Focus on Pharm: Mannitol

Mannitol is a common medication you are likely to see on nursing school exams, NCLEX, and in the clinical setting. Because it requires careful administration, assessment, and follow-up, you will most likely see it utilized in the critical care environment. In this article, we’ll go through mannitol using the Straight A Nursing DRRUGS framework.  Tune in to episode 208 of the Straight A Nursing podcast to listen to this information on the go.

D: Drug Class

Diuretics work on the kidney to remove excess water from the body. Mannitol is classified as a specific type of diuretic called an osmotic diuretic. It is essentially a sugar that is administered intravenously to raise serum osmolality. This has the effect of pulling water from the intracellular space for rapid excretion by the kidneys. Note that mannitol does not raise blood glucose levels in the way that dextrose does. It is only mildly metabolized by the body and excreted quickly by the kidneys. 

R: Routes of administration

The main route of administration for mannitol is intravenous. It comes in various strengths of increasing osmolality – 5%, 10%, 15%, 20% and 25%. Note that 5% mannitol is also sometimes used as an irrigation fluid for transurethral prostate surgery.

R: Regular dose range

According to Davis Drug Guide, the dose range for Mannitol in adults is 0.25 to 2 g/kg and will vary a bit based on usage, which is discussed below. In children the dose range is 1-2 g/kg.

U: Uses of mannitol

Intravenous mannitol has two main uses – to decrease intracranial pressure and to decrease intraocular pressure. It may also be used to flush the kidneys of harmful substances, improve blood pressure during dialysis, and promote diuresis in the early stages of acute renal failure. Let’s talk briefly about each of these. 

• Decrease intracranial pressure and cerebral edema – Mannitol increases serum osmolality which pulls water from the fluid-overloaded cells of the brain into the intravascular space for excretion by the kidneys. The effects of mannitol take effect within 5 to 10 minutes and can last up to 6 hours.
• Decrease intraocular pressure – Mannitol draws water from the vitreous humor of the eye and pulls it into the intravascular space for excretion by the kidneys. With less water in the vitreous humor, intraocular pressure decreases. You’ll most likely see this used prior to ophthalmological procedures and in glaucoma. Again, effects are noticeable within 5 to 10 minutes and last approximately 6 hours.
• Excretion of toxic substances or materials – The increased excretion of water that occurs with mannitol can also promote the flushing of harmful substances such as toxins or hemoglobin/myoglobin in cases of rhabdomyolysis.
• Improve blood pressure during dialysis – Many patients undergoing dialysis become hypotensive or experience dialysis disequilibrium syndrome. Mannitol raises serum osmolality and can prevent these conditions from occurring.
• Acute renal failure – You may see mannitol used in acute renal failure to prevent or decrease the severity of the oliguric phase and the associated fluid overload that occurs. Note that this is not a common use as mannitol can injure the tubules and lead to acute tubular necrosis. 
• Irrigation during prostate surgery – Depending on the type of equipment used in surgery, mannitol or combinations of sorbitol and mannitol may be used as an irrigation fluid. Monopolar electrosurgery devices cannot be used with electrolyte-containing fluids such as 0.9% NaCl or Lactated Ringers solution. In these cases, a non-electrolyte solution is utilized for irrigation. The problem with this is the risk for a hypoosmolar fluid entering systemic circulation and causing significant fluid-related complications such as pulmonary edema and hyponatremia. A mannitol-based solution helps reduce the incidence of these complications. It is important to note, however, that advances in surgery and equipment enable surgeons to use electrolyte-containing solutions for irrigation.

Some other uses of mannitol are: 

• To help diagnose asthma – The “mannitol challenge test” is used to help diagnose asthma. The test involves a baseline spirometry test and repeat tests conducted after the individual breathes in powdered mannitol through an inhaler. The substance can cause bronchospasm, so the test is conducted in the healthcare setting. Though it is highly specific for asthma, it is not highly sensitive. This means a negative test does not rule out asthma, but a positive test means it’s very likely the patient has asthma with ongoing airway inflammation.
• As a component of cardiopulmonary bypass surgery – Mannitol is often used to prime the cardiopulmonary bypass circuit.
• To protect sharp objects – Mannitol can be applied to the tips of​​ sharp objects such as pacemaker wires to protect them from being dulled as they are introduced into the vascular space. The mannitol dissolves quickly, exposing the sharp tip once the device is in place.
• As a food additive – Mannitol is actually a naturally occurring substance and is also used as a food additive due to its ability to provide sweetness while being poorly absorbed by the intestine. The result is that it doesn’t raise blood sugar in the same way other dietary sugars like glucose do. Mannitol also occurs naturally in various fruits and vegetables including mushrooms, cauliflower, peas and celery.

