Physiological responses of female fire fighters to repeated work bouts.

Abstract

Often times in the field, fire fighters work for a period of time, exit the burning building, recover, and then reenter the building to work again. Performing fire-fighting activities during multiple work bouts in the heat suggests that fire fighters may elicit higher heart rates and core body temperatures during a second work bout. However, whether or not they elicit these responses when given adequate recovery time and adequate fluid replacement has not been researched. PURPOSE: This study determined the physiological responses of female fire fighters to two similar bouts of work. Peak oxygen consumption (V[O.sub.2]max), peak heart rates (HR), peak mean skin temperatures (MST), peak rectal temperatures (Trec), and ratings of perceived exertion (RPE) were compared between two work bouts. METHODS: Seven female fire fighters walked for 23 minutes while dressed in complete fire fighting ensemble at 50% of their V[O.sub.2] max in a 40[degrees]C environment. After the first work bout (W[B.sub.1]), subjects rehydrated and rested until their [T.sub.rec] returned to baseline levels. They completed a second work bout (W[B.sub.2]) similar to W[B.sub.1]. V[O.sub.2], HR, [T.sub.rec], MST and RPE were measured throughout the experimental period. RESULTS: The differences in [T.sub.rec] and HR between W[B.sub.1] and W[B.sub.2] were significantly higher during W[B.sub.2]: H[R.sub.1]=149[+ or -]18 bpm vs. HR2=161[+ or -]12 bpm, (p=0.008), and [T.sub.rec1]=37.77[+ or -]0.19[degrees]C vs. [T.sub.rec]2=38.15[+ or -]0.25[degrees]C, (p=0.004). CONCLUSION: The increase in [T.sub.rec] in W[B.sub.2] was small (0.38[degrees]C), and it was attributed to the higher baseline [T.sub.rec] prior to W[B.sub.2]. The higher HR in W[B.sub.2] was attributed to the higher [T.sub.rec] and cardiovascular drift.

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Introduction

This study investigated the physiological responses of female fire fighters to two 23-minute bouts of work in a 40[degrees]C (105[degrees]F) environment at an intensity of 50% of their maximal working capacity (V[O.sub.2]max). They were dressed in full fire-fighting ensemble and walked on a treadmill for 23 minutes twice, while oxygen consumption, heart rate and rectal temperature were measured.

When actually fighting structural fires, fire fighters work for a period of time, exit the burning building, recover, and then reenter the building to work again. Performing fire-fighting activities during multiple work bouts in the heat suggests that fire fighters may elicit higher heart rates and core body temperatures during a second or third work bout. However, whether or not they elicit these responses when given adequate recovery time and adequate fluid replacement has not been studied.

Methods

Seven incumbent female fire fighters (30-42 years) from local fire departments participated as subjects. They were dressed in full fire ensemble: Morning Pride[R] pants and jacket, Fireguard Crosstech[R] gloves, a Nomex[R] hood, a helmet, a facemask and a self-contained breathing apparatus (SCBA). To facilitate easier walking on the treadmill, athletic shoes were worn; however, the weight of boots was added to each ankle to reproduce the weight of the boots. The full fire-fighting ensemble weighed a total of 21 kilograms.

Prior to the study, subjects performed a maximal treadmill test to determine V[O.sub.2]max using open circuit spirometry. After the V[O.sub.2]max was determined, 50% was calculated for the intensity of the work during the study. The protocol consisted of the subjects walking on a treadmill at a speed and grade that produced 50% of their V[O.sub.2]max for 23 minutes in an environmental chamber controlled at 40[degrees]C (19% RH). This was done twice with a rest period between each work bout. During each work bout, oxygen consumption, heart rate, rectal temperature and rating of perceived exertion (RPE) were continuously monitored. Following each work bout, subjects exited the environmental chamber and sat in front of a fan with all fire protective clothing and equipment removed until their rectal temperature returned to within 0.5[degrees]C of their pre-exercise temperature. The average length of time it took for them to recover was twelve minutes after the first work bout and 18 minutes after the second bout. During the recovery period, they ingested sufficient Gatorade[R] to replace the fluids lost as sweat during the work bout.

The amount of sweat lost during the work bout was determined on the previous day by comparing the nude body weight before and after a walk under identical conditions. A kilogram of body weight lost was equivalent to a liter of sweat. The average weight loss was 0.56 kilograms. One liter of Gatorade[R] was given for every kilogram lost; therefore, the average fluid replacement volume was 0.56 liters.

Results and Conclusions

A paired t-test revealed no significant difference between the peak V[O.sub.2] of the first and second work bouts (p=0.124). Oxygen consumption during the first and second work bouts was the same, indicating that subjects were working at the same intensity for both work bouts.

