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GENERAL INTRODUCTION
Eighty percent of the energy metabolized during exercise in a hot
environment is liberated as heat in active muscles.(4) The heat is
transferred from warm muscle tissue to the blood, and is then transferred
to the skin, where it is dissipated to the environment. The body
increases the dissipation of heat to the environment by redirecting
cardiac output, regulating skin blood flow and altering the rate of
sweat secretion.(5) If this heat is not removed from the body's
core, it may result in a potentially dangerous increase of internal
temperature. (4)
The body dissipates heat to the surrounding environment by either
radiation, convection or evaporation. Radiation (heat exchange via
waves) and convection (heat exchange between a surface and a fluid)
occur when warm blood flows through the skin, releasing heat to the
surrounding air. Evaporation (change of water from liquid to vapor,
which absorbs heat from the skin) occurs when sweat evaporates from
the skin. Conduction (heat exchange between two solid surfaces
which are in direct contact) accounts for less than 2% of heat
loss in most situations.(4)
Fluid intake is important before exercise because, if an athlete is
dehydrated, his or her blood volume shrinks and less blood is pumped
through the skin, thereby reducing heat loss via radiation and
convection. The sweat rate also decreases if an athlete is
dehydrated. Evaporation accounts for 80 to 90 percent of heat
dissipation during upright exercise.(4)
Sweating begins just minutes after the onset of exercise in a hot
environment, but the role of water consumed during exercise is not as
apparent. Water that was consumed during exercise is involved in
wetting the skin, and in the important process of cooling the body,
just minutes after it was consumed.(4)
Thirst alone is not a good indicator of the need for fluids.
Research demonstrates that exercise in hot adverse conditions can
cause dehydration in as little as 15 minutes. Drinking when in a
dehydrated state can cause gastrointestinal distress. Athletes who
say they cannot drink during a workout because it gives them a stomach
ache may be allowing themselves to become dehydrated before they take
their first drink.(19) Dehydration is cumulative. If lost fluids
and minerals go unreplaced fatigue and heat illness can result.(19)
The time required for fluid to move from the mouth to the sweat
glands not only includes uptake at the intestine, but also includes
the rate at which fluid empties from the stomach. Factors that
increase stomach emptying include: carbohydrate content
(optimal 6 to 10%), upright (versus seated) posture, low
(versus high) exercise intensity and mild (versus hot) environmental
temperature.(4)
The speed at which a beverage travels from the stomach in to the
small intestine (the gastric emptying rate) depends on the energy
content (calories) and volume of the beverage consumed. A small
concentration of carbohydrate will encourage rapid absorption, but
too much carbohydrate will slow gastric emptying and can result in
distress.(19)
Carbohydrate/electrolyte beverage may be better than water when
continuous exercise lasts longer than 50 minutes, a carbohydrate
deficiency exists, an electrolyte deficiency exists or after
exercise.(4)
The body regulates its temperature differently in different
environments and during different activity states. In a cool
environment, radiation and convection are the major avenues of
heat loss, at rest and during exercise. In a hot-dry environment
(eg. 90F, 20% relative humidity) evaporation accounts for nearly all
of heat dissipation during exercise, emphasizing the importance of
sweat production and lightweight, loose-fitting closing.(5)
The evaporation of sweat from the skin surface is greatly
diminished in a hot-set environment (eg. 90F, 70% relative humidity)
especially when relative humidity is greater than 70%. The body's
increased reliance on radiation and convection in this environment
requires that a greater portion of cardiac output be diverted to skin
blood vessels, thereby decreasing the venous return of the blood to
the heat and increasing cardiovascular strain.(5)
An 8-14 day period of gradual exposure to a hot environment will
improve heat dissipation by increasing blood volume and optimizing
the ratio of blood flow directed to the skin, internal organs and
exercising muscles.(5)
HEAT CRAMPS
Heat cramps begin as feeble twitchings and progress to localized
contractions of skeletal muscles of the legs, arms, or abdomen.
They occur in one motor unit and rarely involve an entire muscle.
