Q: 54 year old male is in ICU after Traumatic Brain Injury. Follow up CT scan shows cerebral edema. Resident ordered Mannitol. After 4 doses of Mannitol patient oxygen requirement on ventilator increased and CXR shows pulmonary edema. Resident ask you: If we are using mannitol to relieve cerebral edema than why does it cause the pulmonary edema?

Answer:

In patients with underlying cardiac or/and renal insufficiency, circulatory volume overload may occur due to expansion of extracellular fluid after serial mannitol administration causing pulmonary edema. It is true that mannitol is an osmotic diuretic but overwhelming hydrostatic pressure due to poor urinary output and underlying compromised cardiac function offsets the increased oncotic pressure and may lead to extravasation of fluid.