Lecture 7 Kin. PDF - Thermal Stress (2024-11-11)

2024-11-11 Thermal Stress November 11th, 2024 0 Lecture Objectives uExplain how thermal balance is maintained in the body, uDescribe fou...

2024-11-11 Thermal Stress November 11th, 2024 0 Lecture Objectives uExplain how thermal balance is maintained in the body, uDescribe four physical factors that contribute to heat balance, uUnderstand the cardiac output, heart rate, and stroke volume response during physical activity in hot weather, uExplain how acclimatization, training, age, gender, and body fat modify heat tolerance during physical activity 1 1 Click to add footer 1  2024-11-11 Lecture Objectives - II uUnderstand symptoms, causes, and treatment for heat cramps, heat exhaustion and heat stroke, uExplain the purpose of the wind-chill temperature index and factors that comprise it, uDiscuss immediate and long physiologic adjustments to cold stress. 2 2 What is temperature? Temperature represents the mean kinetic energy of a substance’s atoms as they move. Functional definition of temperature: Potential for heat exchange between substances (e.g., blood to capillary walls) or objects (playing surface to participant’s body) Humans can tolerate a decline in deep body temperature of -10°C, but only an increase of +5°C 3 3 Click to add footer 2  2024-11-11 Body temperature measurement Core temperature (Tco): 36.1 - 37.8 ∘ C (97 – 100 ∘ F) Tre Teso Ttym Skin temperature (Tsk) – 33.3∘ C (91.4 ∘ F) 4 4 Thermoregulation “The process whereby the body temperature is maintained or controlled under a wide range of environmental conditions”. 5 5 Click to add footer 3  2024-11-11 Assessing High Environmental Temperatures Relative Humidity – The moisture in the air relative to how much moisture (water vapor) can be held by the air at any given ambient condition. Heat Stress Index – a measure used to estimate the risk of heat stress based upon the ambient temperature and relative humidity. Wet Bulb Globe Temperature (WBGT) - A scale, developed by the military, used to assess thermal heat load imposed by the environment. Commonly used in industrial settings and athletic situations. 6 6 Heat Stress Index 7 7 Click to add footer 4  2024-11-11 Thermal Balance Core temperature represents a dynamic equilibrium between factors that add and subtract body heat. Core temperature rises if heat gain exceeds heat loss (i.e., during vigorous exercise in warm, humid environment) Core temperature declines in cold when heat loss exceeds heat production 8 8 Heat Balance During Physical Activity 9 9 Click to add footer 5  2024-11-11 Heat Exchange During Exercise (An example) 60 min exercise, performed at 900 kpm-min-1, at different ambient temperatures M = Metabolic heat production THL – Total heat loss R + C = Radiation plus conduction E = Evaporation 10 10 Effect of Clothing (Uniforms) Football equipment and clothing seal off 50% of body from evaporative cooling benefits. Football gear exacerbates increase in rectal and skin temperatures with exercise. Thermal stress of football gear in exercise 11 11 Click to add footer 6  2024-11-11 What happens when you get overheated? 1 12 2 Hyperthermia The increase in body temperature that occurs with exercise. Physiological mechanisms used to dissipate heat. Increase in sweating rate (evaporation) Vasodilation of the cutaneous (skin vessels) 13 13 Click to add footer 7  2024-11-11 Thermoregulation Hypothalamus contains coordinating center for temperature regulation Acts as thermostat set and regulated at 37°C ± 1°C. Initiates responses to protect from buildup or loss of heat Activation of heat-regulating mechanisms: Thermal receptors in skin provide input to central control center Temperature of blood perfusing hypothalamus directly stimulates 14 14 Thermoregulation – Short term 15 15 Click to add footer 8  2024-11-11 Role of Skin and Subcutaneous Tissues 16 16 Role of the CV system Circulatory system “fine tunes” temperature regulation. Increases in internal heat dilate peripheral vessels as warm blood