Thermoregulation and exerciseassociated heatstress Part 2

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Thermoregulation and exercise-associated
heat-stress Part 2

• AVS 435 Equine Exercise Physiology

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• Midterm 1
• Cal Poly average 73 (45.5 89)
• UCD - average 81.8 (55 97)
• Overall average both campuses) 78.2
• Midterm 2 March 5
• Same general format as MT 1

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• Correction to previous lecture!
• At maximum efficiency, evaporative sweat up to
  65 total heat loss
• Convection (not 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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• Heat exchange pathways
• Convection ( or -)
• Example standing in front of a cool fan (-),
  running a cold water hose across skin (-)
  running HOT water across skin surfaces ()
• Conduction ( or -)
• Example Holding an icepack (-) or a heating pad
  () against skin surface a cat climbing into a
  pile of fresh warm laundry ()

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• Radiation ( or -)
• Example sunbathing () standing next to a hot
  concrete wall () standing next to a frozen wall
  of ice (-)

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• Evaporation (- only)
• Examples sweat drying on hot skin surface a dog
  panting a rabbit (or kangaroo) licking its paws
  to spread saliva
• Most important pathway in horses
• 65 of total heat loss

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• Available evaporative pathways
• Via sweat formation (primary pathway)
• 85 (55 of total heat loss)
• Via respiratory tract (secondary, less important)
• 15 (10 of total heat loss)

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• Species under 100 kgbody mass tend torely more
  on evaporativepanting as primaryheat loss
  pathway
• Sweating may causelethal dehydration insmall
  mammals, therefore rodents dont sweat to lose
  heat

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• Species over 100kg body masstend to rely
  moreon evaporativesweating as primary
  heatloss pathway, as panting is less efficient,
  and large animals possess larger fluid reserves,
  less likely to become dangerously dehydrated

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• Equine evaporation via respiratory tract
• Upper respiratory tract
• Large surface area, turbinates
• Highly vascularized
• Guttural pouches
• 300-500 ml outpouching of auditory (Eustacian)
  tubes

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• Also rhino, tapirs,hyrax

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• Resonating chamber? Pressure regulation?
  Hearing aid?

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• Closely associated with internal carotid artery
  (this proximity can sometimes be a problem, ie
  mycosis)
• Acts to cool blood flowing to brain
• Temp of blood may drop 3F as flows past guttural
  pouches
• Blood temp at hypothalamus up to 2F cooler than
  core temp

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• This adaptation may be unique to horses
• May be adaptation to avoid central nervous heat
  stroke

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• Critical core temp at which voluntary exercise
  ceases
• Humans 104F
• Horses esophageal 108.5 109.4F
• Muscle temps may reach 111 - 113F during high
  intensity exercise

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• Normal respiratory rate 10-20 breaths per minute
• Panting increases heat losses
• Rapid, shallow breathing (up to 180 breaths per
  min) greater heat loss
• Primary phase panting
• Volume shallow, involves dead space, minimal
  changes to acid-base

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• Deeper, slower respiration
• 50-80 breaths/min
• Second phase panting
• Much more likely to affect acid-base (panting
  becomes more alkalotic)

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• Racehorses tend towards acidosis
• Jugging, illegal administration of oral bicarb
  to offset acidosis from LA
• Acidosis (increased H) decreases ATP
  production gt fatigue
• Slows rate and force of muscle contractions gt
  fatigue
• Most muscle pathologies are associated with
  acidotic conditions

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• Endurance horses dont accumulate much LA, but
  high resp rate (panting) to dissipate heat,
  especially w/ dehydration
• Tends towards alkalosis (low H)
• Affects metabolism, ie ionized Ca
• Less bioavailable Ca gt exertional
  rhabdomyolysis, synchronous diaphragmatic flutter
  (thumps)

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• Respiratory losses
• Respiratory rate increases with core temp
• Expired air is saturated, 100 humidity
• In hot, dry conditions, significant fluid losses,
  but NO addl elyte losses via respiratory
  secretions

