Lecture 11 ExPhys

Introduction to Heat Stress and Homeostasis

• Internal Heat Generation: Our bodies generate internal heat primarily through metabolism and muscle contractions.

• Beneficial Heat Production: Generating heat is highly beneficial during exercise in cold environments, as it serves to maintain a normal body temperature.

• Heat Stress Definition: Within the context of this study, heat stress is defined as any environmental condition that increases body temperature and jeopardizes homeostasis.

• Environmental Strain: When metabolic heat production is high (due to exercise) and is combined with a hot environment, the internal and external factors create a significant strain on the human body.

Physiological Mechanisms of Thermoregulation

• Homeothermic Nature: Humans are homeothermic, meaning our internal body temperature is regulated through specific physiological mechanisms. This regulation typically maintains the resting core temperature within the range of $97-100\,^\circ F$, regardless of changes in the surrounding environment.

• Methods of Heat Transfer: Body heat is transferred between the body and the environment through four primary methods:     - Conduction: Transfer of heat through direct physical contact.     - Convection: Transfer of heat by the movement of air or liquid across the skin.     - Radiation: Transfer of heat via electromagnetic waves.     - Evaporation: Transfer of heat as liquid sweat turns into gas.

• Rest vs. Exercise states:     - At rest, the majority of heat is lost through radiation and convection.     - During exercise, evaporation becomes the most critical avenue for heat loss.

• Environmental Humidity Factors:     - When air temperature is close to skin temperature, evaporation of sweat is the only available means of cooling the body.     - High Humidity: Results in a decreased ability to sweat effectively for cooling.     - Low Humidity: Results in a greater ability to sweat effectively for cooling.

• The Hypothalamus: The Preoptic-anterior area of the hypothalamus (POAH) serves as the body’s primary thermoregulatory center.

• Thermoreceptors: Located in the skin and within the body core, these receptors detect temperature fluctuations.

• Automatic Responses:     - If body temperature is too high: The body activates sweating mechanisms.     - If body temperature is too low: The body activates shivering and increases metabolism.

Cardiovascular Function and Limitations in the Heat

• Incremental Demand: Exercise naturally increases demands on the cardiovascular system; however, adding the need to regulate core temperature in the heat enhances this burden significantly.

• Dual Demand of Blood Flow: During exercise in hot conditions, the circulatory system must perform a dual role:     1. Transporting blood to the working muscles to provide oxygen and nutrients.     2. Transporting blood to the skin to dissipate heat via sweating.

• Cardiovascular Compensations: To manage this dual demand, two specific changes occur:     1. Increased Cardiac Output: Cardiac output rises above the levels required for the same intensity in cool conditions. This is achieved by increasing both heart rate and contractility.     2. Blood Shunting: Blood flow is shunted away from nonessential areas and redirected toward the skin.

• Limitations to Performance:     - There is a point where the cardiovascular system cannot compensate for the dual demands of exercise and thermoregulation.     - Factors that overload the cardiovascular system or prevent heat dissipation drastically impair performance and increase the risk of overheating.     - Exercise is often limited when heart rate reaches its maximum, which occurs sooner in the heat, particularly for untrained or non-heat-acclimated individuals.

• Critical Temperature Theory: The brain sends signals to cease exercise when the core temperature reaches a range of $104-105.8\,^\circ F$.

• Dehydration Interaction: Blood flow to working muscles is generally well-maintained at high temperatures unless significant dehydration occurs.

The Role of Dehydration in Cardiovascular Strain

• Pre-existing Dehydration: Risks are high for those exercising in the heat or starting in a hypohydrated state.

• Physiological Consequences:     - Decrease in total body fluid leads to a reduction in sweat rate and skin blood flow.     - This results in increased body heat storage and heightened physiological strain.

• Clinical Signs:     - Increase in heart rate.     - Increase in Total Peripheral Resistance (TPR).     - Significant reductions in stroke volume and cardiac output.     - Overall decrease in blood flow.

