PA 5- Hyperthermia. Hypothermia.

Homeothermia

Definition: Mechanisms maintaining a constant internal body temperature (T), regardless of ambient temperature and physical effort.

Homeothermia Processes

  • Heat Generation: Thermogenesis

  • Heat Maintenance and Loss: Thermolysis. Balance between these processes is crucial; an imbalance leads to thermoregulation disorders, resulting in hyperthermia or hypothermia.

Body Temperature

  • Normal Central Body Temperature: 37°C (range: 36.2°C - 37.7°C)

  • Peripheral temperature is approximately 0.5°C lower than central temperature. This disparity is important for diagnosing temperature-related illnesses.

Thermoregulation

  • Thermic Homeostasis: Regulation through feedback control mechanisms, including neuroendocrine responses to changes in temperature.

  • Importance of Thermoregulation: Vital for enzyme activity, metabolic processes, and overall cell survival.

  • Hormonal mechanisms involved: hypothalamus, endocrine system, adrenal glands, which regulate thermogenesis and thermolysis in response to physiological signals.

Thermogenesis

  • Sources of Thermogenesis:

    • Cellular Energy Metabolism: The primary energy source, providing 70% of thermogenic energy through metabolic reactions.

    • ATP-dependent Na/K Pump Activity: Essential for maintaining cellular ion gradients, contributing to heat production.

Heat Conservation Mechanisms

  • Sympathetic Nervous System Activation: Induces vasoconstriction to conserve heat.

  • Stimulation of Adrenal Cortex: Increases secretion of hormones like norepinephrine, enhancing metabolic rate and heat generation.

  • Increased Muscle Tone & Involuntary Muscle Contraction: Shivering response generates additional heat.

Thermolysis

Mechanisms that result in heat loss through:

  • Radiation: Heat loss to the surrounding environment.

  • Convection: Heat loss through airflow.

  • Conduction: Direct contact with cooler surfaces resulting in heat transfer.

  • Evaporation: Important for cooling, especially in high temperature conditions, through sweat production.

Hyperthermia

Definition: Occurs when thermogenesis exceeds body capabilities, raising central temperature above the normal range after exposure to heat or intense exercise. Normal central temperature values lie between 36.2°C to 37.7°C.

Symptoms of Mild Hyperthermia (T < 39°C)

  • Vasodilation

  • Increased sweating

  • Reduced circulating blood volume

  • Decreased venous return and peripheral resistance

  • Symptoms: muscle weakness, vertigo, nausea, altered consciousness, heightened anxiety, and cognitive impairment.

Symptoms of Moderate Hyperthermia

  • Central Temperature: 40°C - 40.5°C

  • Enhanced thermolysis mechanisms with increased sweating and heat loss noted.

  • Risk of heat exhaustion and heat-related illnesses increases significantly during this stage.

Severe Hyperthermia

  • Central Temperature: > 40.5°C - 43°C

  • Consequences include cessation of thermoregulatory center function, brain edema, risk of thermal shock, multi-organ failure, and potential death if untreated.

Malignant Hyperthermia

Definition: Severe reaction to certain anesthetic drugs characterized by rapid increase in body temperature and severe muscle contractions.

  • Mortality Rate: Up to 5% with treatment; 75% without.

  • Etiopathogenesis: Genetic susceptibility due to defects in calcium transport affecting the ryanodine receptor leads to dysregulation in calcium homeostasis during anesthesia.

Mechanism of Malignant Hyperthermia

A genetic defect causes uncontrolled calcium release from the sarcoplasmic reticulum, leading to excessive muscle contraction, hypermetabolism, extreme heat production, and subsequent metabolic complications.

Signs and Symptoms of Malignant Hyperthermia

  • Muscle rigidity, rhabdomyolysis, myoglobinuria, acute renal failure, metabolic acidosis, and hyperkalemia with dysrhythmias, leading to cardiac complications.

Treatment for Malignant Hyperthermia

  • DANTROLENE: A muscle relaxant that inhibits calcium release from the sarcoplasmic reticulum, effectively halting the hypermetabolic state and restoring normal muscle function.

Thermic Myalgia

Definition: Muscle spasms caused by intense and prolonged physical activity in a warm environment, often requiring rest and hydration for recovery. Symptoms may include cramping and localized pain.

Insolation

Definition: The direct effect of solar radiation on the head, which can lead to serious complications such as cerebral edema and heat stroke.

  • Complications: Serious risks include serous meningitis, sunburn, and other heat-related illnesses, necessitating preventive measures during sun exposure.

Hyperthermia Treatment Principles

  • Move from warmth to cooler environments.

  • Cooling methods: Utilize ice packs, cool baths, and air conditioning.

  • Hydration: Employ enteral and parenteral routes to counteract fluid loss.

  • Correct hydro-electrolyte imbalances: Monitor hydration status and electrolyte levels to prevent complications.

  • Urine alkalinization: Administer bicarbonate if indicated to manage acidosis.

  • Arrhythmia management: Continuous cardiac monitoring is crucial to address any dysrhythmias that may develop.

Hypothermia

Definition: Central temperature < 35°C after prolonged exposure to cold, which can lead to diminished thermoregulation.

  • Commonly Affected Groups: Elderly, infants, alcoholics, patients with certain medical conditions, including diabetes and cardiovascular diseases.

Pathophysiological Mechanisms of Hypothermia

Failure of the mechanisms of heat production and conservation leads to excessive heat loss or inadequate production, leading to systemic hypotension and metabolic derangement.

Metabolic Effects of Hypothermia

Increased oxygen consumption, decreased hepatic glycogen stores, and reduced metabolic reaction rates can impair vital organ function.

Signs and Symptoms of Hypothermia

  • Peripheral vasoconstriction, which may lead to cyanosis.

  • Bradycardia: Heart rate often decreases as hypothermia progresses.

  • ECG changes, such as a prolonged QT interval and potential asystole, indicating severe cardiac compromise.

Stages of Hypothermia

  1. Agitation Stage (excitation)

  2. Exhaustion Stage (inhibition)

  3. Paralysis Stage (critical)

Agitation Stage Symptoms

Central temperature 32°C - 35°C: chills, hyperglycemia, confusion, and impaired cognitive function, often mistaken for intoxication.

Exhaustion Stage Symptoms

Central temperature 32°C - 28°C: impaired energy sources, rhythm disturbances, altered consciousness with lethargy.

Paralysis Stage Symptoms

Central temperature < 28°C: coma, cardiovascular complications, and irreversible cellular damage leading to death.

Long-term Effects of Hypothermia

Potential complications include organ failure, myocardial infarction, pancreatitis, cognitive impairments, and hemostatic issues.

Treatment Principles for Hypothermia

  • Gradual rewarming to avoid shock; use of blankets and warm fluids.

  • Oxygen therapy as needed to support breathing.

  • Vital Function Support: Monitoring heart rate, blood pressure, and respiratory function is critical.

  • Glucose Infusion: Administered in cases of hypoglycemia.

  • Monitoring Diuresis: Electrolyte imbalances are common; fluid management is key.

Regulation of Core Body Temperature

Reference: Hutchison's Clinical Methods, detailing the mechanisms and pathways involved in thermoregulation.