Thermoregulation: Fever, Hyperthermia, Hypothermia - Quick Reference

Thermoregulation: Quick Reference

  • Thermoregulation = body’s process to balance heat production and heat loss; regulated by the hypothalamus. Core temperature target ≈ 98.6∘F98.6^{\circ}\mathrm{F}. Normal range varies: 97−99∘F97-99^{\circ}\mathrm{F}. Core temp remains stable; surface temp varies. Daily pattern: highest in evening, lowest in early morning.

  • Major concepts:

    • Hypothalamus detects deviations and initiates responses to restore core temp.

    • Core temperature vs. skin temperature dynamics.

  • Temperature transfer mechanisms:

    • Conduction: heat transfer through direct contact with a cooler/heater surface (e.g., OR table).

    • Convection: fluid motion of air or water (e.g., fan, baths).

    • Radiation: heat transfer without contact (e.g., sun).

    • Evaporation: heat loss via vaporization (e.g., sweat).

  • Cold response in the body:

    • Receptors detect cold; sensors are more responsive to cold than heat.

    • Responses: shivering, vasoconstriction.

  • Factors that affect thermoregulation:

    • TBI, age, nutrition, surgery, infection, radiation, brain tumors, medications, chronic diseases.

  • Terminology you should know:

    • Afebrile = without fever

    • Tmax = highest temperature recorded

    • Hypothermia = < 95∘F95^{\circ}\mathrm{F}

    • Febrile/Pyrexia = > 101.3∘F101.3^{\circ}\mathrm{F}

    • Hyperthermia = > 104.0∘F104.0^{\circ}\mathrm{F}

  • Temperature assessment: common routes

    • Oral, Axillary-air pit, Rectal- most accurate (butt), Temporal- across head, Tympanic- ear temperature

    • Core (invasive): Esophageal

  • Life-span considerations (overview):

    • Older adults: higher risk due to loss of subcutaneous fat, inactivity, impaired thermoregulation; fever may be blunted; nutrition matters.

    • Young children: can tolerate high temps but febrile seizures possible in infants/young children.

    • Pregnancy: temp fluctuations—1st trimester ↑ temp; 2nd/3rd trimester ↓ temp; returns to baseline ~12 weeks postpartum.

Fever: Definition, Causes, and Management

  • What is fever?

    • Temp > 101.3∘F101.3^{\circ}\mathrm{F}

    • Low-grade fever varies by individual

    • Body’s natural response to infection; not always bad.

  • Causes of fever:

    • Immune response, stress, inflammation

    • Drugs: amphetamines, drug fever, alcohol withdrawal

    • Overexposure to heat, autoimmune diseases (e.g., RA)

  • Fever pathophysiology (brief):

    • Rapid rise in core temp due to a change in the set point (infection, dehydration, stress)

    • Chills with cold sensation, then plateau at new set point

    • Vasodilation may occur with heat loss; risk of hypotension; skin warm to touch.

    • Metabolic demand ↑ (e.g., ~1° F temp rise → ~7% ↑ O2 need).

  • Fever types (patterns):

    • Intermittent: fever with normal periods

    • Remittent: wide fluctuations in a 24 h period

    • Relapsing: brief afebrile periods between febrile spells

    • Constant: persistently above normal

    • Fever spike: rapid rise to fever then return to normal within hours

  • Fever signs/symptoms:

    • Flushed skin, warm skin, weakness, fatigue, malaise

    • Headache, myalgias, poor appetite, vomiting/diarrhea

    • Tachypnea, tachycardia, hypotension; cognitive changes; seizures (in severe cases)

  • Diagnostic labs/testing:

    • Initial: CBC, UA, CXR

    • Additional: BUN/creatinine, glucose, electrolytes, blood cultures

  • Non-pharmacologic interventions:

    • Cooling techniques (fan, cooling rag, ice packs)

    • Cooling blankets; maintain room temperature

    • Cool fluids; oral intake increased; identify source

  • Pharmacologic treatment:

    • Acetaminophen (Tylenol): antipyretic/analgesic; typical dose 325 mg–1 g; max 4 g/day4\,g/day; liver metabolism

    • Aspirin: 325–650 mg; max 4 g/day4\,g/day; bleeding risk; kidney metabolism

    • Ibuprofen (NSAID): 200–800 mg; bleeding risk; renal metabolism

Hyperthermia: Overview and Management

  • Definition: extremely elevated body temperature, typically > 104∘F104^{\circ}\mathrm{F}; includes heat stroke, heat exhaustion, drug-induced hyperthermia, TBI, stroke, SCI.