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• Horse exercising in hot, dry conditions
• Relies heavily on heat loss via evaporation of
  sweat and respiratory fluids
• Heat loss is more efficient
• Less fluid and elyte loss

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• Horse exercising in hot, humid conditions
• Evaporation less effective, may be totally
  ineffective
• Sweat rate exceeds evaporation rate dripping
  sweat with much less heat loss per liter of sweat
• Much more total fluid and elyte loss

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• Evaporation via sweat
• Primary heat loss pathway in relatively few
  species
• Horses, humans, some cattle breeds/species
• Poorly developed in dogs/pigs
• Rodents/lagomorphs dont sweat, (but they do
  drool on themselves)

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• Sweat triggered via two stimuli pathways
• Adrenaline/epinephrine
• Increased sympathetic tone
• Response to excitement or exercise
• Initial sweating prior to increase in core
  temperature
• More dilute sweat than temp-triggered

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• Sweat triggered via two stimuli pathways
• Response to increase in body temp
• Sweat rate proportional to increase in body temp

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• Two types of sweat glands in mammalian species
• Both located in dermis layer of skin
• Apocrine glands _at_ hair follicles
• Hoofed species, include equine
• Apocrine glands in horses cover most of haired
  and hairless areas of the body

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• Apocrine glands secrete latherin proteins along
  with water, electrolytes, lactic acid, urea
• Helps spread water along hairs, increases surface
  area for more efficient heat loss
• Foamy, lathery sweat
• Glands can become exhausted, latherin secretion
  decreases

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• Eccrine (merocrine) glands produce more aqueous
  (watery) secretions
• Primary sweat gland in primates
• Horses only have eccrine glands on frog of hoof ,
  apocrine elsewhere
• Ruminants on nose
• Pigs on nose and knees
• On footpads of carnivores

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• Eccrine secretions more dilute, so species (ie,
  humans) that rely on eccrine sweat glands lose
  less electrolytes per liter of sweat than do
  species (ie, horses) that rely on apocrine sweat
  glands
• Take-home message - Electrolyte replacement is
  more important in horses than humans

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• Sweat formation
• Human thermogenic sweat is hypotonic relative to
  plasma
• Sweat is less salty than blood
• Equine thermogenic sweat is hypertonic
• Sweat is SALTIER than blood
• Neurogenic sweat is dilute

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• Chemical composition of equine sweat does not
  vary significantly between breeds
• Does vary based on
• Excitement (adrenaline more dilute)
• Individuals (minimal differences)
• Intensity of exercise
• Intense more dilute, may be partially related
  to excitement again

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• How does conditioning affect sweat mechanisms?
• Sweat rate increases (more L/hr)
• Proportional to increase in core temp
• Start and stop sweating more quickly
• More sensitive to changes in core temperatures
• More capillaries (cutaneous vascularization)
  more efficient heat loss
• Less total fluid and ion loss vs unconditioned
  horses

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• Olympic-class human athletes in response to
  conditioning
• Increased sweat rate
• Also true in horses
• Start sweating earlier
• Also true in horses
• Produce more dilute sweat
• Unknown in horses

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• How does duration of exercise affect sweat
  composition?
• HIGHLY VARIABLE based on balance of fluid and
  elyte losses and gains
• Prolonged exercise variable fluid losses based
  on fitness, air temp, humidity, excitement,
  intensity of exercise, etc gt dehydration gt
  increased osmolarity (concn of salts in fluid)

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• Regardless of core temp, sweat rate decreases and
  may cease with dehydration
• Heat loss via sweat 85 of evaporative loss (up
  to 55 of total heat loss)
• Heat loss via respiratory 15 of evaporative loss
  (up to 10 of total heat loss)
• Dehydration much higher risk of heat exhaustion

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• How to calculate dehydration
• dehydration kgs/liter of fluid removed from
  body weight
• Example Horse pre-exercise weighs 500 kg horse
  pre-exercise
• Post-exercise weight 450 kg
• 500 450 50 kg weight loss 50 liters of body
  fluids
• 50 500 kg 10 dehydrated