They are most often observed during repetitive, high intensity exercise
when athlete has lost a large volume of sweat.(3)
Treatment involves rest and drinking mildly salted fluid
(two 10 grain salt tablets crushed in one quart of water or 0.5 to
1 liter of normal saline).(3)
HEAT EXHAUSTION
Although heatstroke is the third leading cause of death among young
athletes, heat exhaustion is observed 8-12 times more often than heat
stroke. By definition heat exhaustion is the "inability to continue
exercise in a hot environment" and primarily results from seat loss
and/or inadequate fluid intake.(2) It is a form of shock due to
depletion of body fluids.( 21) When dehydration (especially greater
than 3% of bodyweight) is superimposed on exercise heat stress; the
cardiovascular system is simply unable to pump sufficient blood to
meet all of the body's needs. Signs and symptoms can include:
profuse sweating, "heat sensations" on the head/shoulders/chest,
weakness, "rubbery" legs, chills or gooseflesh on the neck and
shoulders, anxiety, irritability, slight confusion, upper body
swaying, nausea, vomiting, muscle cramps, fainting,(2) rapid and
weak pulse, pale or flushed skin, excessive fatigue, disturbance
of vision and incoherence. (21)
Treatment consists of rest in a cool environment, elevation of legs,
pouring cool water on the skin and replacement of fluids orally or
intravenously. Recovery is usually complete within 12 to 48 hours,
without further complications.(2) Athletes suffering from heat
exhaustion should not be allowed to practice or compete for the
remainder of that day. (21) Heat exhaustion may be prevented by
proper heat acclimatization; consuming water, electrolytes and
carbohydrates to replace losses; and monitoring ambient conditions
(temperature and humidity) to allow practice sessions to be tailored to
the environment.(2)
HEATSTROKE
Between 1959 and 1985 there were 77 heatstroke fatalities at high school
and college level. Heatstroke is caused by an "overload" of the body's
temperature regulation system or a "failure" of the cardiovascular system
during exercise in a hot environment. This loss of thermoregulatory
control heatstroke is a medical emergency. Medical care must be
obtained at once; a delay in treatment can be fatal. Overload occurs
when the rate of heat production exceeds the rate of heat dissipation,
even in the presence of profuse sweating. Failure of the body's
temperature regulation suggest dysfunction of either the brain, sweat
glands of skin blood vessel aspects of human heat loss.(6)
Heatstroke results from extreme hyperthermia (core body temperature
over 104F). The extent of multi-system tissue injury and the mortality
rate are closely related to the core temperate at collapse and the tie
that elapses prior to the initiation of cooling therapy.(6)
Heatstroke commonly involves more than one factor: obesity, insufficient
heat acclimatization, dehydration, lack of sleep, fever, drug or alcohol
abuse. Other factors are air temperature, time of day, type of
activity, exercise intensity, exercise duration, and clothing. It
usually occurs among young, motivated males who push their bodies
beyond the point at which they would normally stop exercise if
discipline, competition, or peer pressure were not involved.
Few cases of exercise-induced heatstroke have been reported among
females. (7)
Heatstroke can occur when the air temperature is as low as 41F and
often occurs during the early morning. Victims may be wearing shorts
and a T-shirt.(7)
It is characterized by very high body temperature, hot dry skin
(which indicates the failure of the primary temperature-regulating
mechanism - sweating) and possibly seizure or coma.(6)
First aid includes immediate cooling of the body without causing the
athlete to shiver. Recommended methods for cooling include using
wetting the body and fanning vigorously. Victims of heatstroke should
be hospitalized and monitored carefully.(6)
Recovery from heatstroke is a highly individualized process. The
majority of patients recover completely within one to six months.
