Physiology · Year 1 · from Physiology
Case 1: Heat Stroke - Thermoregulatory Failure
Clinical Image
Source: Wikimedia Commons - Hyperthermia - CC BY-SA 4.0
Patient Presentation
A 72-year-old man is brought to the emergency department by his daughter after she found him confused and unresponsive in his apartment during a summer heat wave. The apartment had no air conditioning, and the ambient temperature was estimated at 38°C (100°F). The patient lives alone and has been taking a new medication for urinary frequency that was started 2 weeks ago.
Demographics
- Age: 72 years
- Sex: Male
- Past Medical History: Hypertension, benign prostatic hyperplasia
- Medications: Lisinopril, oxybutynin (anticholinergic, started 2 weeks ago)
Chief Complaint
Found unresponsive with altered mental status during heat wave
Physical Examination
- Core temperature: 41.2°C (106.2°F) via rectal probe
- Blood pressure: 88/52 mmHg (hypotensive)
- Heart rate: 128 bpm
- Respiratory rate: 28/min
- General: Unresponsive to verbal commands, responds to painful stimuli
- Skin: Hot, dry, flushed (absence of sweating)
- Neurological: GCS 9, pupils reactive, no focal deficits
Workup
- Serum creatinine: 2.8 mg/dL (acute kidney injury)
- CK: 12,500 U/L (rhabdomyolysis)
- AST/ALT: 245/312 U/L (hepatic injury)
- Lactate: 6.2 mmol/L (tissue hypoperfusion)
- PT/INR: Prolonged (early DIC)
- Urinalysis: Myoglobinuria
Diagnosis
Exertional/Classic Heat Stroke with Multi-Organ Dysfunction
Treatment
- Immediate aggressive cooling: ice packs to groin/axillae, cold IV fluids, evaporative cooling
- Continuous core temperature monitoring (goal: reduce to <39°C within 30 minutes)
- IV fluid resuscitation for hypotension
- Treat rhabdomyolysis: aggressive hydration, monitor for compartment syndrome
- ICU admission for monitoring of multi-organ function
- Discontinue anticholinergic medication
- Social services consultation for living situation
Physiological Principles Demonstrated
- Hyperthermia vs. fever: In heat stroke, the temperature set point is NOT elevated (unlike fever). The body's heat dissipation mechanisms are overwhelmed by environmental heat or impaired by medications, causing uncontrolled temperature rise.
- Thermoregulatory mechanisms: Normal cooling depends on sweating (evaporation) and cutaneous vasodilation (convection/radiation). Anticholinergic medications block sweating, impairing the most effective cooling mechanism.
- Homeostatic failure consequences: When thermoregulation fails, the resulting hyperthermia causes protein denaturation, cellular injury, and multi-organ dysfunction (brain, liver, kidneys, muscle).
- Feedback loop breakdown: The hypothalamic thermoregulatory center cannot compensate when effector mechanisms (sweating) are pharmacologically blocked or overwhelmed.