flows to the cooler periphery. Heat conservation occurs when blood shunts rapidly to deep body cavities and muscle mass 17 17 Click to add footer 9  2024-11-11 Blood Flow Redistribution Thermoneutral (25.0 ∘ C) Heat (43.3 ∘ C) 18 18 Exercise in the Heat: Cardiac Output Heart Rate Stroke Volume 19 19 Click to add footer 10  2024-11-11 OLD vs Young; Men vs women 20 20 Exercise Intensity and Body Temp 21 21 Click to add footer 11  2024-11-11 Example of a Heat Stress Study The Impact of Heat Exposure and Repeated Exercise on Circulating Stress Hormones 22 23 23 Click to add footer 12  2024-11-11 Core Body Temperature Response 24 24 Heart Rate 25 25 Click to add footer 13  2024-11-11 Plasma and Blood Volume 26 26 Catecholamine Response 27 27 Click to add footer 14  2024-11-11 Cortisol and HGh Response 28 28 Factors Affecting the CV Response to Heat Age and Gender Hydration Dehydration increases physiological strain Fitness Level Fitness improves thermoregulatory function and heat tolerance Body Composition Adipose tissue interferes with heat dissipation and acts as insulation; increases metabolic cost of activity Acclimatization The adaptive changes that occur when an individual undergoes prolonged or repeated exposure to a stressful environment; these changes reduce the physiological strain produced by such an environment 29 29 Click to add footer 15  2024-11-11 Acclimatization to Heat 30 Physiologic Adjustments To Heat Acclimatization Acclimatization Physiological effect Improved cutaneous blood flow Transports metabolic heat from deep tissues Effective redistribution of cardiac output Circulation to skin and muscles Lowered threshold for sweating Evaporative cooling begins early in exercise Redistribution of sweat over skin surface Use of body surface for evaporative cooling Increased sweat output Maximizes evaporative cooling Lowered salt concentration of sweat Preserves electrolytes in extracellular fluid Lower skin and core temperatures and heart Frees greater proportion of cardiac output to rate for standard exercise the active muscles Less reliance on carbohydrate catabolism Carbohydrate sparing 31 during exercise 31 Click to add footer 16  2024-11-11 Acclimatization Effects Tco HR Sweat Rate 32 32 Hydration Level in Various Sports 33 33 Click to add footer 17  2024-11-11 Fluid Ingestion During and after Exercise 70 kg, 2:30 marathoner looses 1-2 L H2O per hour (5 L) Gastric emptying and intestinal absorption are influenced by the composition of ingested fluid. Gastric emptying influenced by caloric content and volume ingested. Fluid with 2.5 – 10% CHO 200- 400 mL q 15 – 30 min Glucose increases intestinal absorption Exercise-induced hyponatremia Plasma Na < 135 mmol/L 34 34 Heat Illness “Probably the greatest stress ever imposed on the human CV system is the combination of exercise and hyperthermia. Together these stresses can present life-threatening challenges, especially in highly motivated athletes who drive themselves to extremes in hot environments.” Rowell, 1986 A spectrum of disorders that range in intensity and severity from mild cardiovascular and central nervous system disruptions to severe cell damage, including the brain, kidney and liver. 35 35 Click to add footer 18  2024-11-11 Minor exertional heat illness Heat Cramps Heat Syncope An acute disorder consisting of A temporary disorder brief, recurrent, and characterized by circulatory excruciating pain in the failure due to pooling of blood voluntary muscles of the legs, in the peripheral veins and the arms, or abdomen subsequent decrease in ventricular filling, which leads to a decrease in cardiac output 36 36 Major Exertional Heat Illness Heat Exhaustion Heat Stroke Characterized by a rapid and A serious medical emergency: weak pulse, fatigue, weakness, characterized by elevated skin profuse sweating, and core temperatures, psychological disorientation, tachycardia, vomiting, diarrhea, and fainting. hallucinations and coma. 37 37 Click to add footer 19  2024-11-11 Exertional Heat Illness 