A 7-year-old boy is brought to the emergency department after being pulled out of a swimming pool. He had been submerged for approximately 4 minutes. On arrival, he is unconscious, cyanosed, and not breathing. His body temperature is 34°C. After resuscitation, he regains consciousness but remains confused. Over the next 6 hours, his respiratory distress worsens despite supplemental oxygen, and bilateral crackles are heard on auscultation. Chest X-ray shows bilateral pulmonary edema. What is the most likely pathophysiological mechanism responsible for his deteriorating respiratory status?

Explanation

## Pathophysiology of Drowning and Secondary Pulmonary Edema **Key Point:** In fresh water drowning, water is hypotonic and rapidly absorbed across the alveolar membrane into the pulmonary capillaries, causing hypervolemia, hemodilution, and pulmonary edema. This is the mechanism of "secondary drowning" or "delayed pulmonary edema." ### Mechanism in Fresh Water Drowning 1. Hypotonic water enters alveoli 2. Osmotic gradient draws fluid into pulmonary capillaries 3. Hypervolemia and hemodilution develop 4. Pulmonary edema worsens over hours (6–24 hours post-rescue) 5. Surfactant washout increases alveolar surface tension 6. Ventilation-perfusion mismatch and hypoxemia result **High-Yield:** Fresh water drowning → **hypervolemia** → pulmonary edema (often delayed 6–24 hours). This is why the child deteriorates *after* initial resuscitation. ### Comparison: Fresh Water vs. Salt Water Drowning | Feature | Fresh Water | Salt Water | |---------|-------------|------------| | Osmolarity | Hypotonic | Hypertonic | | Fluid movement | Into lungs (absorption) | Out of lungs (osmotic pull) | | Hemodynamic effect | Hypervolemia, hemodilution | Hypovolemia, hemoconcentration | | Pulmonary edema | Yes (secondary, delayed) | Yes (osmotic, earlier) | | Electrolyte disturbance | Hyponatremia, hypokalemia | Hypernatremia, hyperchloremia | | Cardiac dysrhythmia risk | Ventricular fibrillation (cold water) | Asystole (salt water) | **Clinical Pearl:** The child's worsening respiratory distress 6 hours after rescue is classic for secondary drowning in fresh water—a delayed phenomenon due to progressive pulmonary edema from hypervolemia and surfactant washout. **Mnemonic:** **FRESH = Fluid Reabsorption Edema, Sodium Hyponatremia** (fresh water pulls fluid into lungs and dilutes serum sodium). ### Why This Child Deteriorates - Initial resuscitation may restore consciousness and vital signs - Continued fluid absorption into pulmonary capillaries over 6–24 hours - Surfactant denaturation reduces compliance - Progressive hypoxemia despite oxygen therapy - May progress to acute respiratory distress syndrome (ARDS)