Physiological Responses to Prolonged Exercise in Extreme Heat Conditions: a Case Study

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JEPonline

Physiological Responses to Prolonged Exercise in Extreme Heat Conditions: A Case Study

Mark Hosler1, Victoria Hosler2, Chase Tobin3, Brandon Strop4, Walter Schroeder5, Lee Beckwith1

1Department of Pathology, Southeast Hospital, Cape Girardeau, MO, USA, 2Truman State University, Kirksville, MO, USA, 3Eastern Virginia Medical School, Norfolk, VA, USA, 4Department of Biology, Southeast Missouri State University, Cape Girardeau, MO, USA, 5Cape County Otolaryngology, Cape Girardeau, MO, USA

ABSTRACT

Hosler MW, Hosler VC, Tobin CA, Strop CB, Schroeder WA, Beckwith LG. Physiological Responses to Prolonged Exercise in Extreme Heat Conditions: A Case Study. JEPonline 2014;17(4):1-9. The purpose of this case study was to examine the physiological responses of a male subject 59 yrs of age while running eight 1-mile circuits under extreme heat conditions. The subject’s body was previously acclimated to exercise in hot conditions with adequate fluid intake. Body temperature increased very slightly from 37.83°C before exercise to 37.94°C during exercise. Serum sodium level increased from 140 mmol·L-1 before exercise to 149 mmol·L-1 after exercise, which is consistent with mild dehydration. The body of this well-trained athletic elderly male responded well to exercise under extreme heat conditions through physiologic mechanisms that included copious sweating and reduced renal blood flow.

Key Words: Heat Acclimation, Running, Sweating, Renal Blood Flow

INTRODUCTION

An extensive amount of literature addresses the dangers of exposure to outdoor heat during the summer months (2-4,7,10,14,15,17). In the present study, extreme conditions of heat combined with exercise could be ethically explored because the subject was one of the authors (MWH) and the standard circumscriptions of experimental human protocols could be suspended.

The purpose of this case study was to test the hypothesis that a fit elderly male’s body, previously acclimatized to exercise in hot conditions, with adequate fluid intake, could physiologically respond well to exercise under extreme heat conditions.

METHODS

Subject

The subject was a male 59 yrs of age with a body weight of 68.2 kg. Past medical history included tonsillectomy at age 9 and severe unipolar affective disorder at age 58. The subject’s only exercise activity was long distance running with an emphasis on racing performance. The subject had a 26-yr history of running and road racing with approximately 105,000 total miles logged. The subject was well adapted to heat and had previously run many midday miles at high temperatures and for prolonged periods of time. He had demonstrated the ability to sweat copiously and to lose 1 L (2.2 lbs) of sweat every 16 min.

Procedures

The subject ran intervals of 1 mile each, for a total of 8 miles, under extreme heat conditions over the course of 90 min. Elapsed time for each mile, and heart rate were recorded for each of the 8 miles. Body temperature was recorded before exercise and after miles 2, 4, 6, and 8. Refer to Table 3 for elapsed time per mile, body temperature, and heart rate.

In addition, blood and urine samples were taken and analyzed before exercise and immediately after exercise. Blood, but not urine, samples were analyzed immediately after 10 min of forced fluid intake after exercise. The collected blood and urine samples were submitted for tests for a battery of laboratory analytes considered to be most affected by dehydration. Blood and urine samples were analyzed in the laboratory of Southeast Hospital in Cape Girardeau, MO, USA (using a Siemens Bayer ADVIA ® 1650 Chemistry Analyzer).

Glomerular Filtration Rate was estimated by Meditech Laboratory Information System using the simplified Modification of Diet in Renal Disease (MDRD) formula: glomerular filtration rate (ml·min-1 ÷ 1.73 m2) = 175 x [plasma creatinine]-1.154 x [age]-0.203 x [1.212 if patient is black] x [0.742 if patient is female](5,6). The analyte laboratory results are presented in Table 4. Subjective responses were also recorded. The study was undertaken in southeast Missouri and in outdoor conditions purposefully intended to replicate the most extreme heat of ambient summer temperatures. The aerobic exercise trial began at 3:45 pm Central Daylight Savings Time and finished at 5:15 pm on July 18, 2006. Conditions were sunny without clouds during the entire run (Table 1 and Table 2).

Table 1. Pre-Exercise Outdoor Environmental Conditions.

Table 2. Post-Exercise Outdoor Environmental Conditions.

The running track was black, which maximized the absorption of electromagnetic waves. The dark track also increased the track-level temperature as the sun’s energy waves were transduced and re-emitted as heat. The track surface temperature was 51°C (123.8°F). The subject wore a black GORE-TEX® suit with extra layers of clothing to increase body heat retention.

