Mannitol adverse effects: pulmonary oedema and dehydration
Mannitol, a diuretic that works by osmosis, first draws water into the circulation, risking pulmonary oedema (fluid on the lungs), and then drives water out in the urine, risking dehydration.
Remember
Mannitol first pulls water into the blood, so pulmonary oedema is the early risk, especially in heart failure, and the same osmotic action then drives water out in the urine, so dehydration comes later.
Memory aid
what mannitol draws in, the kidney washes out
It reminds you that the water mannitol pulls into the circulation is later removed by the kidney.
Osmotic diuretics such as mannitol work by osmosis. They are freely filtered at the glomerulus (the kidney's filter) and then stay in the nephron tubule (the narrow tube where urine forms), holding water there. In the blood they raise the osmolality (the concentration of dissolved particles, towards which water moves), so water is drawn out of the tissues, including the swollen brain. This is how intracranial pressure (the pressure inside the skull) falls.
The first effect is a rapid, brief rise in intravascular volume (the volume of fluid inside the blood vessels), because water moves into the circulation before the kidneys have cleared the extra. A heart that cannot accommodate the added load passes the pressure back to the lungs. Fluid is forced into the alveoli (the air sacs), and pulmonary oedema (fluid on the lungs) results. This is why mannitol needs caution in heart failure.
Water loss tends to appear later. Mannitol holds water in the tubule and increases the volume of urine, so the water drawn out of the tissues is passed out of the body. Repeated or prolonged use therefore drains total body water and produces dehydration. Because osmotic diuretics increase the kidneys' excretion of water relative to sodium, prolonged use can also cause hypernatraemia (a high blood sodium). If the losses are not replaced, dehydration can progress to hypovolaemia (a low circulating volume) and hypotension (low blood pressure). Fluid balance, electrolytes and kidney function therefore need monitoring.
Key facts
- Mechanism
- mannitol is freely filtered at the glomerulus (the kidney's filter) and holds water in the nephron tubule, while in the blood it draws water out of the tissues.
- Early effect
- water moves into the circulation, so intravascular volume (the volume of fluid inside the blood vessels) rises quickly and briefly.
- Early risk
- pulmonary oedema (fluid on the lungs), in patients with heart failure especially.
- Later effect
- water is lost in the urine, which can cause dehydration, hypovolaemia (a low circulating volume) and hypernatraemia (a high blood sodium).
- Monitoring
- fluid balance, electrolytes and kidney function need watching during treatment, because mannitol can cause dehydration and electrolyte disturbances.
Exam traps
- A direct effect on how strongly the heart muscle contracts does not explain pulmonary oedema after mannitol: the risk comes from overexpansion of the fluid volume overloading the heart.
- Pulmonary oedema after mannitol is not a toxic injury to the lung's thin gas-exchange membrane: it is a pressure problem from overexpansion of the fluid volume.
- A low circulating volume from water lost in the urine is a real consequence of osmotic diuresis (the extra urine an osmotic diuretic produces), but it does not come first: it follows the diuresis, because initially the circulating volume rises rather than falls.
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