• Outcome: Ultimately increases the risk of heat stroke or force-stops exercise due to fatigue.

Health Risks and Conditions Related to Heat

• Failure of Defenses: Despite thermoregulatory defenses, the combination of metabolic heat and environmental heat can raise internal temperatures to levels that impair cellular function.

• Index of Stress: Air temperature alone is not an accurate index of physiological stress.

• Six Variables of Heat Stress:     1. Metabolic heat production.     2. Air temperature.     3. Humidity.     4. Air velocity.     5. Radiant heat stress.     6. Clothing.

• Wet-Bulb Globe Temperature (WBGT): This is the best method for measuring total heat stress on the body.

• Heat-Related Disorders:     - Heat Cramps: Characterized by severe, painful cramping of large skeletal muscles. Theories for the cause include the Fluid and Electrolyte Theory and the Altered Neuromuscular Theory.     - Heat Exhaustion: Symptoms include fatigue, dizziness, nausea, vomiting, fainting, and a weak, rapid pulse. Accompanied by a core temperature between $97-104\,^\circ F$. It is caused by the cardiovascular system's inability to meet blood flow demands due to low blood volume.     - Heat Stroke: A life-threatening emergency caused by the failure of the thermoregulatory mechanisms. Characterized by core temperatures exceeding $104\,^\circ F$ (often $>105\,^\circ F$) and altered mental status (confusion, disorientation, unconsciousness). If untreated, it leads to coma and death.

• Heat Exhaustion Treatment:     - Rest in a cool environment.     - Elevate feet to improve venous return to the heart.     - Provide fluids (but do not "push" or force fluids).

• Heat Stroke Treatment:     - Immediate immersion in cold water (preferably with ice).     - If immersion is unavailable: wrap the body in cold wet sheets and apply ice packs to the groin, armpits, and neck.     - Use fans or stir the water to increase convective cooling.     - Monitor rectal temperature.     - Policy: The individual should not be transported until the body has begun to cool.

Sickle Cell Trait and Environmental Complications

• Sickle Cell Anemia: A genetic disease where red blood cells (RBCs) become oblong and inefficient at carrying oxygen. This requires two copies of the recessive gene.

• Sickle Cell Trait: Inheriting only one copy of the gene. These individuals typically do not display sickled cells under normal conditions but are at high risk for pathologies exacerbated by exercise or dehydration.

• Statistics and Risk:     - College football players with the sickle cell trait have a $15\times$ increased risk for exertional death compared to those without the trait.     - Sudden deaths usually occur during high-intensity exercise in high heat and humidity due to extreme cardiovascular strain.     - Potential mechanisms include defects in kidney function.

Strategies for Preventing Hyperthermia

• Scheduling: Exercise when the environment is less stressful (early morning/late evening) or inside with air conditioning.

• Intensity: Decrease the intensity of the workout.

• WBGT Thresholds: Outdoor events should require special precautions if the Wet-Bulb Globe Temperature exceeds $82.4\,^\circ F$.

• Operational Precautions:     - Fluids must be readily available.     - Hydration/rest breaks should occur every $20-30\,minutes$.     - Utilize more frequent substitutions in team sports.     - Provide cold towels, cooling tubs, fans, and tents.

• Safety Protocols:     - Remove anyone mentioning or showing signs of feeling unwell immediately.     - Educate all staff and athletes on symptoms and risks.     - Ensure proper clothing and equipment are used.

Physiological Adaptations: Heat Acclimation vs. Acclimatization

• Definitions:     - Heat Acclimation: Physiological adaptations resulting from multiple repeated exposures over a short period (days to weeks).     - Acclimatization: Gradual adaptations occurring from living in a specific environment for months to years.

• Process of Acclimation:     - Requires exercise in the heat; sitting in a sauna is insufficient.     - Dependent on environmental conditions, duration of exposure, and exercise intensity.     - Typically takes $9-14\,days$ of exercise in the heat for full effect.     - Well-trained individuals acclimate faster than untrained individuals.