RESULTS

As planned, the subject ran a series of 1-mile circuits and ran to complete exhaustion. The subject was able to complete 8 circuits (8 miles) before experiencing exhaustion (Table 3). At the conclusion of exercise, the subject experienced no pain. But, the subject did experience the degree of discomfort associated with any maximal physical effort lasting longer than 1 min (e.g., such as running an 800 m race or racing for 10,000 m).

Neither during the exercise nor after completion of the exercise did the subject experience a sensation of heat, being hot, or being overheated. The subject noted that his pace per mile was slower than the pace that he could have been maintained under more normal conditions and with similar effort (1,2,4,9,11,16). This observation correlated with the subject’s previous experiences when running during extreme heat.

Table 3. Mile Time, Body Temperature, and Heart Rate.

Table 4. Physiologic Analytes of Interest from Peripheral Blood and Urine Samples.

Unless otherwise stated, analytes are from peripheral blood samples. QNS = quantity of sample not sufficient for test. N/A = Not Applicable / Test Not Performed. uL= microliter; fL= femtoliter; pg = pictogram

DISCUSSION

The body of this well-trained and heat-acclimated subject responded physiologically very well to exercise under extreme heat conditions. Copious sweating cooled the body well, which is indicated by the relatively little change in body temperature. As to the subject’s renal function, glomerular filtration rate decreased. This latter response is typical of strenuous exercise (12). Subjective findings also support the physiological response.

Discussion of Subjective Findings

The cessation of exercise was determined by the subject after completing eight 1-mile circuits and at the point of complete exhaustion. At that point, the subject felt neither hot nor uncomfortable but was unable to continue exercising. Throughout the test the running rate per mile remained steady. The slight decrease in elapsed time for mile 8 was due to the subject’s perception that his energy reserve was nearly depleted and that additional exertion during mile 8 would achieve complete exhaustion.

Although the subject did not experience any sense of heat or warmth, exhaustion was reached much sooner and at a faster rate than would have occurred with more normal ambient temperature (1,2,4,9,11,16). Measured body temperature increased very slightly from 37.83°C at rest to 37.94°C during exercise (2,8,11,18). Thus, the copious volume of fluid loss was effective in cooling and maintaining the subject’s body temperature (1,7). Heart rate increased along with the subject’s body temperature (i.e., until mile 8), then, body temperature decreased while heart rate continued to increase steadily (9,11).

Although the fluid loss and analyte changes are impressive, they were not noticed by the subject and were easily reversed. The subject did not experience any sensation of heat. After hydration the subject felt completely normal with the exception of thirst (secondary to the elevated serum sodium of 149 mmol·L-1) and exercise fatigue (16). These symptoms disappeared within minutes and with immediate hydration. In the following days the subject felt normal, without headache, fatigue, irritability, and/or other symptoms.

Discussion of Objective Findings

An increase in the concentration of serum analytes after exercise is consistent with dehydration due to loss of water in the form of sweat (17,18). Given that sweat contains electrolytes at a hypotonic concentration (i.e., more electrolytes present than in free water but less than in normal serum), the loss of free water during sweating is greater than the loss of electrolytes and other analytes. Thus, serum sodium increased from 140 mmol·L-1 to 149 mmol·L-1 (17). Similarly, serum total protein and serum albumin increased markedly (15). However, serum potassium, chloride, and carbon dioxide did not show similar increases in concentration (17). Since the vast predominance of body potassium is extravascular and intracellular, the amount of potassium excreted in the subject’s sweat was insignificant in terms of total body stores.

Changes in other analytes were more dramatic. Glucose levels increased from 82 mg·dL-1 to 119 mg·dL-1. Blood urea nitrogen levels increased from 16.8 mg·dL-1 to 19.8 mg·dL-1, and creatinine levels increased from 0.8 mg·dL-1 to 1.6 mg·dL-1 (4,13,17). Serum calcium, which is one of the most closely guarded analytes in the human body, showed a surprisingly large change from 9.4 mg·dL-1 to 11 mg·dL-1 (17). Similarly, red blood cell counts and white blood cell counts showed marked changes. The white cell count increased dramatically from 6.5x103/microliter to 10.4x103/microliter. This increase would be expected since no white blood cells are excreted in normal eccrine sweat. Of course, some of the leukocytosis could be due to release caused by systemic stress.

Also, with regards to the subject’s renal function, glomerular filtration rate decreased. This response is typical of the influence of strenuous exercise on the body. The estimated glomerular filtration rate fell from 105 mL·min-1·1.73 m2 to less than 47 mL·min-1·1.73 m2. The change represents an excellent adaptive mechanism of the human body to conserve free water by reducing blood flow to the kidneys (12,15,17).