Chapter 15 Marini Pharmacotherapy

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SECTION I • Techniques and Methods in Critical Care

Transcutaneous/Transdermal Administration (Patches)

Enteral Administration Bioavailability of enterally administered drugs can be limited by gastric pH, bowel wall edema, enteral feeding interactions, and first-pass metabolism. Effective enteral therapy requires gut motility, mucosal perfusion, and epithelial integrity. Patients with ileus, gut hypoperfusion, or atrophic or injured epithelium are poor candidates for enteral therapy because absorption will be limited. Drugs given in aqueous solutions are more rapidly absorbed than those given in oily solutions, and nonionized drugs are more readily absorbed than ionized drugs. A few poorly absorbed drugs (e.g., vancomycin, poly- myxin) are intentionally given enterally to act selec- tively in the gut. Drugs destroyed by an acidic pH may be partially protected by enveloping them with an enteric coating. Conversely, other drugs require acid for activation or absorption (e.g., sucralfate, ketoconazole, iron), a point that deserves consider- ation in patients receiving acid-suppressive therapy. Liquid preparations are the preferred formula- tions when possible because they are readily absorbed and are less likely to clog feeding tubes. Elixirs and suspensions are preferred over syrups because syrups tend to cause more clumping when exposed to enteral feedings. Many liquid preparations are extremely hyperosmolar or contain large amounts of sorbitol, increasing the risk of GI intolerance. Suspensions generally contain less sorbitol, and even though they still have high osmolality, diluting them with water will help decrease tonicity and thereby make them a more desirable formulation. Some formulations are not appropriate for enteral administration. These include lansoprazole oral suspension granules and mineral oil (which are too viscous and may occlude the feeding tube), sucralfate suspension (may cause an insoluble mass or bezoar formation), and extended, sustained, or delayed-release formulations (listed below). Although solid dosage forms (e.g., tablets, capsules) can be used, the tablets should be crushed and the capsules should be opened and the contents mixed with 15 to 30 mL of water before delivery. It is important to note that many sustained or delayed-release drug formulations (e.g., any drug that has ER, DR, SR, XL after its name) should not be crushed and given via the enteral route. It is often best to use the immediate release of these formulations and divide them appropriately

Cutaneous drug absorption depends on skin per- meability, temperature, blood flow, moisture con- tent, and the presence of dermatologic disorders. There are many drugs available in a patch formu- lation, including nitroglycerin, clonidine, fentanyl, scopolamine, lidocaine, and many others. Patches are a great option in stable patients who are unable to take enteral formulations or may want the con- venience of prolonged dosing (some patches are changed on a weekly basis). In the ICU setting, however, patches tend to be problematic for sev- eral reasons. First, as stated above, drug release from a patch is regulated by temperature, so if the patient has an elevated temperature, the drug will be more rapidly released from the patch, potentially resulting in toxic levels. Second, the patch takes time to start working after placement (anywhere from 6 to 18 hours depending on the drug in the patch because of diffusion through the skin), and therefore, once the patch is removed, it will take about that same amount of time for the drug effect to stop working. This could be problematic if you have placed a clonidine patch, for example, and your patient becomes hypotensive. Intraocular Drugs Although ophthalmic preparations (e.g., drops, oint- ments, suspensions, emulsions) may contain a very small amount of drug, it should be remembered that these medications can still have significant systemic side effects. When administered, eyedrops come into contact with the conjunctiva and the secretory membranes of the tear canals, eventually reaching the circulatory system through the mucosa of the nose and throat. Atropine eyedrops, which are administered to achieve mydriasis for retinal examination, may cause fever, headache, facial erythema, dry mouth, high blood pressure, difficulty in concentration, cramps, and blurred vision. Glaucoma medications, like ophthalmic beta-blockers, may cause bradycar- dia, reduced blood pressure, dry eye, and exacerbate asthma and congestive heart failure. Therefore, any drug given via the ophthalmic route should be reviewed for its potential side effects and monitored by both the patient and physician.