Mannitol is an osmotic diuretic, and its mechanism of action is a little different than the typical loop diuretic you are most familiar with (Lasix). One key difference besides how it works -- is where it works. We will touch more on that in a second. Diuretics regardless of type enhance diuresis, which is the body’s natural method of removing water and salt from the circulation via urination.
During diuresis, blood filters through the kidneys, and water and other waste products are removed, siphoning out of the body through the ureters, bladder, and urethra. All diuretics accomplish the same goal of removing excess fluid from the circulation by increasing urinary output. Mannitol is special is that is also improved renal blood flow.
Some diuretics are more potent than others. The difference in diuretics stems from something called their “mechanism of action.” This refers to the specific ways in which diuretics act on the kidneys. The various sites in the kidneys where diuretics exert their effect can influence how powerful they are and also how quickly they produce the intended effect of increased urination.
Lasix works specifically on the thick ascending loop of Henle, they prevent the reabsorption of sodium, so sodium gets excreted through the urine. As good rule of thumb to remember is that water follows sodium. So based on this you can quickly realize who this medication could greatly benefit a CHF patient who is volume overloaded.
Mannitol, however, works throughout the entire nephron, and includes the Loop of Henle, but also the proximal tubules and the glomerulus. It reduces osmotic pressure in the blood and blood vessels. It can also help with removing excess fluid from the brain, which is crucial in situations of increased intracranial pressure (ICP) and brain swelling. Mannitol achieves this by elevating blood plasma osmolality, resulting in enhanced flow of water from tissues, including the brain and cerebrospinal fluid, into interstitial fluid and plasma. As a result, cerebral edema, elevated intracranial pressure, and cerebrospinal fluid volume and pressure may be reduced.
Rarely will mannitol be given in your routine 911 calls; it is most commonly seen in aeromedical operations and/or critical care interfacility transfers via intravenous infusion pumps. The most common use case is for symptomatic elevated ICP management. However, it can be used for other diagnosis such as cystic fibrosis, asthma, and even glaucoma, where intraocular pressures are elevated.
Regarding acute traumatic head injuries, more implications for further research are definitely needed. There are still many unanswered questions regarding the optimal use of mannitol. Its utilization today is fairly commonplace, but what is unclear is how to optimize the administration. Provider clinical judgement is mainly determining mannitol and/or the use of hypertonic saline infusions. Some may argue that 3% saline is superior, but that is still up for debate.
The image below is meant to simulate an old head bleed that is no longer active. Cerebral edema tends to get worse 4-8 days post bleed even though it may be controlled. This image shows lots of swelling and an accompanying shift.... (see the asymmetry) something mannitol can help with. Neurosurgeons will monitor the patient's serum osmolarity in a serial fashion -- The initial target of serum osmolality is often set at 300–320 mOsmol/kg. Acute renal failure can be a consequence that might develop when serum osmolality exceeds 320 mOsmol/kg during mannitol infusion. At that point the mannitol infusion is likely reduced or shut off completely because too much fluid has been pulled into the intravascular space. Therefore, measurement of serum osmolality during hyperosmolar agent infusion is of clinical importance to determine clinical efficacy, adjust dosage and avoid side effects.
**Disclaimer ** As with any topic we discuss on our blog entries -- our content will never supersede your own local protocols -- please continued to be aligned with your medical direction and relevant department policies above all else.
December 16, 2024
Author: Joshua Ishmael, MBA, MLS(ASCP)CM, NRP
Pass with PASS, LLC
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