Intravenous Osmotic Drugs

Intravenous osmotic drugs have a more rapid and reliable osmotic effect compared with oral medications, especially in patients with nausea and vomiting.

8.9.1 Mannitol. Mannitol is an osmotic diuretic that is the drug of choice when an intravenous osmotic drug is required for lowering IOP. The drug has an onset of action of 10 to 30 minutes, a peak effect in 30 to 60 minutes, and a duration of action of approximately 4 to 6 hours.5,6,43,44 After intravenous injection, mannitol is distributed in extracellular water and has poor ocular penetration (table 8.4). These properties enhance the osmotic effect of the drug, especially in inflamed eyes.17

Mannitol is not metabolized and is excreted unchanged in the urine, which is an advantage over glycerol in diabetic patients. A potential disadvantage of mannitol is the increased intravascular volume, because the drug is confined to extracellular water. Also, a relatively large volume of intravenous fluid is required due to the limited solubility of mannitol. Thus, patients with cardiac or renal disease require cautious use of this drug.

8.9.2 Other Intravenous Osmotic Drugs

Urea. is infrequently used as an osmotic drug for lowering IOP. The onset of action, time to maximal effect, and duration of effect of urea are similar to those of mannitol.21-23 The osmotic effect is less pronounced after infusion of urea solution compared with mannitol, because urea is distributed in total body water and penetrates the eye more readily, especially when the eye is inflamed.16 These characteristics may also cause a greater rebound of IOP compared with other drugs. Urea is not metabolized and is excreted rapidly in the urine.

Urea solution must be prepared fresh prior to administration, because old solutions are unstable and decompose to ammonia. The solution should be gently warmed during preparation to compensate for the endothermic reaction of dissolving the drug. However, warming the solution to above 50°C may produce ammonia. To prevent hydrolysis, the drug solution is prepared in a 10% invert sugar. The drug may produce irritation or even thrombophlebitis at the intravenous infusion site, and skin necrosis may occur if the drug extravasates. Because of its disadvantages, urea is rarely used compared with mannitol.

Sodium ascorbate, administered intravenously at a dose of 0.5 to 1g/kg, is ionized in solution and is distributed in total body water, providing 2 mOsm in solution per milliosmole of dry weight.15 This drug penetrates the eye and is unstable, which has limited its clinical usefulness.

Sucrose, administered as a 50% solution, requires large doses to achieve a high osmotic level in the blood and has toxic effects, making it unsuitable for use as an osmotic drug.

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