Thermoregulation and exerciseassociated heatstress Part 1

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Thermoregulation and exercise-associated
heat-stress Part 1

• AVS 435 Equine Exercise Physiology

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• Reading
• Marlin Chapter 12, pgs 133 150
• Hinchcliff Chapter 6.3, pgs 382 - 396

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• Thermoregulation
• Process by which internal body temperature is
  maintained within a narrow physiologic range
• Mammals (37-400C)(98.6 104 0 F)
• Horses 37.2 38.6 0C (99 101.5 0 F)

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• Thermoneutral zone
• Environmental temperature at which body temp can
  be maintained primarily by changes in
  vaso/pilomotor mechanisms
• No additional energy expenditure (shivering,
  sweating, etc) to raise or lower body temp

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• One of most important and closely regulated
  systems in body
• Too high hyperthermia
• Heat production or heat input exceeds heat output
• Body temperature rises to dangerous levels
• 41.5 42.5 0 C (106 108 0 F) potentially
  damaging/lethal

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• Too low hypothermia
• Heat output exceeds heat production
• Hypothalamus ability to regulate greatly
  impaired lt 29 0C (85 0 F)
• Cardiac failure 20 0C (68 0 F)

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• Thermoregulation is a balance between heat
  input/production and heat loss
• Heat input environmental
• Heat production metabolic processes
• Heat loss environmental and/or metabolic

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• Heat production
• Metabolic heat production ()
• Conversion of chemical energy (ie, stored
  substrates) to mechanical energy (ie, muscle
  contraction)
• 20-25 efficiency
• 40-60 fold increase with exercise

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• Environmental gain/loss
• Convection ( or -)
• Transfer of heat between tissue and fluid
  (usually moving fluid)
• Tissue body skin surface or within blood
  vessels
• Fluid liquid or gas
• Wind movement, contact with fluid/water,
  respiratory heat losses (ie, panting)

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• Convection
• Natural convection heat changes arising from
  thermal gradient alone
• Forced convection
• Increased heat changes due to forced movement of
  fluid/air across tissues
• Wind chill, incl/ fans, running speed
• Water current
• Blood circulation

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• Convection losses/gains dependent on thermal
  gradient
• Will you get colder faster in 80 air/water or
  40 air/water?
• Fluid movement increases thermal gradients
• Rapid air/water exchange continuously removes
  heat from tissue surface, maintains greater
  thermal gradients

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• The thermal gradient in convection loss/gain can
  be altered
• Skin surface blood flow
• Increased circulation increased heat loss
• Decreased circulation decreased heat loss
• Insulation (hair, piloerection, SQ fat)
• Behavioral
• Curling up
• Huddling in groups
• Increased respiratory rates, (panting)

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• Conduction ( or -)
• Direct transfer of heat through liquid, solid or
  gas
• Example lying on cold (or hot) concrete,
  contact with cold (or warmed) surgical table
• Relatively minor source of heat loss in horses

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• Radiation ( or -)
• Movement of heat between two objects without
  direct contact
• Solar radiation up to 15 of heat gain
• Heat reflected from hot concrete, sand, pavement,
  etc
• Grass, water absorbs radiation rather than
  reflects, so feels cooler
• Can also lose body heat to cooler surroundings
• Lose body heat to cool walls even when air in
  between is warm

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• Evaporation
• Heat loss through vaporization of sweat, saliva,
  respiratory fluids
• Extremely important in horses
• 85 sweating, 15 respiratory
• May be only form of heat loss available when air
  temp exceeds body temp

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• Evaporation is the only pathway by which the body
  only loses heat, never gains.
• Efficiency of evap varies based on
• Magnitude of thermal gradient
• Hot horse/cool air or hot horse/hot air?
• Velocity of air movement (convection)
• Maintains thermal gradient
• Ambient air humidity
• High humidity decreases vapor pressure gradient,
  less efficient

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• Lots of overlap in heat mechanisms
• Example increased respiration gt
• Convection (forced movement of air across
  tissues), PLUS
• Evaporation of saliva/respiratory secretions

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• Hs Hm Hcd Hc Hr He
• Hs heat stored in tissues
• Hm metabolic heat production
• Directly proportional to oxygen consumption (VO2)
• 20 efficient 80 heat production
• Hcd conduction heat loss/gain
• Hc convection heat loss/gain
• Hr radiation heat loss/gain
• He evaporative heat loss

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• How important is cooling?
• Heat generated during submaximal exercise
  .56F/min gt 33F/hr
• Normal to potentially lethal in lt 15 min of
  exercise if heat not removed
• Enough heat generated during 50-mile endurance
  ride to melt and boil 150 lbs of ice

