Thermoregulation and exercise-associated heat-stress Part 1

1. Thermoregulation and exercise-associated heat-stress � Part 1 AVS 435 � Equine Exercise Physiology

3. 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)

4. 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

5. 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

6. Too low = hypothermia Heat output exceeds heat production Hypothalamus� ability to regulate greatly impaired < 29 0C (85 0 F) Cardiac failure ~ 20 0C (68 0 F)

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

8. 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

9. 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)

10. 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

11. 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

12. 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)

13. 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

14. 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

15. 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

16. 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

17. Lots of overlap in heat mechanisms Example � increased respiration => Convection (forced movement of air across tissues), PLUS Evaporation of saliva/respiratory secretions

18. 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

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

20. Heat generated during 1 � mile race = 1.8 � 2.7�F/min Total heat production in 2 min race 3.6 � 5.4�F � 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 < 5 min

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

22. 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

23. Physiologic factors that increase thermal stress Intensity of exercise Thermic effect of feed Protein > structural carbs > simple carbs > fats Body mass

24. Surface area to body mass ratio As body mass increases, surface area decreases Most heat exchange occurs at skin surface => lower BSA:BM = less heat loss per m2 of skin surface

25. Desert species Small body mass Larger SA:BMratio Dissipate heatbetter Harder for small animals to retain body heat in cold environments

26. Polar species Larger body mass Smaller SA:BM Retain heatbetter Few smallspecies in coldenvironments

27. Compare humans to horses Human body mass 80 kg Skin surface 2 m2 SA:BM = 1:40 Horses 500 kg Skin surface 5 m2 SA:BM = 1:100

28. 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)

29. 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 @ 32�F 580 cal of heat @ 104�F Equiv to 1-2 min max exercise or 5-6 min of submaximal exercise

30. 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

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

32. 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

33. 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

34. 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

35. How to quantify ambient conditions Comfort/heat index Original index used by FEI Easy, no complicated equipment Air temp in shade plus humidity Doesn�t include air currents or radiation Values > 180 = dangerous

36. Cute li�l gadgets Values > 130,use precautions Values > 150,use more precautions Value > 180, cancel

37. 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

38. 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

39. WBGT Index < 28: No add�l precautions, but consider if horses are acclimitized 28 � 30: Reduction in difficulty of course, add�l rest/cooling stops 30 � 32 More reductions in course, more rest/cooling

40. WBGT Index 32 � 33 Even more modifications to course, much stricter veterinary monitoring, more cooling stops, mandatory provision of shade for horses > 33 + Competition may not be safe

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