One study of two cross country runners showed that liver damage
required 11-12 months to heal. Other studies have reported that
victims may be intolerant to exercise in heat for two to 5 years.(7)
NCAA GUIDELINES
The NCAA recommends the following guidelines to prevent heat illness:
1. Take a complete history, including previous heat illness history
and physical examination
2. Perform a yearly health-status questionnaire
3. 7-10 day acclimatization period and gradual increase in
intensity of exercise
4. When protective gear must be worn, frequent rest periods should
be scheduled so that the gear can be loosened to allow evaporation
of sweat. Dark colored clothing increases absorption of solar
radiation. Never use rubberized suits.
5. Take regular measurements of environmental conditions like
wet-bulb temperature, dry-bulb temperature and globe temperature.
A wet-bulb reading higher than 75F or warm-weather humidity
above 90% may represent dangerous conditions that are made more
severe if the sun is shining. A WBGT (wet bulb globe temperature
higher than 82F suggest that careful control of all activity be
undertaken.
See
Heat Index Table to determine heat index.
6. Cool water must be readily available and athletes should be
encouraged to drink as much and as frequently as possible.
For participation periods up to two hours in duration, most
weight loss represents water loss and that fluid loss should be
replaced as soon as possible.
A two pound weight loss represents approximately one quart of fluid
loss. Carbohydrate/electrolyte drinks are appropriate.
7. Record body weight before and after practice. Those who
have lost 5% of their bodyweight or more over a period of several
days should be evaluated medically.
8. Some athletes may be more susceptible to heat injury than
others: those not heat acclimatized, those in relatively poor
physical condition, those with excess body fat, those who
regularly push themselves to capacity, those with a history
of heat illness and those with any febrile conditions.
9. Athletes should be monitored for signs and symptoms of heat
injury: cessation of sweating, weakness, cramping, rapid and
weak pulse, pale or flushed skin, fatigue, nausea, unsteadiness,
disturbance of vision, incoherance.
See
Climatic Heat Stress and the Exercising Child and
Adolescent
2003 USA TRACK AND FIELD GUIDELINES
It is recommended that long distance runners consume 1 liter of fluid
for every liter lost during a race. For any race longer than one hour,
the following procedure is suggested to determine individual fluid needs:
1. Be properly hydrated BEFORE the workout, i.e. your urine should be
very slightly yellow or clear.
2. Do a warm-up to the point of lightly perspiring and urinate if
necessary.
3. Weigh yourself naked.
4. Run for one hour at race intensity.
5. Drink a MEASURED amount of fluid of your choice during the
workout.
6. Do not urinate during the workout.
7. After the workout weigh yourself immediately on the same scale.
8. Urinate and/or drink as necessary.
Use the following formula to calculate your hydration requirements:
Pre-workout body weight (kg) from Step 3: _____ kg
(To convert from pounds to kilograms, divide pounds by 2.2)
Post-workout body weight (kg) from Step 7: _____ kg
........................................................... = _____ kg
of body weight lost during the workout.
.................................. _____ kg x 1000 = ______ grams of body
weight lost during the workout
Grams of body weight lost + fluid (ml) consumed from Step 5 = _____ ml
The above ml of fluid is the ml of fluid per hour needed to
properly hydrate.
To convert oz to ml: oz x 30 = ml.
To convert ml to oz: ml/30 = oz.
GENERAL REHYDRATION GUIDELINES
Water is life. It is the main ingredient in almost every tissue in the
body and it is the most important nutrient. You will only survive a
maximum of 7 days without it. The amount of body water ranges from
50% to about 80%. Muscles are over 70% water; red blood cells are
about 60% water. The daily requirement, excluding exercise, is 8
cups per day. That means water- not coffee, tea, soda, beer, or
even juice. Since thirst is not felt until you have dehydrated,
it is not a good indicator of your need for water.
Athletes exercising in a hot environment can loose over one ounce of
water every minute. Conversely, the body is only able to absorb one
ounce every 3 to 4 minutes. This can result in a loss of 16 ounces
every 20 minutes. A 3% loss of body weight in water can result in
performance losses from 20-30%. Greater that a 5-6% loss can be a
medical emergency. To combat this, pre-event hydration is essential.
Weigh yourself before and after your event or training session. Figure
out the difference and then determine the percentage of body weight
lost in water.