• Physiological Changes:     - Plasma Volume Expansion: Occurs within the first $1-3\,days$ due to fluid shifts and increased proteins/oncotic pressure. This expansion is temporary (returns to baseline in $10\,days$) but supports stroke volume and cardiac output initially.     - Heart Rate and Core Temp: Acclimated individuals have a lower heart rate and core temperature for a given submaximal exercise intensity.     - Improved Sweating: Sweating begins earlier (reducing stored heat) and is more evenly distributed (focusing on arms and legs for dissipation). Sweat becomes more dilute to conserve sodium, aided by increased aldosterone.

Physiological Responses to Cold Stress

• Cold Stress Definition: Any environmental condition causing a loss of body heat that threatens homeostasis.

• Defense Mechanisms:     - Peripheral Vasoconstriction: Sympathetic stimulation of smooth muscle in skin arterioles causes them to contract, reducing peripheral blood flow (limbs/face) to minimize heat loss.     - Non-shivering Thermogenesis: SNS stimulation increases metabolic rate to generate more heat.     - Shivering: Rapid, involuntary skeletal muscle contraction and relaxation that can increase the body's rate of heat production by $4-5\times$.     - Behavioral Responses: Huddling together for warmth.

Factors Affecting Body Heat Loss in Cold Environments

• Thermal Gradient: Generally, heat loss is greater when the difference between skin temperature and environmental temperature is larger.

• Overdressing Risk: Can lead to sweating, which then causes rapid heat loss through evaporation when the exercise stops or the environment is cold.

• Body Composition and Size:     - Subcutaneous fat and inactive peripheral muscles act as excellent insulators.     - Leaner, smaller individuals lose heat faster than those with higher fat mass.     - Surface Area to Mass Ratio: Larger people have a smaller ratio, making them less susceptible to hypothermia.

• Sex and Age Differences:     - Sex: Women usually have more subcutaneous fat (advantageous) but smaller muscle mass (disadvantageous as muscles generate heat via shivering). Overall sex differences are minimal.     - Children: Have a high surface area to mass ratio, leading to greater heat loss.     - Elderly: Often lose muscle mass, reducing insulation and shivering capacity, making them highly susceptible to hypothermia.

• Windchill: An index of the cooling power of the environment based on air movement. It does NOT change the air temperature but increases convective heat loss, increasing the risk of tissue freezing.

• Cold Water Immersion:     - Heat loss by convection is $26\times$ faster in cold water than air of the same temperature.     - At $15\,^\circ C$ ($59\,^\circ F$), rectal temperature decreases by approximately $2.1\,^\circ C$ ($3.8\,^\circ F$) per hour.     - Moving water accelerates this loss further.

Muscle and Metabolic Responses to Cold

• Reduced Muscle Function: Cold muscles produce less force; the periphery is impacted most and first.

• Substrate Utilization Shifts:     - Generally, prolonged exercise increases Free Fatty Acid (FFA) mobilization via Epinephrine/Norepinephrine ($E/NE$).     - However, cold-induced vasoconstriction in skin and subcutaneous fat prevents the effective mobilization of FFAs.     - Consequently, Glucose becomes the primary fuel source during exercise in the cold.     - Shivering is fueled by both glycogen (carbs) and fats.

Health Risks Associated with Cold Exposure

• Hypothermia: Defined as a core temperature dropping below $95\,^\circ F$.

• Cardiorespiratory Effects: Risk of cardiac arrest; breathing in cold air rapidly can damage lung structures.

• Frostbite: Tissue death and freezing of exposed skin due to lack of circulation and oxygen.

• Exercise-Induced Asthma: Caused by the drying of airways or cold-induced bronchospasm; common in winter sport facilities.

