1.3 Absorption

Absorption

The first stage of pharmacokinetics is known as absorption. Absorption occurs after drugs enter the body and travel from the site of administration into the body’s circulation. Medications can enter the body through various routes. Common routes to administer medications include the following examples:

• Oral (swallowing an aspirin tablet)
• Enteral (administering medication into the gastrointestinal tract via a nasogastric tube)
• Rectal (administering an acetaminophen suppository)
• Intranasal (spraying allergy medication into the nose)
• Inhalation (breathing in asthma medication from an inhaler)
• Intramuscular (injecting an influenza vaccine into the deltoid muscle)
• Subcutaneous (injecting insulin into the subcutaneous tissue in the abdomen)
• Transdermal (wearing a nicotine patch that is absorbed through the skin)
• Intravenous (administering antibiotics directly into a vein)

First-Pass Effect

When a medication is administered orally or enterally, absorption may be significantly hindered in the gastrointestinal (GI) tract. For example, when medications made of protein are introduced into the GI tract, they can be quickly deactivated by enzymes as they pass through the stomach and duodenum. If some of the drug is absorbed from the intestine into the bloodstream, part of the absorbed portion may be broken down by liver enzymes, whereas the remaining part escapes into the general circulation. The portion of the drug that enters the general circulation will either become protein-bound (and thus inactive) or remain free to circulate and create an action at a receptor site. This entire process that results in reduced concentration of active drug available in an individual’s circulation is known as the first-pass effect. Due to the first-pass effect, prescribing providers and nurses administering medications must understand that several doses of an oral medication may be needed before enough free drug stays active in the circulation to exert the desired effect. These metabolic effects are further described in the “Metabolism” section later in this chapter.

Alternate Routes

A workaround to the first-pass effect is to administer medication using alternate routes to the GI tract. Examples of alternative routes that avoid the first-pass effect include transdermal, nasal, inhalation, injection, or intravenous administration of medication. Alternative routes of medication administration bypass the first-pass effect by entering the bloodstream directly or via absorption through the skin or lungs. For example, pain relievers may be administered directly into the bloodstream (referred to as intravenous medications) so they are quickly available for distribution to tissues within the body.

Alternative routes of medication have other potential considerations. For example, injections are often painful and cause a break in the skin, an important barrier to infection. They can also be costly and difficult to administer daily, may cause localized side effects, or contribute to unpredictable fluctuations in medication blood levels.

Transdermal application of medication is an alternate route that has the primary benefit of slow, steady drug delivery directly to the bloodstream, without passing through the liver first. See Figure 1.2^[1] for an image of a client self-administering a transdermal patch. Drugs delivered transdermally enter the blood via a meshwork of small arteries, veins, and capillaries in the skin. This makes the transdermal route of drug delivery particularly useful when a medication must be administered over a longer period of time to control symptoms. For example, transdermal application of fentanyl, a pain medication, can provide effective pain management over a several hours; a scopolamine patch can control motion sickness over the duration of a cruise ship vacation; and a nitroglycerin patch is used to prevent chronic chest pain. Despite their advantages, transdermal patches have a significant drawback in that only very small drug molecules can enter the body through the skin, making this application route inappropriate for some types of medications.

Inhaling drugs through the nose or mouth is another alternative route for rapid medication delivery that bypasses the liver. See Figure 1.3 for an image of a client self-administering an inhaler.^[2] Metered-dose inhalers have been a mainstay of asthma therapy for several years, and nasal steroid medications are often prescribed for allergy and sinus problems.

Emerging Discoveries and Recent Developments

Researchers are currently exploring alternative methods of drug delivery such as the use of inhaled insulin powders. Afrezza® is an example of an inhaled insulin approved by the Food and Drug Administration (FDA) to assist with blood sugar control. This technology stems from novel uses of chemistry and engineering to manufacture insulin particles of just the right size for absorption. If too large, the insulin particles could lodge in the lungs; if too small, the particles will be exhaled.^[3]

Life Span Considerations

Neonate & Pediatric

Gastric absorption in neonates and pediatric clients varies from adults. In infants, the acid-producing cells of the stomach are immature until around the age of one to two years. Additionally, gastric emptying may be decreased because of slowed or irregular peristalsis (coordinated muscle movements of the intestines).

The liver of infants and children is not fully mature, resulting in a decrease in first-pass elimination and subsequently higher drug levels in the bloodstream.^[4]

Older Adult

As a natural result of aging, older adults will experience decreased blood flow to tissues within the GI tract. In addition, there may be changes in the gastric (stomach) pH that may alter the absorption of certain medications. Older adult clients may also experience variations in available plasma proteins, which can impact drug levels of medications that are highly protein-bound.

Consideration must also be given to the use of subcutaneous and intramuscular injections in older clients experiencing decreased cardiac output because decreased drug absorption of medications can occur when peripheral circulation is decreased. Additionally, as adults age, they often have less subcutaneous fat, resulting in decreased absorption of medication from transdermal patches that require adequate subcutaneous fat stores for proper absorption.^[5]

The box below summarizes route considerations that a nurse should consider when administering medication.