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• Humans 1 dehydration 10 decrease in
  performance
• Cannot extrapolate directly to horses
• Average fluid losses in eventing 3-5
• Average fluid losses during 50-100 mile endurance
  rides 5
• Both numbers would be MUCH higher if there is no
  fluid replacement during rest stops

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• Cellular function affected _at_ 2-3 dehydration,
  but difficult to identify clinically
• Impaired muscle contraction/force gt fatigue
  faster
• Decreased sweating rate
• Impaired heat loss pathways
• Increases rate of core heat temperature (approach
  critical temps faster)

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• 6-8 dehydration
• Progressive impairment of cellular function,
  thermoregulation, heat loss
• Increased blood viscosity
• Remember resistance is related to length of tube,
  radius of tube and/or VISCOSITY of fluid
• Heart has to work harder to maintain same cardiac
  output
• Decreased cardiac stroke volume
• Increased HR, slower HR recovery

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• Clinical symptomsof dehydration
• Positive (slowskin tenting
• Not very repeatable(different results from
  people all thinking theyre doing the same thing)
• More accurate at multiple places on point of
  shoulder vs neck
• Watch out for age, neck position

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• Mucous membrane
• Tacky
• Reduced salivaproduction
• Abnormal mmcolor usuallymore related toshock,
  oxygen content (cyanotic), sepsis (toxic line),
  dont use color as strong indicator of hydration
  state

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• Capillary refill time
• Press finger to gums to blanch, how quickly does
  it pink up?
• Function of hearts ability to circulate blood
  through small peripheral vessels in oral mucosa
• Good relationship to hydration state

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• Capillary refill time
• 1- 1 ½ sec normal
• 2 sec moderatedehydration
• 3 sec substantial dehydration
• 4 sec critical condition, very bad juju

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• Poor heart rate recoveries following cessation of
  exercise
• Cardiac Recovery Index
• Take baseline HR
• Trot horse out 125, back 125
• Take second HR 1 minute after baseline
• Is 2nd value same (good), lower (better) or
  higher (bad)?

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• Dehydrated horses have poor gut motility
• As hydration is compromised, blood is drawn away
  from non-vital tissues to supply heart, brain,
  muscles, etc

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• Circulation is shunted away from splanchnic
  (visceral) organs, including GI tract
• Decreased blood circulation less GI motility gt
  continued gas production from ongoing microbial
  fermentation gt higher likelihood of colic

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• Hind gut can acts as a fluid reservoir for 30-40
  liters of fluid
• Important in prolonged events
• Water reabsorbs throughout large bowel
• Too much increased chances of impaction

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• Hind gut fill is just extra weight during racing
• 30 - 40 liters fluid 66 88 lbs
• Race track pre-race practices to empty gut of
  ingesta/fluid fill
• Linseed oil via NG tube projectile diarrhea
• Poop 75 water
• Empties the gut, but not very smart
• Even 2-3 dehydration fatigue faster

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• Anhidrosis
• Pathologic absence of sweating despite increases
  in core temperature
• May occur due to dehydration
• Body conserves remaining plasma volume, stops
  sweating
• Rapid increase in core temp

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• Primary disease
• Most common in hot, humid regions 20 of horses
  in Florida
• Exact cause still unknown
• Sweat glands reduced response to norepinephrine
• Reduced or absent ability to sweat
• May still sweat under saddle, mane, between hind
  legs

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• Diagnose by local SQ administration of
  epinephrine
• Normal horses localized sweat within 30 minutes
• Dry coat
• Lack of insensible sweat, hair coat very dry and
  harsh

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• Anhidrotic horses VERY susceptible to heat
  exhaustion
• Tx Manage horses environment
• Air conditioning, water misters, fans
• Move horse to cooler region
• Exercise/turnout only at night or during cool
  weather
• Check thyroid function, electrolyte status,
  supplement accordingly

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• Nutritional supplements?
• Tyrosine, cobalt, iodine
• 30-70 effective
• Maybe some supportive data, but not peer-reviewed
  journal
• Treat claims cautiously
• Acupuncture?
• Anecdotal support only