Age-related changes in thermoregulation

Basal & Central Control - The Aging Thermostat

• Basal Metabolic Rate (BMR): ↓ BMR from ↓ muscle mass (sarcopenia) and altered thyroid function, reducing basal heat production.
• Hypothalamic Dysfunction: The preoptic area of the hypothalamus experiences neuronal loss and ↓ sensitivity to neurotransmitters (e.g., norepinephrine).
  • Leads to blunted autonomic responses: impaired shivering, ↓ vasoconstriction to cold, and ↓ sweating/vasodilation to heat.
  • Widens the thermoneutral zone, allowing core temperature to drift further before corrective action is initiated.

⭐ A blunted or absent febrile response is common in the elderly with infections; confusion or functional decline may be the only initial signs.

Heat Production & Conservation - Can't Crank the Heat

• ↓ Basal Metabolic Rate (BMR):
  • Primarily from age-related sarcopenia (↓ lean body mass) & ↓ thyroxine levels.
  • Results in lower baseline heat production.
• Impaired Shivering Thermogenesis:
  • ↓ Muscle mass reduces the intensity and efficacy of shivering.
  • Delayed onset of shivering in response to cold stress.
• Inefficient Vasoconstriction:
  • Blunted response of cutaneous blood vessels to cold stimuli.
  • Leads to persistent heat loss by failing to shunt blood away from the periphery.
• Behavioral & Sensory Deficits:
  • Reduced perception of cold temperatures.
  • Immobility may prevent seeking warmer environments.

⭐ A normal or even low body temperature in an elderly patient can mask a severe underlying infection. Always consider sepsis even in the absence of fever.

Heat Dissipation & Sensation - Faulty Coolant System

• Impaired Sweating:
  • ↓ Number and functional capacity of eccrine sweat glands.
  • Leads to a significant ↓ in evaporative cooling, the primary mechanism for heat dissipation in high ambient temperatures.
• Reduced Skin Blood Flow:
  • Blunted cutaneous vasodilation in response to ↑ core body temperature.
  • Results from age-related endothelial dysfunction and ↓ nitric oxide availability.
  • This impairs convective heat loss from the core to the periphery.
• Sensory & Thirst Deficits:
  • ↓ Thermoreceptor sensitivity leads to a delayed perception of being hot.
  • Thirst sensation is blunted, predisposing to dehydration, which further reduces sweat output and cardiovascular reserve.

⭐ Clinical Pearl: Elderly patients often have a blunted febrile response to infection. A persistent core temperature >37.2°C (99°F) can signify a serious infection that would cause high fever in a younger individual.

Clinical Implications - Hypo & Hyperthermia Risks

• Hypothermia Risk (Core Temp <35°C / 95°F):

  • Elderly are more susceptible due to:
    • ↓ Shivering & non-shivering thermogenesis.
    • Impaired vasoconstriction.
    • ↓ Metabolic rate & subcutaneous fat.
    • Reduced perception of cold.
  • Often precipitated by sepsis, hypothyroidism, or immobility.

• Hyperthermia Risk (Heat Stroke/Exhaustion):

  • Higher risk in elderly because of:
    • ↓ Sweating capacity (gland atrophy).
    • Impaired cutaneous vasodilation.
    • ↓ Thirst drive, leading to dehydration.

⭐ Medication Alert: Many drugs worsen thermoregulation in the elderly. Diuretics can cause volume depletion, β-blockers can limit cardiac response to heat, and anticholinergics impair sweating.

High‑Yield Points - ⚡ Biggest Takeaways

• Elderly have a narrower interthreshold range, making them susceptible to both hypothermia and hyperthermia.
• Impaired afferent sensing leads to a decreased perception of temperature changes, delaying behavioral responses.
• Reduced shivering capacity and a lower basal metabolic rate blunt the thermogenic response to cold.
• Impaired cutaneous vasoconstriction increases heat loss, while impaired vasodilation and decreased sweating limit heat dissipation.
• Polypharmacy and chronic diseases frequently exacerbate these age-related vulnerabilities.