38 38 Treatment of Heat Illness 39 39 Click to add footer 20  2024-11-11 40 40 Cold Stress 41 Click to add footer 21  2024-11-11 COLD EXPOSURE Human exposure to extreme cold produces significant physiologic and psychologic challenges Further compromise of core temperature with: chronic exertional fatigue sleep loss inadequate nourishment reduced tissue insulation depressed shivering heat production 42 Hypothermia § Decrease in body’s temperature to a point that normal physiological function is impaired. Occurs in 3 stages: § Stage 1: Body temp drops 1 to 2ºC below normal (35-34ºC) § Loss in ability to perform complex motor tasks § Breathing becomes rapid & shallow § Stage 2: Body temp drops 2 to 4ºC below normal (34-32ºC) § Neuromuscular function is affected § Stage 3: Body temp drops below 32ºC § Body systems shut down, organs fail, brain dies 43 43 Click to add footer 22  2024-11-11 Physiological Changes with Reduced Core Temperature Stage °F °C Physiological Changes Normothermia 98.6 37.0 No noticeable effect Mild hypothermia 95.0 35.0 Maximal shivering, increased blood pressure 93.2 34.0 Amnesia; dysarthria; poor judgment; behavior change 91.4 33.0 Ataxia; apathy Moderate 89.6 32.0 Stupor hypothermia 87.8 31.0 Shivering ceases; pupils dilate 85.2 29.6 Cardiac arrhythmias; decreased cardiac output 85.2 29.6 Unconsciousness Severe hypothermia 82.4 28.0 Ventricular fibrillation likely; hypoventilation 80.6 27.0 Loss of reflexes and voluntary motion 78.8 26.0 Acid-base disturbances; no response to pain 77.0 25.0 Reduced cerebral blood flow 75.2 24.0 Hypotension; bradycardia; pulmonary edema 73.4 23.0 No corneal reflexes; areflexia 66.2 19.0 Electroencephalographic silence 64.4 18.0 Asystole 59.2 15.1 Lowest infant survival from accidental hypothermia 56.7 13.7 Lowest adult survival from accidental hypothermia 44 Body Fat, Physical Activity, and Cold Stress Differences in % body fat influence physiologic function in cold during rest and exercise Successful ocean swimmers possess larger amounts of subcutaneous fat than highly trained non-ocean swimmers; additional fat increases effective insulation in cold water Physiologic strain from cold-water and cold-land environments depends on metabolic level and resistance to heat flow through body fat 45 Click to add footer 23  2024-11-11 Body Fat, Physical Activity, and Cold Stress Differences in % body fat influence physiologic function in cold during rest and exercise Successful ocean swimmers possess larger amounts of subcutaneous fat than highly trained non-ocean swimmers; additional fat increases effective insulation in cold water Physiologic strain from cold-water and cold-land environments depends on metabolic level and resistance to heat flow through body fat 46 46 HOW COLD IS “TOO COLD”? Increased peripheral vasoconstriction during cold exposure causes dangerously low skin and extremity temperatures, particularly when compounded by increases in convective and conductive heat loss Predisposing factors to frostbite include: Alcohol use, low physical fitness, fatigue, dehydration, poor peripheral circulation Early warning signs of cold injury include: Tingling and numbness in fingers and toes or burning sensation in nose and ears Wind Chill Index - Developed as a public health tool to help prevent frost-bite and cold-induced injuries. Takes into account wind velocity measured at 1.3 m (5ft) 47 47 Click to add footer 24  2024-11-11 Wind-Chill Temperature Index Wind makes a difference The wind-chill temperature index provides a useful way to understand dangers from winter winds and freezing temperatures and provides frostbite threshold values 48 48 § Physiological thermoregulation in the cold monitored by § Cold receptors § Found in the skin, abdominal viscera, & spinal cord § Monitor change & rate of decrease in temperature § Fewer in number than heat receptors § Signal many different actions to occur 49 49 Click to add footer 25  2024-11-11 What happens when you get cold? 50 51 Click to add footer 26  2024-11-11 