Check out the
Momsteam.com Fluid Guidelines for Young Athletes
Pre-event Hydration
1. Athletes should consume 50 to 100 ounces of fluid above and beyond
their normal intake the day before the event.
2. The athlete should consume 16 to 32 ounces of water 2 hours prior
to the event (16oz 2 hours prior and 16 oz 1 hour prior).
Check for the
Heat Index Table to determine see if it is too hot
or too cold to participate in the event.
3. Eliminate your bladder 15 minutes prior to the event.
4. Drink cool water during the event as it is absorbed faster and cools
the body better than warm water.
During-event Hydration
1. Drink no more than 8-10oz per 15-20 minutes. Do not take any ephedra
products. Sip the water, do not gulp it down.
Post-event Hydration
1. Replace each pound lost with one pint of water.
2. Sip the water, do not gulp it down.
3. For multi-day events: Weigh yourself before the event and every
morning. If you lose 1%-3% of body weight, re-hydrate by drinking at least
8oz per pound lost. If you lose 3%-6%, re-hydrate and back off on your
training intensity the next day. If you are light-headed, fatigue more
easily or sweat profusely, STOP training and do not train the rest of the
day or the next day. If you lose greater than 7%, you must seek medical
attention immediately!
HYPONATREMIA
"Hyponatremia is caused by overhydrating, which
results in a dangerous decrease in the body's
sodium levels."
RESEARCH
12 runners underwent two treadmill tests running at 240 and 270m/min,
20 minutes total, one in a 23 C and one at 38 C. The heart rate,
plasma lactate and plasma glucose differences between the 23 and 38
C tests were used to describe each runner's relative EHT.(1)
During strenuous, upright exercise, blood flow is redistributed away
from visceral organs. The amount of blood diverted from these tissues
is directly proportional to the %VO2 max at which exercise is performed.
The added demands of a hot environment are met by redistributing an even
greater fraction of cardiac output away from central organs, which may
result in greatly elevated lactic acid and premature exhaustion.(1)
Authors recommend to begin exercise well hydrated; minimize plasma
volume losses and acclimatize to heat. They also recommend that HR
be evaluated as a predictor of EHT.(1)
For more information on Hyponatremia see
See
Hyponatremia in Athletes
CARBOHYDRATE BEVERAGES
Carbohydrates consumed immediately after and two hours after exercise
can enhance muscle glycogen restoration.(12) Most oral rehydration
solution contain less than 6% carbohydrates (about 55 calories per 8oz.)
because research has shown that the greatest rates of fluid absorption
are usually stimulated by solution containing from 2-6% carbohydrates.
Increasing carbohydrate content beyond 6% often reduces the rate of fluid
absorption.(20) Research shows the 6% carbohydrate concentration is
absorbed at least 30% faster than water. Higher glucose levels are
maintained with ingestion of this type of beverage.(9)
Gatorade (6% carbohydrate/electrolyte solution) was absorbed
significantly faster than water during both exercise and recovery
periods. Exercise did not influence the absorption process.(11)
Exercise performance improved significantly with carbohydrate
feedings. This study recommends ingesting 30-60g/hour of
carbohydrate during exercise to improve performance.(13)
Solutions with multiple transportable substrates that can stimulate
several different solute transport mechanisms produce greater solute
and water absorption than solutions with only a single transport
medium. The 6% CHO solution containing a combination of free glucose
and fructose are suggested for maximizing waster and CHO
absorption.(18)
To calculate the carbohydrate percentage of any beverage:
Divide the number of grams of carbohydrate per serving (in milliliters)
and multiply by 100, e.g. 14 grams per 8oz. (1oz.x30=milliliters)
14/240 mlx100= 5.83 or 6%
ELECTOLYTES
Potassium 30mg/8oz
Sodium 110 mg/8oz
FRUCTOSE BEVERAGES
When the subjects ingested the beverage every 20 minutes during two hours of exercise, the fructose beverage was not as readily available for energy as the glucose-containing beverages. The glucose polymer did not provide any metabolic advantage over the glucose-free beverage.(14) Consumption of fructose was associated with greater incidence of gastrointestinal distress due to its slow absorption rate, a greater loss of plasma volume, greater stress hormone response and substantially poor exercise performance.(17)
