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Journal of Exercise Physiologyonline
August 2014
Volume 17 Number 4
Editor-in-Chief
Official Research Journal of
Tommy
the American
Boone, PhD,
Society
MBA
of
Review
Board
Exercise
Physiologists
Todd Astorino, PhD
Julien Baker,
ISSN 1097-9751
PhD
Steve Brock, PhD
Lance Dalleck, PhD
Eric Goulet, PhD
Robert Gotshall, PhD
Alexander Hutchison, PhD
M. Knight-Maloney, PhD
Len Kravitz, PhD
James Laskin, PhD
Yit Aun Lim, PhD
Lonnie Lowery, PhD
Derek Marks, PhD
Cristine Mermier, PhD
Robert Robergs, PhD
Chantal Vella, PhD
Dale Wagner, PhD
Frank Wyatt, PhD
Ben Zhou, PhD
Official Research Journal
of the American Society of
Exercise Physiologists
ISSN 1097-9751
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,
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
2Truman
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):19. The purpose of this case study was to examine the physiological
responses of a male subject 59 yrs of age while running eight 1mile 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
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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 26yr 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).
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Table 1. Pre-Exercise Outdoor Environmental Conditions.
Temperature
33.9°C (93°F)
Humidity
51%
Heat Index
38.9°C (102°F)
Dew Point
73°
Barometric Pressure
29.95 in
Table 2. Post-Exercise Outdoor Environmental Conditions.
Temperature
32.8°C (91°F)
Humidity
55%
Heat Index
37.2°C (99°F)
Dew Point
73°
Barometric Pressure
29.98 in
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.
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Table 3. Mile Time, Body Temperature, and Heart Rate.
Mile
Time
Body Temperature
Heart Rate
(min, sec)
(degrees Celsius, Fahrenheit)
(beats·min-1)
Pre-Exercise
N/A
37.83°C (100.1°F)
N/A
1
9 min 51 sec
N/A
110
2
9 min 41 sec
37.89°C (100.2°F)
112
3
9 min 42 sec
N/A
117
4
9 min 41 sec
37.94°C (100.3°F)
124
5
9 min 46 sec
N/A
127
6
9 min 47 sec
37.94°C (100.3°F)
127
7
9 min 39 sec
N/A
140
8
9 min 18 sec
37.83◦°C (100.1°F)
150
Table 4. Physiologic Analytes of Interest from Peripheral Blood and Urine Samples.
Physiologic Analyte
Normal Range
PreExercise
PostExercise
PostExercise + 10
min + fluids
Na (mmol·L-1)
136-145
140
149
148
K (mmol·L-1)
3.4-5.1
4.2
4.5
4.2
Cl (mmol·L-1)
98-111
109
111
109
CO2 (mmol·L-1)
20.0-31
23
23
25
Glucose (mg·dL-1)
70-115
82
116
119
Blood Urea Nitrogen (mg·dL-1)
6.0-24
16.8
20.2
19.8
Creatinine (mg·dL-1)
0.5-1.5
0.8
1.5
1.6
Calcium (mg·dL-1)
8.4-10.5
9.4
10.9
11
Total Protein (g·dL-1)
6.0-8.4
6.6
7.6
8
Albumin (g·dL-1)
3.20-5.20
4
4.8
4.9
5
Total Bilirubin (mg·dL-1)
0.3-1.3
0.7
1.1
1.1
Alkaline Phosphatase (U/L)
25-117
68
73
72
Aspartate (U/L)
0-39
29
35
37
Aminotransferase
Alanine Aminotransferase (U/L)
0-40
23
24
23
Calculated Glomerular Filtration
Rate (mL·min-1·1.73 m2)
60-93
105
51
47
Creatine Phosphokinase (U/L)
24-195
137
202
187
MB Fraction of Creatine
Phosphokinase (ng·mL-1)
0.0-5.5
4.5
6
5.4
Troponin I (ng·mL-1)
0.00-1.00
0.01
0.02
0.01