ACUTE ADAPTATIONS TO COLD STRESS Physiological Responses Behavioural responses Sweating ceases Curl-up – Shivering - ↑ BMR Add more clothing Eat more – Peripheral vasoconstriction Seek warmer environment – Thyroid hormone release Increase physical activity level – Sympathetic stimulation – Vasoconstriction and 52 piloerection 52 Exercise in the Cold Decreased nerve conduction Exercise-induced asthma velocity and neuromuscular activity Increased ventilation Reduction in force production Higher oxygen consumption Diminished power output Metabolic changes Increased reliance on CHO Decreased time to peak power 53 53 Click to add footer 27  2024-11-11 Cold Acclimatization § Acclimatization (natural environment)/Acclimation (artificial environment) § Behavioral changes § Warm-up § layering clothing & dressing properly § Psychological adjustments: ability to endure cold § Physiological adaptations § Higher basal metabolic rates 54 § Vasodilation in response to high altitude cold exposure 54 Cold Stress Study 55 Click to add footer 28  2024-11-11 56 56 Exercise and Cold Stress “Control” Sit in thermoneutral water (35° C) “Passive Heating” COLD EXPOSURE ENTRY Sit in warm water (2 h) Informed Consent, (38° C) PRE-TREATMENT Anthropometric Measures, (1 h). VO2 max “Thermal Clamp” Sit in cold air Exercise in cold water (18 °C) (5°C, 40 % r.h.) ° Exercise in 35 C water Castellani et al., (2002). Med. Sci. Sports Exerc. 34: 2013-2020 Brenner et al., (1999) J. Appl. Physiol. 87: 699-710 57 Click to add footer 29  2024-11-11 Rectal Temperature 58 Signs/Symptoms of Lowered Body Temperature Constant shivering Blue lips and fingers Confused behavior Poor co-ordination Poor decision making (decreased mental capacity) Decreased physiological performance 59 59 Click to add footer 30  2024-11-11 Risk Factors for Cold Injury Age Previous cold injury Disease (diabetes) Injury Raynaud’s phenonmenon Alcohol consumptions Decreases circulation Alters mental state 60 60 COLD STRESS AND SKIN § Non-freezing § Chilblain § Occurs with repeated exposure to air temps of 0-16°C § Skin appears red, swollen, may be sore or tingle § Trench foot § Occurs with prolonged contact in wet environment (0-10°C) § Freezing § Frostnip - firm, cold, white area § Frostbite – inflammed, tissue damage § Toes, fingers, checks, ears and nose most susceptible 61 61 Click to add footer 31  2024-11-11 Cold Stress and the Skin – Nonfreezing Injuries Chillbain Trenchfoot www.en.wikipedia.org www.ByeByeDoctor.com 62 62 Cold Stress and the Skin – Freezing Injuries Frostnip Frostbite www.eknow.info www.beforeitsnews.com 63 63 Click to add footer 32  2024-11-11 Mechanism of Frost Bite 64 64 Example Jan 17-18, 2019, Ottawa Valley (-31∘C) 120 Soldiers with the Royal Canadian Dragoons participated in basic Winter Survival Training (12 km March with overnight stay outside) 20 Soldiers suffered frostbite 65 65 Click to add footer 33  2024-11-11 Cold-Weather Clothing and Thermoregulation Six factors affect clothing’s clo (insulation) value: Wind speed: higher speed disturbs zone of insulation Body movements: moving disturbs zone of insulation Chimney effect: loosely hanging clothing ventilates trapped air layers away from body Bellows effect: vigorous body movements increase ventilation of air layers that conserve body heat Water vapor transfer: clothes resist water vapor exit Permeation efficiency factor: how well clothing absorbs liquid by capillary action 66 66 COLD-WEATHER CLOTHING Several layers of light Clothing layer against Mesh of cloth fibers Up to 40% of body clothing, or garments skin should wick traps air that then heat dissipates lined with animal fur, moisture. warms. through highly or synthetic fabrics vascularized head that with numerous layers represents only 8% of of trapped air, provide body’s total surface. better insulation than a single bulky clothing layer. Establishes a barrier to heat loss because cloth and air conduct heat poorly. 67 Click to add footer 34  2024-11-11 Questions? 68 Click to add footer 35