Recording-2025-03-09T23:31:31.524Z

Normal Body Temperature Regulation

• Core Body Temperature:

  • Normal values are 37 degrees Celsius (98.6 degrees Fahrenheit).

  • Regulated within a narrow range of ±0.2 degrees Celsius.

  • Core temperature reflects the temperature of deep tissues in the thorax, abdomen, and central nervous system and is typically 2-4 degrees Celsius warmer than skin temperature.

• Hypothalamus Role:

  • Detects temperature deviations outside the interthreshold range, instigating homeostatic mechanisms to restore normal temperature.

  • Critical for normal organ function, enzymatic activity, and cellular processes.

Thermal Sensors

• Locations of Heat Sensitivity:

  • Sensitive cells located in the hypothalamus, brainstem, spinal cord, deep abdominal tissue, thoracic tissue, and skin.

• Types of Fibers:

  • Efferent fibers for cold receptors: A delta fibers.

  • Efferent fibers for warm receptors: C fibers.

• Communication Pathways:

  • Both receptors send signals to the anterior hypothalamus via the lateral spinothalamic tract or cranial nerve V.

Central Thermal Regulators

• Comprising of the hypothalamus and spinal cord.

Response to Hypothermia

• Detection & Transmission:

  • Cold detected by thermal sensors and transmitted to central regulators, prompting efferent responses:

    • Behavioral modification (e.g., dressing warmly).

    • Vasoconstriction mediated by alpha-1 adrenergic receptors.

    • Shivering increases metabolic rate, potentially up to six times the basal rate.

  • Nonshivering thermogenesis occurs mainly in neonates via beta-3 adrenergic receptors in brown adipose tissue.

Response to Hyperthermia

• The body counteracts by sweating, vasodilation, and behavioral changes.

• **Threshold and Gain: **

  • Threshold: Temperature where effectors activate.

  • Gain: Rate of response to temperature changes.

Body Heat Distribution

• Core and Periphery:

  • 33% of body heat in the periphery; fluctuations here are wider compared to the core.

  • 66% of heat lies within the central core (brain and trunk organs) maintained between 36.5 and 37.5 degrees Celsius.

Unintended Perioperative Hypothermia

• Defined as a core temperature below 36 degrees Celsius during surgery; affects up to 20% of patients.

• Classification of Hypothermia:

  • Mild: 32 to 36 degrees Celsius.

  • Moderate: 28 to 31.9 degrees Celsius.

  • Severe: < 28 degrees Celsius.

Mechanisms of Heat Loss During Anesthesia

• Radiation:

  • Primary avenue for heat loss (up to 50%).

  • Occurs via emission of infrared waves; exacerbated by cool surroundings.

  • Loss of heat attributed to the temperature gradient between body and surrounding objects.

• Convection:

  • Accounts for up to 30% of heat loss.

  • Heat is lost via movement of air or liquid; disruption of trapped warm air layer by air currents increases heat loss.

  • Increased air exchanges in operating theaters promote greater heat loss via convection.

• Evaporation:

  • Accounts for 15-50% of heat loss, especially during surgeries with large exposed surfaces.

  • Mitigated by covering wet surfaces and improving humidity, warming inspired gases.

• Conduction:

  • Relatively insignificant (about 5%); occurs from direct contact with cool surfaces.

  • Increased during anesthesia if patients are on cold conductive surfaces or IV fluids.

• Respiration:

  • Contributes to 5-10% of heat loss; related to humidification and warming of inspired air.

  • Minimized by using heat and moisture exchanges (HME).

Causes of Hypothermia During Anesthesia

• Loss of behavioral response to cold.

• Impaired thermoregulatory mechanisms; anesthesia widens the interthreshold range from 0.4 to 4 degrees Celsius.

• Peripheral vasodilation from anesthesia leads to rapid core heat loss to periphery.

• Extended surgeries, large body surface area exposure, and use of unwarmed IV fluids worsen heat loss.

• Neonates:

  • Higher surface area to volume ratio, immaturity in thermoregulation, and reduced insulating fat lead to increased heat loss.

Phases of Hypothermia During Anesthesia

• Phase 1:

  • Immediate redistribution of body heat after induction.

  • Core temperature drops due to anesthesia-induced vasodilation.

  • Duration: 30-45 minutes; drop of 1-1.5 degrees Celsius.

• Phase 2:

  • Gradual core temperature reduction; lasts 2-3 hours with a drop of 1 degree Celsius.

  • Result of heat loss exceeding heat gain from metabolism.

  • Negative heat balance resulting in hypothermia.

• Phase 3:

  • Plateau where heat loss equals heat production; stabilization begins after the core temperature falls to about 34 degrees Celsius.

Clinical Implications of Hypothermia

Cardiorespiratory Effects:

• Reduced cardiac output and oxygen delivery; leftward shift in the oxygen-hemoglobin dissociation curve.

• Increased incidence of arrhythmias and myocardial infarction risk among hypothermic patients.

Hematological Effects:

• Impairment of coagulation function; increased blood loss and tendency for transfusion requirements raised at lower temperatures.

Metabolic Effects:

• Reduced metabolic rates; altered pharmacodynamics resulting in prolonged drug effects.

CNS Effects:

• Cognitive functions worsen progressively with temperature drops, leading to potential EEG inactivity.

Treatment of Shivering:

• Strategies include warming the patient; pharmacological options such as Platidine and Clonidine may reduce shivering without increasing hypothermic risk, provided patients are warmed simultaneously.

Surgical Outcomes:

• Impaired wound healing and increased infection risk; longer hospital stays and higher costs associated with hypothermia.

Pediatric Population Considerations:

• Neonates and children exhibit increased metabolic and hypothermic susceptibility compared to adults; managing perioperative hypothermia is critically important in this demographic.