Why isn't mannitol used in pulmonary edema? And why is it used in cerebral edema?
Mannitol would expand the intravascular volume, increasing cardiac output and causing pulmonary edema (more fluid going to the lungs than it can drain.)
(Assuming the pulmonary edema is due to CHF:) The increased hydrostatic pressure proximal to the left atrium causes transudation in the lungs. Although mannitol can act as a diuretic, it initially increases plasma volume due to its effects on elevating plasma oncotic pressure.
Increased plasma volume --> increased left atrial preload in the face of decompensation that already occurred even at a lower preload --> increased LAP (PCWP) with further decompensation --> exacerbation of pulmonary venular transudation.
So basically, it causes edema by volume overload.
It's blood brain barrier (BBB) that allow us to use mannitol for brain edema. Since no such barrier is there in lungs, mannitol can cross capillaries into alveoli and worsen it. Even in cerebral edema, we give mannitol only when the BBB is intact. Otherwise, mannitol can create havoc there too.
Infusion of hypertonic solutions of any effective small molecular weight solute (eg hypertonic saline, mannitol or urea) will dehydrate the brain. In the peripheral capillaries, these solutes are not effective at exerting an osmotic force because they can easily cross these capillary membranes.