptive characteristics and heart dimension measurements of the subjects are given in Table 1. Subjects were young, apparently healthy male recruit firefighters. Aortic diameter, assessed by echocardiography, was 3.3 cm. Cross-sectional area (calculated from a geometric model based on diameter) averaged 8.6 cm². Measures of septal thickness, post wall thickness, and left ventricular chamber dimensions were all within normal limits.¹⁶

HR increased significantly throughout the protocol. At the end of Trial 1, mean HR was 175 b-min^-1 and increased to an average of 189 b-min^-1 at the end of Trial 3. The HR at the end of Trial 3 was identical to age-predicted maximum HR based on mean age of 31 years (220131 = 189). By 16 minutes of recovery, the HR was not significantly different from pre-task values.

Although SV increased by 25 percent from the pre-task values to immediately post-Trial 1, this difference did not achieve statistical significance. There was a significant decrease in SV at the end of Trial 2 and Trial 3, compared with the end of Trial 1. SV was approximately 35 percent lower at the end of Trial 3, compared with Trial 1. This represents a statistically significant and clinically important reduction in the heart’s pumping capacity.

All three perceptual variables showed significant changes over time, indicating greater distress with progressive work. Perceived exertion increased significantly from Trial 1 to Trial 3. RPE increased progressively across trials, with the second trial evincing greater perceived effort than the first, and the third more than the second.

Respiratory distress and thermal sensations showed significant increases across trials. In both cases, the increase from Trial 1 to Trial 3 was statistically significant.

DISCUSSION

This study was conducted to characterize the cardiovascular and psychological responses of recruit firefighters to strenuous firefighting activities. Our data suggest that repeated, short bouts (approximately seven minutes) of strenuous firefighting activity in a building that contained live fires resulted in age-predicted maximal HRs and a significant reduction in SV in this group of firefighters. The cardiovascular responses were mirrored by changes in psychological variables.

The HRs reported in this study are similar to data that have been reported during live fire situations and live fire training drills. (7),¹⁷, (8) (9) (10) Other studies have found lower HRs than we are reporting during simulated firefighting activities.^{18, 19, 20} and during actual emergencies. (8) The differences in HR values reported by different authors are likely attributable to differences in the intensity of the firefighting task and radiant heat load.

SV was lower following the second and third trials compared with the end of the first trial. In fact, SV decreased approximately 35 percent from the end of the first trial to the end of the third trial. Furthermore, the dramatic decrease in SV at the end of the third trial occurred despite a 10-minute rehabilitation period during which firefighters consumed fluid. It is known that SV is lower during prolonged exercise in the heat as compared with the same activity performed under thermoneutral conditions. (20) What is notable in this study is that the marked reduction in SV occurred after three short bouts of firefighting activities (totaling approximately 21 minutes) in the hot environment. The reduction in SV may be caused by decreased venous return (caused by vasodilatation in the vessels in the working muscles and skin) or a reduction in plasma volume (due to profuse sweating) or, as is most likely the case, a combination of these factors. Psychological responses to firefighting activities indicate progressively greater stress with subsequent trials.

This study has documented that strenuous firefighting activities performed in a hot, hostile environment result in sizable and significant changes in both cardiovascular and psychological parameters. Given what we know about the magnitude of the cardiovascular response, particularly the reduction in SV, it is critically important that

• firefighters are fully hydrated prior to beginning firefighting activities,
• firefighters be encouraged to drink plenty of fluids at the fire scene, and
• firefighters aggressively rehydrate following fire suppression activities.

Development of and adherence to standard operating procedures for on-scene rehabilitation should play an important role in addressing the adverse cardiac responses to firefighting activities.

We acknowledge the National Volunteer Fire Council for financially supporting this research through a research grant. We also wish to thank the staff of the University of Illinois Fire Service Institute, particularly David Clark, Brad Bone, and Jim Straseske, for their invaluable support during this project. Finally, we would like thank the subjects who volunteered to participate in this study.

References

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D.L. SMITH, Ph.D., is a member of the Department of Exercise Science and Dance at Skidmore College, Saratoga Springs, New York.

T.S. MANNING, M.S. , is with the Department of Medicine: Division of Clinical Pharmacology, School of Medicine and Biomedical Sciences, State University of New York at Buffalo.

S.J. PETRUZZELLO, Ph.D., is a member of the Department of Kinesiology, University of Illinois, Urbana.