## Pathophysiology of Fresh Water Drowning **Key Point:** Fresh water is hypotonic relative to plasma. When aspirated in large quantities, it is rapidly absorbed across the alveolar membrane into the pulmonary capillaries, causing acute hemodilution and hypoosmolality. ### Mechanism of Death 1. **Acute hypoosmolality** → water moves into cells (including neurons) by osmosis 2. **Cerebral edema** develops → increased intracranial pressure 3. **Seizures, loss of consciousness, and brainstem herniation** may ensue 4. **Pulmonary edema** (frothy fluid in airways) compounds hypoxemia ### Biochemical Changes in Fresh Water Drowning | Parameter | Change | Mechanism | | --- | --- | --- | | Serum osmolality | ↓ (hypoosmolal) | Water absorption into circulation | | Serum Na^+^ | ↓ (hyponatremia) | Dilutional effect | | Serum K^+^ | Variable | Hemolysis is minimal in fresh water | | RBC hemolysis | Minimal | Fresh water enters cells, not lysing RBCs | | Cerebral edema | ✓ Present | Cytotoxic edema from water influx | **High-Yield:** The classic teaching is that fresh water causes **cerebral edema** (cytotoxic) due to hypoosmolality, whereas salt water causes **pulmonary edema** (cardiogenic) due to hypernatremia and fluid shift into alveoli. **Clinical Pearl:** Drowning victims who aspirate fresh water may develop sudden cardiac arrhythmias (cold water immersion reflex) or seizures from cerebral edema, even after apparent rescue.
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