CARBONATED BEVERAGES
Carbonated soft drinks contain carbohydrates in a concentration of 10-11%. Compared to the typical sport drinks, such beverages are absorbed slowly. In addition, the carbonation turns into carbon dioxide gas when warmed in the stomach and can cause gastrointestinal distress. The caffeine found in many soft drinks actually increased fluid lost by stimulating urine production.(19)
SUPER ORAL REHYDRATION
A new concept of super oral rehydration has been proposed in which nutrients other than carbohydrates are also included in the beverage like amino acids. The theory needs to be confirmed.(20)
VOLUME OF FLUID INGESTED
The volume of fluid ingested is a powerful regulator of gastric emptying. There was greater gastric emptying with rehydration of 150% of fluid lost. However, this was attained via relatively large gastric fluid volume, with concomitantly large urine volumes. Thus, forced intake of a relatively dilute solution is not an effective method of rapid rehydration and may be detrimental to subsequent performance.(16)
EXERTIONAL HEAT ILLNESS
Recruits at higher risk for developing EHI (exertional heat illness) had a BMI of >22/kg/mm and 1.5 mile run time of >12 minutes. These recruits had an eightfold higher risk for developing EHI during basic training when compared to those with BMI <22 and a 1.5 mile run time of <10 minutes. Only 1/5 of male recruits met these criteria for high risk, but they accounted for nearly half of the EHI cases occurring during the 12 weeks basic training course.(10)
CHILDREN
Studies have shown that the main maturational differences to exercise in the heat (mostly sweating rate and composition) occur at late puberty and adulthood, rather than from pre- to mid-puberty.(15)
WHEELCHAIR
Individuals with spinal cord injury possess limited
autonomic control of heat dissipation (sweat gland
secretion, redistribution of cardiac output, vasodilation
in cutaneous vessels) below the level of injury. The
magnitude of their thermoregulatory impairment is related
to the level and completeness of the spinal cord lesion.
Other factors are impaired sympathetic cardiac stimulation
which results in lower stroke volume and cardiac output and
decreased venous return to the heart than that of able-bodied
individuals; reduced thermoregulatory response for a given
core temperature; unique movements of upper-body wheelchair
propulsion, when compared to lower body exercise of equal
intensity, produce enhanced strain, evidenced by greater
plasma volume loss and catecholamine concentrations;
medications (oxybutynin and phenoxybenzamine for bladder
control) may negatively affect sweating and
thermoregulation.(8)
What We Can Learn From the Korey Stringer Episode
1. There was no doctor nor an ambulance on the field.
2. Listen to weather warnings
See
See
Heat Index Table
Don't practice in full pads and wear light colored jerseys!
Have a shaded area to go to when not in a play.
Have ice available and make use of it (head and neck).
Have mist sprayers available.
3. Less shouting at players, more encouragement.
4. Come to practice in shape - get in shape during the off season.
5. Stringer vomited 5 times on the first day of practice - this is
a good clue that the player is not in shape and is not heat acclimated!
6. If you continue to sweat profusely after practice and the remainder
of that day and night - don't practice the next day.
7. Weight the players before and after practice. Stringer lost 6lbs
the first day of practice.
8. Why was Stringer in the trailer for 45 minutes and not on his way
to the hospital immediately?
9. Why was a 22 year old trainer attending to multi-million dollar
professional football players?
10. Why wasn't Stringer's temperature taken (the trainer did not even
have a thermometer in the trailer)? If he was "unresponsive"
get him to the hospital!
For more information see
Heat Illness Prevention Tips
Outdoor Action Guide to Heat Related Illnesses and Fluid Balance
and
Attacking Heat Related Death and Illness in Football Players
and
Tackling The Heat
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HYDRATION GUIDELINES AND HEAT INJURY