The comparison of the heart rate and rectal temperature during work in the heat between the two work bouts revealed that rectal temperature and heart rate were significantly higher in the second work bout when compared to the first work bout. A paired t-test revealed a significant difference between the peak heart rates of the first and the second work bouts (p=0.008). Peak heart rates were determined by averaging the last minute of each work bout. The peak heart rate (Figure 1) of the second work bout (H[R.sub.2]=161[+ or -]12 bpm) was significantly higher than the first work bout (H[R.sub.1]=149[+ or -]18 bpm). A difference of 12 bpm indicated that the cardiovascular system was under greater stress during the second work bout.

A third paired t-test revealed that the peak rectal temperature (Figure 2) of the second work bout ([T.sub.rec2]=38.15[+ or -]0.25[degrees]C) was significantly higher than the first work bout ([T.sub.rec1]=37.77[+ or -]0.19[degrees]C, p=0.004). Although the peak rectal temperature in the second work bout was statistically higher, the increase was small (0.38[degrees]C). The higher rectal temperature in the second work bout was mainly attributed to the higher baseline rectal temperature prior to the second work bout.

The significantly higher heart rates observed in the second work bout were attributed to the increase in rectal temperature, cardiovascular changes and dehydration. There is a positive relationship between rectal temperature and heart rate (1). Therefore, since rectal temperature was higher in the second work bout, heart rate would also be expected to be elevated. The increase in rectal temperature resulted in an increase in blood flow to the skin due to subcutaneous vasodilation, which is the thermoregulatory mechanism to help dissipate heat through the skin. This mechanism shunts blood away from the working muscles, and causes a decrease in venous return and stroke volume. Therefore, the heart rate increases to compensate for the decrease in stroke volume. This is termed cardiovascular drift, where the increase in heart rate and the decrease in stroke volume are equal in magnitude, but opposite in direction (2). Cardiovascular drift is attributed to the competition for blood flow between the working muscles and skin, as well as the decreasing plasma levels during dehydration; however, research has documented that sufficient fluid replacement attenuates cardiovascular drift (3).

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The increased heart rate can also result from dehydration and/or the incomplete absorption of fluid replacement into the plasma during recovery. Although subjects adequately replaced their sweat losses between work bouts, the extent of fluid absorption into plasma was not measured. Hematocrit measurements were used to determine the changes in plasma volume. In this study, hematocrit was measured before the exercise and during the two recovery periods. There were no significant changes in hematocrit measurements, indicating that subjects did not have a significant water deficit.

Since it is widely accepted that dehydration is detrimental to work performance (4,5,6), adequate rehydration between multiple work bouts in the heat is mandatory. Subjects adequately rehydrated ab libitum when given one serving of fluid; they were not forced to finish the drink. Kenney (7) explains this:

"Surprisingly, if cold palatable beverages are within arm's length of well-trained, experienced, exercising athletes, most drink enough to offset sweat losses. However, if they have to move even a short distance to get a drink bottle, voluntary dehydration usually occurs. This suggests that a complex behavioral component is involved in determining fluid consumption by exercising athletes. In less experienced athletes, and in almost all athletes after exercise, fluid intake rarely balances sweat losses."

Since subjects did not have large sweat losses (0.56 L), they were able to adequately rehydrate. Sweat rate rises proportionately to environmental heat stress and the metabolic work rate (8). Therefore, an increase in environmental temperature and workload caused the subjects' sweat rates to increase accordingly. If subjects worked in hotter temperatures and performed more strenuous work, a greater sweat loss would occur and they would require a greater fluid replacement. Therefore, the probability that they would adequately rehydrate in the twelve- or eighteen-minute recovery period is reduced.

The combined effect of exercise and hyperthermia poses the second greatest stress on the cardiovascular system (9). These adverse affects are two-fold: First, since the skin and muscles compete for blood flow, their demands can be too large for the cardiac output. Second, cardiac filling and the resultant stroke volume decrease as cutaneous vasodilation shunts the blood to the skin. Ultimately the increase in skin blood flow will result in the failure to maintain adequate blood flow to the working muscles. Failure to maintain adequate blood flow to the muscles results in the termination of work; failure to maintain adequate blood flow to the skin results in hyperthermia (10).

During rehydration, an effective way of replacing the decreasing blood volume due to sweating is by combining sodium chloride with water (11). Fluid retention during a three-hour period of plain water rehydration is 51% of what was lost, whereas salt in conjunction with the water resulted in a 71% net fluid retention (10). When salt is ingested with the water, plasma sodium concentrations are sustained for a greater duration of the rehydration period. The intake of sodium and water also resulted in a significantly higher plasma sodium concentration when compared to water alone. Sodium chloride delays urine production and maintains the salt-dependent thirst drive, leading to a more complete restoration of body water content (12).