  • Pathophysiology (conceptual):

    • Cellular dysfunction with release of intracellular fluid

    • Increased blood flow and work on the heart; potential failure of cardiac output

    • Risk of hypotension and organ failure if not treated

  • Initial management (ABC):

    • Airway, Breathing, Circulation first

    • Antipyretics/NSAIDs

    • External cooling: ice packs, cooling blankets, cold IV fluids, cold lavages

    • Remove excess clothing

  • Diagnostic testing:

    • CBC, renal panel, lactic acid, CT scan (if indicated)

    • Monitor vital signs continuously

  • Hyperthermia patterns and causes:

    • Heat exhaustion vs heat stroke distinctions (see below)

  • Heat exhaustion vs heat stroke (brief):

    • Heat exhaustion: hypovolemia, hypotension, fatigue, headaches, dizziness; sweating present; treat with fluids and rest

    • Heat stroke: hot, dry skin or absence of sweating, confusion, seizures, hypotension, stupor- loss of coronation, not making sense, cerebral edema, swelling of the brain, death, potential organ failure; requires aggressive cooling and support

Lifespan Considerations for Hyperthermia

  • Children/Adolescents:

    • Fever often infection-related; consider mononucleosis in adolescents with pharyngitis/lymphadenopathy, ear infections, hand, foot & mouth, strep throat, febrile seizures- rapid increase in temperature

    • Febrile seizures more common in infants/young children

  • Older adults:

    • May not mount fever with infection; risk of delayed treatment

Hypothermia: Definition, Mechanisms, and Stages

  • Definition: temp < 95∘F95^{\circ}\mathrm{F} (therapeutically used in some contexts); organ death risk below 82.4∘F82.4^{\circ}\mathrm{F} (≈ 28∘C28^{\circ}\mathrm{C})

  • Mechanisms:

    • Excessive heat loss

    • Inadequate heat production to counteract loss

    • Impaired hypothalamic thermoregulation

    • When were cold we vasoconstrict

  • Stages of hypothermia:

    • Mild: 32−35∘C32-35^{\circ}\mathrm{C} (89.6-95°F)

    • Moderate: 28−32∘C28-32^{\circ}\mathrm{C} (82.4-89.6°F)

    • Severe/Profound: < 28∘C28^{\circ}\mathrm{C} (< 82.4°F)

  • Mild hypothermia signs:

    • Fatigue, slurred speech, poor coordination, confusion, irritability, shivering, tachycardia, tachypnea

  • Moderate hypothermia signs:

    • Depressed mental status, no shivering, bradycardia, bradypnea, hypotension, pallor/cyanosis, hallucinations, possible coma

  • Severe hypothermia signs:

    • Apnea, absent pulse, ventricular arrhythmias, dilated pupils, coma

  • Common causes/risk factors:

    • Outdoor exposure, trauma, alcohol/drugs- makes you feel warm when you’re not, endocrine disorders (thyroid), chronic diseases, medications, sepsis, age, social isolation- homelessness, going on a walk and no one knows, burns

  • Signs and symptoms to monitor:

    • Fatigue, altered mental status, vital sign abnormalities, poor perfusion, frostbite risk if exposed to cold, weak pulse, shivering, confusion, piloerection- hair on your arms stick up

  • Hypothermia labs:

    • Potassium (K+), electrolytes, coagulation studies, arterial blood gas, cardiac enzymes,

  • Lifespan considerations:

    • Newborns/children: high surface-to-volume ratio; rapid cooling risk; caregiver/neglect concerns, lose 4x the amount of heats than adults due to decreased subcutaneous tissue and increased surface area.

    • Older adults: reduced subcutaneous fat and thermoregulation; comorbidities (diabetes, neuropathy- can’t feel fingers going numb, hypothyroidism)

    • Cognitive impairment and sensory decline increase risk

Frostbite: Definition and Care

  • Frostbite = freezing injury to skin and subcutaneous tissues; occurs below 32°F (0°C)

  • Common on exposed/peripheral areas due to vasoconstriction; body preserves core temperature at periphery

  • Signs: numbness, itching, prickling; skin cyanotic, reddened, or white; later stiffness, paresthesias

  • Thawing phase: skin may turn yellow/white; loss of elasticity; burning pain; edema; blisters; necrosis may occur

  • Treatment:

    • Remove from cold, dry clothing; cover head, warmth blanket

    • Rapid rewarming (do not rub); use moist warm packs (avoid heating pads)

    • Monitor for infection; pain control; TIME IS TISSUE

    • Necrosis- tissue death

    • Gangrene