B-type Natriuretic Peptide (pg·mL-1)
0.0-100.0
10.7
QNS
QNS
Urine Albumin
/Creatinine (mg·g-1)
0.0-40.0
5.4
7.2
N/A
Urine Albumin (mg·L-1)
0.0-37.0
8.9
11.4
N/A
162
158
N/A
Urine Creatinine (mg·L-1)
White Blood Cell Count (x106/uL)
3.5-11.0
6.5
10.4
7.8
Erythrocyte Count (x106/uL)
4.40-5.90
4.64
5.58
5.33
Hemoglobin (g·dL-1)
13.3-17.7
14.7
17.5
16.9
Hematocrit (%)
40.0-52.0
40.9
49
47.3
Mean Corpuscular Volume (fL)
80.5-99.7
88.3
87.9
88.6
Mean Corpuscular Hemoglobin
(pg)
Mean Corpuscular Hemoglobin
Concentration (g·dL-1)
26.4-34.0
31.7
31.3
31.7
31.4-36.3
35.9
35.6
35.7
Red Blood Cell Distribution Width
11.0-15.0%
13.3%
12.3%
12.4%
Platelet Count (x103/uL)
150-400
254
344
297
Neutrophils (% of Total White
Blood Cells)
50.0-75.0
51.8
46.3
55.3
Lymphocytes (%)
19.0-48.0
39
46.8
36.5
Monocytes (%)
0-10
4.4
3.5
4.4
6
Eosinophils (%)
0.0-6.0
2
1.3
1.6
Basophils (%)
0.0-2.0
1
0.6
0.7
Large Unstained Cells (%)
0.0-5.0
1.8
1.3
1.5
Absolute Neutrophil Count
(x103/uL)
1.5-7.0
3.36
4.8
4.32
Absolute Lymphocyte Count
(x103/uL)
1.5-4.0
2.53
4.85
2.85
Absolute Monocyte Count
(x103/uL)
0.2-1.0
0.28
0.36
0.34
Absolute Eosinophil Count
(x103/uL)
0-0.35
0.13
0.14
0.12
Absolute Basophil Count
(x103/uL)
0-0.20
0.07
0.07
0.05
Absolute Large Unstained Cell
Count (x103/uL)
0.12
0.14
0.12
Urinalysis
Color
Clarity
Yellow
Clear
Yellow
Clear
N/A
N/A
Specific Gravity
1.005-1.030
1.027
1.026
N/A
pH
5.0-9.0
5
5
N/A
Leukocyte Esterase
Negative
Negative
Negative
N/A
Nitrite
Negative
Negative
Negative
N/A
Protein
Neg-Trace
Negative
Negative
N/A
Glucose
Neg-Trace
Negative
Negative
N/A
Ketones
Neg-Trace
Negative
Negative
N/A
Bilirubin
Negative
Negative
Negative
N/A
Blood
Negative
Negative
Negative
N/A
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
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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 6.5x10 3/microliter to
10.4x103/microliter. This increase would be expected since no white blood cells are excreted in
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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).
CONCLUSIONS
In sum, the body of this elderly well-trained athletic male responded well to exercise under extreme
heat conditions. His response was due primarily to the physiologic mechanisms that included
copious sweating and reduced renal blood flow. The fact that high ambient temperatures can be
life threatening cannot be denied. However, in the well-trained heat-acclimated elderly athletes
with adequate fluid intake, they are not invariably so.
ACKNOWLEDGMENTS
The authors would like to thank Lynne Hosler, Norman Anderson, BS, MT(ASCP), Director of the
Laboratory at Southeast Hospital, Cape Girardeau, MO, and the Southeast Hospital and
Laboratory for assisting in collecting and analyzing blood and urine samples. We would also like to
thank Stan Hosler and Paul Cordes, MD for helpful suggestions regarding the manuscript.
Address for correspondence: Lee G. Beckwith, MD, Department of Pathology, Southeast
Hospital, 1701 Lacey Street, Cape Girardeau, MO, 63701, Email: LeeBeckwith507@gmail.com
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