G: Guidelines for mannitol administration

There are quite a few guidelines for mannitol administration, so they are categorized here as safety, assessment and labs.

Safety

Like any other hypertonic solution, mannitol can easily cause extravasation injury. For this reason it is recommended that mannitol be administered through a large peripheral vein or central line.

Note that mannitol can crystallize in the bottle, especially when exposed to cool temperatures. If this occurs, the safest thing to do is get another bottle. If that is not possible, the Davis Drug Guide recommends warming the bottle in hot water and shaking vigorously to dispel the crystals. The bottle should then be allowed to cool to body temperature and only be administered if crystals do not appear. When in doubt, always consult your pharmacist and/or the prescribing physician. You also want to be sure to use an in-line filter to prevent any precipitate or crystals that you can’t see from entering circulation. Even small crystals can cause vascular and end-organ damage. 

It’s also important that you know your hold parameters for mannitol. This will usually be a serum osmolality of above 320 mOsm (specific levels will be written into the medication order). If the order is not written with hold parameters, clarify with the prescribing physician.

Mannitol is contraindicated in cases of pulmonary edema, active intracranial bleeding, and significant hypovolemia. Other contraindications include heart failure, renal failure, renal insufficiency, and uncorrected electrolyte imbalances. Children with hyperemia should not be given mannitol as it can increase intracranial pressure to fatal levels.

Be careful when giving mannitol to patients who also take digoxin, as the hypokalemia that can result will increase the risk of digoxin toxicity. There’s also an increased risk for renal failure if the patient is also taking other nephrotoxic drugs including aminoglycosides and diuretics. Additionally, there’s an increased risk for neurotoxicity when taken with aminoglycosides such as gentamicin.

Assessment

Your nursing assessments will include: 

• Monitor the IV site for signs of infiltration or extravasation.
• Monitor for signs of electrolyte imbalance.
• Monitor urine output; an indwelling catheter will most likely be utilized for “strict I/O” monitoring (not usually indicated for one-time doses to decrease intraocular pressure pre-operatively). If urine output is not at least 30-50 ml/hr the patient will likely need reevaluation by the MD. Note that mannitol causes a pretty significant diuresis in most cases.
• Monitor ICP if given for increased intracranial pressure. This patient will have a sensor inside a ventricle or parenchyma and be admitted to the ICU. In some cases, the patient may have a lumbar drain instead.
• Assess neurological status closely; the desired outcome is an improvement in neurological status as cerebral edema and ICP are reduced. Note that when first administered, mannitol can cause a transient hyponatremia, and then as water is excreted, hypernatremia can result. Both of these conditions can cause alterations in neurological status.
• Monitor cardiac function as electrolyte imbalances can cause dysrhythmias.
• Monitor hemodynamics as fluid shifts can lead to heart failure, while diuresis can cause hypovolemia . Both conditions can cause hemodynamic instability.
• If your patient has received mannitol for increased intraocular pressure, assess visual acuity and ocular pain.

Labs

• Draw labs at designated times, especially if mannitol is being given repeatedly. 
• Serum sodium and potassium are typically measured every 4 to 6 hours with a goal of maintaining electrolyte balance.
• Serum osmolality will be measured every 4 to 6 hours and/or prior to each dose. 

S: Side effects of mannitol administration

One of the most significant adverse effects of mannitol administration is heart failure caused by rapid shifts of fluid into the intravascular space. This is why it is typically contraindicated in patients with heart failure.

Another significant adverse effect is worsening cerebral edema. Though generally given to decrease intracranial pressure, mannitol can worsen cerebral edema especially if vessels are damaged (as is the case with intracranial hemorrhage). Additionally, repeated doses of mannitol can allow it to leach into the blood brain barrier over time. This would then cause fluid to be pulled into the brain and out of the vascular space, worsening cerebral edema. Other CNS side effects to watch for are confusion, lethargy, and headache.

Dehydration or worsening of existing dehydration due to water losses (diuresis). The patient may complain of thirst as the effects of dehydration are felt. This should warrant a more thorough assessment.

Electrolyte imbalances – hyponatremia, hypokalemia and hypocalcemia are common. As diuresis continues, note that hypernatremia can result if the patient becomes dehydrated.

Hypersensitivity reactions to mannitol are possible. Though not the most frequent adverse reaction, all are life threatening if they do occur. These include dyspnea, hypotension and anaphylaxis.

TL;DR

Your quick summary for mannitol is that it is an osmotic diuretic that is used in cases of increased intracranial pressure to pull fluid out of the brain and into the vascular space. The goal is decreased ICP and improved neurological function. You will monitor electrolytes and check serum osmolality routinely and before each dose. Mannitol is given via an inline filter and you must always check for crystals prior to administration. It is contraindicated in heart failure, pulmonary edema and intracranial hemorrhage.

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