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In this episode, I’ll discuss how to administer mannitol for elevated intracranial pressure.

The definitive treatment for elevated intracranial pressure is to fix the underlying cause. Common causes are traumatic brain injury, intracranial hemorrhage, CNS infection, or intracranial neoplasm.

Until definitive treatment can be arranged, temporary treatments to lower intracranial pressure such as mannitol may be employed.

When given as a bolus, mannitol is an osmotic diuretic that does not cross the blood-brain barrier. Mannitol lowers intracranial pressure by osmotically drawing free water out of the brain and into circulation where it can be eliminated. If given as a continuous infusion, mannitol will eventually cross the blood-brain barrier and have no effect on intracranial pressure.

Onset, duration & dose

Onset – Minutes

Duration – Hours

Dose – 1g/kg bolus repeated as needed with 0.25 to 0.5g/kg

Mannitol is needed STAT

Often, the decision to administer mannitol is made due to an acute, severe change in the patient’s neurological status that leads the physician/provider to believe the intracranial pressure is too high. The request is usually for mannitol to be given “STAT.” Variability in dose and administration technique can cause significant delays between the time mannitol is needed and the time it actually gets administered.

Administration

Mannitol is available in 25% 50mL vials (12.5g per vial) and 20% 500mL bags (100g per bag). Both preparations may crystallize at room temperature. Mannitol should be carefully inspected for crystals before it is administered, and it should be administered with an in-line filter, typically 0.22 micron in size.

Ideally, the 1g/kg dose of mannitol is administered as a bolus over 30 minutes. Comparing the available forms of mannitol against this dosing regimen, it becomes clear that the 12.5g vials are not practical. For a 75kg patient, the nurse would have to draw up six 12.5g vials and stand at the bedside pushing them for 30 minutes.

That leaves the 20% 500mL infusion as the most reasonable option for delivering a 1g/kg bolus of mannitol over 30 minutes.

While the 20% infusion option is the best, it too has some drawbacks. Many patients will receive less than 100g, and it is somewhat unusual to dispense an infusion that contains more than the intended dose of an IV medication. It is also not practical to transfer the exact amount of mannitol needed to a second container due to the time this takes and the urgency of the need for mannitol.

Fortunately, features available on “smart pumps” can facilitate administering the exact amount of mannitol needed. Smart pumps are able to specify a limit on the total volume of any medication that is to be infused, regardless of the rate of infusion. Nurses at my institution routinely use this feature on all IV infusions to ensure that the infusion rate is assessed frequently. In the context of a mannitol infusion, the nurse can set the pump to stop administration after the exact amount of mannitol has been delivered.

My ’20-10-5 rule’ for quick administration

In order to quickly determine the appropriate smart pump settings at the bedside for a 500mL bag of 20% mannitol to deliver 1g/kg over 30 minutes use the following steps:

1. Multiply the patient’s weight in kg by 10, and make this the rate of infusion in mL/h on the smart pump.

2. Multiply the patient’s weight in kg by 5, and make this the total volume to be infused on the smart pump.

Using the 75kg patient as an example, set the smart pump to infuse 20% mannitol at a rate of 750mL/h, with a total volume to be infused of 375 mL. This will deliver 75g of mannitol over 30 minutes.

If a patient weighs over 100kg, the infusion rate may need to be capped at 999mL/h, depending on the maximum rate the smart pump allows. Don’t worry, as this will still allow the mannitol to infuse at a rate sufficient to lower the intracranial pressure.

Starting Thursday, March 2, 2017, members of my Critical Care Pharmacy Academy will have access to my new Masterclass: Principles of infectious disease for ICU pharmacists. To learn more about the Academy and sign up go to pharmacyjoe.com/academy.

If you like this post, check out my book – A Pharmacist’s Guide to Inpatient Medical Emergencies: How to respond to code blue, rapid response calls, and other medical emergencies.

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