To decrease the risk of progressive dehydration during exercise, water and electrolytes should be replaced at the same rates at which they are lost (12). Furthermore, the optimal replacement fluid should contain about one gram of carbohydrate per minute of work, and an adequate amount of electrolytes that will maintain serum osmolality and plasma volume (13).

Montain and Coyle (14) demonstrated that fluid replacement maintained blood volume. Therefore, fluid replacement between fire suppression work bouts is necessary in order to maintain blood volume and venous return. As blood volume decreases with dehydration, there is a further decrease in venous return to the heart, which decreases stroke volume. This decrease in stroke volume decreases the physical working capacity of the fire fighters.

In conclusion, subjects in this study were allowed to recover until rectal temperatures returned to 0.5[degrees]C above baseline temperatures. The time it took for rectal temperatures to meet this criterion was twelve and eighteen minutes for the first and second recovery periods, respectively. Since rectal temperatures did not return to the original baseline levels after the first work bout, the rectal temperature in the second work bout was higher. The higher rectal temperature led to a higher heart rate, thus increasing the stress on the cardiovascular system.

This study was conducted in a controlled laboratory setting, where the workload and working conditions were understandably less stressful than an actual situation in the field. However, results of this study indicate that subjects exhibited higher rectal temperatures and heart rates in the second work bout even when given adequate fluid replacement between work bouts. Although the ambient temperature, the workload and the recovery time are not easily controlled in the field, ample fluid replacement during recovery periods should be provided to offset sweat losses, and attenuate cardiovascular drift.

This study was supported by the Las Vegas Fire Union, Local 1285; Las Vegas Fire and Rescue Department; and the University of Nevada, Las Vegas, Graduate Student Association.

REFERENCES

1. Skoldstrom B. Physiological responses of fire fighters to workload and thermal stress. Ergonomics 1987; 30(11): 1589-1597.

2. Guyton AC, Hall JE. Textbook of medical physiology (10th ed). Philadelphia, W.B. Saunders Company, 2000.

3. Coyle EF, Montain SJ. Thermal and cardiovascular responses to fluid replacement during exercise. In: Perspectives in exercise and sports medicine, vol. 6: Exercise, Heat, and Thermoregulation, Gisolfi CV, Lamb DR (Eds). Indianapolis, IN, Benchmark Press, 2001.

4. Murray R. Fluid needs in hot and cold environments. International Journal of Sport Nutrition 1995; 5: S62-S73.

5. Montain SJ, Smith SA, Matott RP, Zientara GP, Jolesz FA, Sawka MN. Hypohydration effects on skeletal muscle performance and metabolism: A 31P MRS study. Journal of Applied Physiology 1998; 84: 1889-1894.

6. Sawka MN, Pandolf KB. Effects of body water loss on physiological function and exercise performance. In: Perspectives in Exercise and Sports Medicine, vol. 3: Fluid Homeostasis during exercise, Gisolfi CV, Lamb DR (Eds). Indianapolis, IN, Benchmark Press, 1990.

7. Kenney WI. Sports Science Exchange Roundtable: Why don't athletes drink enough during exercise, and what can be done about it? Gatorade Sport Science Institute 2001; 12(1): 2.

8. Robinson S, Robinson AH. Chemical Composition of Sweat: Physiological Reviews 1954; 34: 202-220.

9. Rowell LB. Circulatory adjustments to dynamic exercise and heat stress: competing controls. In: Human Circulation During Physical Stress. New York, Oxford University Press Inc., 1986.

10. Nose H, Mack GW, Shi X, Nadel ER. The role of osmolality and plasma volume during rehydration in humans. Journal of Applied Physiology 1988; 65(1): 325-331.

11. Nadel ER, Mack GW, Takamata A. Thermoregulation, exercise, and thirst: Interrelationships in humans. In: Perspectives in Exercise and Sports Medicine vol. 6: Exercise, Heat, and Thermoregulation, Gisolfi CV, Lamb DR, Nadel ER (Eds). Traverse City, MI, Cooper Publishing Group, 2001.

12. Ladell WS. Water and salt (sodium chloride) intakes. In: The Physiology of Human Survival, Edholm O, Bacharach A (Eds). New York, Academic Press. 1965.

13. Noakes TD. Fluid replacement during exercise. Exercise and Sport Sciences Reviews 1993; 21: 297-330.

14. Montain SJ, Coyle EF. The influence of graded dehydration on hyperthermia and cardiovascular drift during exercise. Journal of Applied Physiology 1992; 73(4): 1340-1350.

by Paulette M. Yamada, M.S. and Lawrence A. Golding, Ph.D., FACSM 

Laboratory of Exercise Physiology, Department of Kinesiology, University of Nevada, Las Vegas