Atmospheric and Thermal Characteristics of Altitude

• Terminologies:     - Barometric Pressure: Total pressure exerted by the atmosphere.     - Partial Pressure of Oxygen ($PO_2$): The portion of pressure exerted specifically by oxygen molecules.     - Hypobaric: Environment with reduced barometric pressure.     - Hypoxia: Low $PO_2$ in the air.     - Hypoxemia: Low $PO_2$ in the blood.

• Pressure at Sea Level: $760\,mmHg$.

• Pressure at Mount Everest: $250\,mmHg$.

• Air Composition: At any elevation, the air contains $20.93\%\text{ Oxygen}$, $0.03\%\text{ CO}_2$, and $79.04\%\text{ Nitrogen}$. The percentage of oxygen does not change; rather, the molecules are spread thinner, exerting less pressure.

• Thermal Gradient: Air temperature decreases at a rate of approximately $1.8\,^\circ F$ for every $500\,ft$ of ascent. Everest's summit is estimated at $-40\,^\circ F$.

• Solar Radiation: The intensity of solar radiation increases as altitude increases.

Acute Physiological Responses to Altitude Exposure

• Ventilation: Pulmonary ventilation ($V_e$) increases immediately as chemoreceptors detect low $PO_2$. This involves increased tidal volume and respiratory rate.

• Respiratory Alkalosis: High ventilation causes blood $PCO_2$ to fall and blood pH to increase. This helps oxygen reach tissues but prevents ventilation from over-accelerating.

• Renal Compensation: Kidneys excrete bicarbonate ions to offset respiratory alkalosis.

• Pulmonary Diffusion: The actual ability of oxygen to diffuse in the lungs does not change; low blood oxygen is purely due to the low environmental $PO_2$.

• Oxygen Transport:     - Less oxygen binds to hemoglobin.     - To compensate, the oxyhemoglobin dissociation curve shifts to the left.

• Gas Exchange at Muscles: Reduced $PO_2$ in blood means less oxygen diffuses into muscles, making them highly susceptible to fatigue.

• Blood Adjustments:     - Volume: Initial decrease in blood volume due to respiratory water loss and increased urine production, which increases hematocrit.     - EPO: Erythropoietin is released from the kidneys to stimulate RBC production (takes weeks to months).

• Cardiac Output: Increases at rest and submaximal levels (via increased heart rate driven by $E/NE$) to compensate for low oxygen. However, maximal cardiac output is decreased due to lower stroke volume and lower maximal heart rate.

• Metabolism: Increased Basal Metabolic Rate ($BMR$), greater reliance on carbohydrates, initially increased lactate, and decreased blood pH.

Exercise Performance and Clinical Risks at Altitude

• Performance:     - Endurance events are impaired beginning at roughly $5,000\,ft$.     - $VO_2\max$ decreases as altitude rises due to lower arterial $PO_2$ and maximal cardiac output.     - Anaerobic activities ($<1\,minute$) are generally not impaired because they do not rely on oxygen transport.

• Health Risks:     - Acute Mountain Sickness (AMS): Usually happens above $8,000\,ft$. Symptoms: Headache, insomnia, Cheyne-Stokes breathing. Prevention: Gradual ascent.     - High-Altitude Pulmonary Edema (HAPE): Life-threatening accumulation of fluid in the lungs and blood clots. Symptoms: Cough, chest tightness, fatigue, cyanosis (blue lips), syncope. Treatment: Oxygen, descent, hyperbaric bag.     - High-Altitude Cerebral Edema (HACE): Rare but fatal ($>14,000\,ft$); fluid in the cranial cavity. Symptoms: Confusion, lethargy, difficulty walking, unconsciousness. Treatment: Oxygen, descent, hyperbaric bag.

General Hydration Guidelines for Environmental Extremes

• Drinking Strategy: Drink primarily when thirsty; do not force fluids if not thirsty.

• Symptom Awareness: Headaches, cramps, or disorientation do not always indicate dehydration.

• Baseline: Start exercise well-hydrated. Urine should be a light-yellow color.

• Environmental Risk: Dehydration can occur easily at altitude and in the cold due to increased respiratory rates and dry air.