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• Heat generated during 1 ½ mile race 1.8
  2.7F/min
• Total heat production in 2 min race 3.6 5.4F
  high but not dangerous due to short duration of
  exercise
• Most of heat stored, little time to dissipate
  large amounts of heat during race, but decreases
  quickly after cessation of exercise
• If continued to gallop at maximal speeds, and
  heat were not dissipated, could reach lethal
  temps in lt 5 min

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• Environmental factors that increase thermal
  stress (Hs)
• High ambient air temps
• Sunny conditions
• Lack of wind/air movement
• High humidity

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• Physiologic factors that increase thermal stress
• Unfit condition
• Higher cardiovascular load, less capillary
  density, less developed sweating ability
• Insulation
• Hair, body fat, heavy muscling
• Dehydration
• Increased blood viscosity, decreased convection

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• Physiologic factors that increase thermal stress
• Intensity of exercise
• Thermic effect of feed
• Protein gt structural carbs gt simple carbs gt fats
• Body mass

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• Surface area to body mass ratio
• As body mass increases, surface area decreases
• Most heat exchange occurs at skin surface gt
  lower BSABM less heat loss per m2 of skin
  surface

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• Desert species
• Small body mass
• Larger SABMratio
• Dissipate heatbetter
• Harder for small animals to retain body heat in
  cold environments

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• Polar species
• Larger body mass
• Smaller SABM
• Retain heatbetter
• Few smallspecies in coldenvironments

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• Compare humans to horses
• Human body mass 80 kg
• Skin surface 2 m2
• SABM 140
• Horses 500 kg
• Skin surface 5 m2
• SABM 1100

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• Horses body mass 625 larger than humans, but
  only 250 increase in skin surface
• Must develop more efficient methods of heat
  dissipation
• Human max sweat rate 17 ml/m2min
• Horses max sweat rate 50 ml/m2min or up to 15
  L/hour (faster than any other species)

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• Amount of heat loss via evaporation of sweat is
  dependent on temperature and vapor pressure
  (humidity) of surrounding air
• 1 liter evaporated water removes
• 598 cal of heat _at_ 32F
• 580 cal of heat _at_ 104F
• Equiv to 1-2 min max exercise or 5-6 min of
  submaximal exercise

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• At maximum efficiency, evaporative sweat up to
  65 total heat loss
• Respiratory losses up to 25 of heat loss
• Conduction, radiation, etc remainder
• If insufficient heat loss, the result is
  increasing core body temp, heat exhaustion

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• Evaporative efficiency
• Maximized in cool, dry conditions
• Less efficient hot, dry conditions
• LEAST efficient hot, humid conditions
• Potentially INEFFECTIVE

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• Rate of Hs (heat storage in tissue) in hot,
  humid conditions may be twice as rapid as under
  cool, dry conditions
• Core temp can reach dangerous levels twice as
  quickly

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• Rate of evaporation is a function of the gradient
  of vapor pressure between ambient air and at skin
  surface
• Sweat rate exceeds evaporation rate sweat drips
• Drips only 10 as effective at removing heat as
  evaporation
• Most likely to occur when ambient conditions are
  hot and humid

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• Why quantify ambient conditions
• Increasing precautions against heat exhaustion
  (FEI)
• Decrease length, difficulty of course
• Provide rest/cooling stops
• Additional veterinary monitoring
• Limit/cancel competition if conditions are
  extreme

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• How to quantify ambient conditions
• Comfort/heat index
• Original index used by FEI
• Easy, no complicated equipment
• Air temp in shade plus humidity
• Doesnt include air currents or radiation
• Values gt 180 dangerous

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• Cute lil gadgets
• Values gt 130,use precautions
• Values gt 150,use more precautions
• Value gt 180, cancel

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• Thermal stress is relative to acclimitisation
• 1996 Race of Champions
• 100 mile elite endurance race
• Horses from both coasts
• Lots of treatments of horses from west coast
• No problems in horses from SE

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• Wet Bulb Globe Thermometer (WBGT)
• Adapted for use for 1996 Atlanta Olympic Games
• Much better reflection of conditions
• Air temp in shade
• Relative humidity
• Radiation
• Air movement
• Multiple sites, position at level of horse

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• WBGT Index
• lt 28 No addl precautions, but consider if
  horses are acclimitized
• 28 30 Reduction in difficulty of course,
  addl rest/cooling stops
• 30 32 More reductions in course, more
  rest/cooling

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• WBGT Index
• 32 33 Even more modifications to course, much
  stricter veterinary monitoring, more cooling
  stops, mandatory provision of shade for horses
• gt 33 Competition may not be safe

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• Highest recorded WBGT 34.7
• Horses did fine, riders had heat stroke