Pharmacokinetics - Absorption: Nursing pharmacology

Pharmacokinetics refers to the movement and modification of a drug or medication inside the body. Once a medication is administered, it's first absorbed into the circulation, then distributed to various tissues throughout the body, metabolized or broken down, and finally, eliminated or excreted in the urine or feces.

This process can be broken down into 4 components with the acronym ADMMI, which stands for absorption, distribution, metabolism, and elimination.

Now, the first step of pharmacokinetics is absorption, which refers to the process of how a medication goes from the site of administration to the bloodstream.

To do so, the medication may need to cross one or more cell membranes, and this can occur via passive transport, which requires no energy, or active transport, which requires energy in the form of ATP.

With passive transport, the medication goes down its concentration gradient. So from an area of higher concentration to an area of lower concentration.

There are two types of passive transport, passive diffusion and facilitated diffusion. Passive diffusion allows small, lipid-soluble and non-polar medications to freely move across the membrane.

Facilitated diffusion, on the other hand, allows larger water-soluble and polar medications to move across the membrane through transport proteins like channels and carrier proteins.

Now, some medications need to be transported through active transport against their concentration gradient. So from an area of lower concentration to an area of higher concentration.

This can be achieved with specific carrier proteins that use ATP as an energy source to pump medications against their gradient.

Now, the rate of absorption, or how quickly this process occurs, as well as the extent of the absorption, or how much of that medication reaches the bloodstream, can be affected by several factors.

One of them is the pH of the environment where the absorption takes place. OK, so most medications are either weak acids or weak bases.

Weekly acidic medications will be better absorbed in an acidic environment, like the proximal duodenum, while weekly basic medications are more likely to get absorbed in an alkaline environment, like the distal ileum of the small intestine.

Note that even though the stomach is acidic, it is not suitable for the absorption of even weak acids, mainly because of its thick mucus layer.

OK, now another factor that affects absorption of a medication is the surface area available. In this regard, a good place for absorption is the small intestine, which has a very large total surface area, approximately the size of a tennis court.

Other factors that can affect the rate and extent of absorption include the amount of blood supply to the absorption site, as well as the presence of other substances, such as certain foods in the gastrointestinal tract, that can either promote or inhibit absorption.

Besides these, the route of administration and pharmaceutical preparation of a medication also have some role in its absorption.

For example, a medication given sublingually has a higher rate and extent of absorption than its oral form, as it bypasses the stomach acids.

Similarly, pharmaceutical preparations like enteric coatings are made in such a way that when taken orally, they protect the medication from stomach acids.

Only after reaching the small intestine, the coating gets dissolved, allowing the medication to be absorbed. A practical measure of absorption is bioavailability, which refers to the proportion of the medication that actually reaches the bloodstream.

For example, if a client takes 100 mg of aspirin orally, and only 60 mg of this is absorbed into the circulation, the bioavailability is 0.6 or 60%.

Alright, before administering any medication to your client, remember the general pharmacokinetic principles that relate to how medication is absorbed into your client's body.

First, review their medical history, taking note of any conditions that can affect medication absorption, like gastroparesis or conditions that may decrease circulation to the gastrointestinal tract.

Next, perform a focused assessment based on the route of medication ordered for your client. For oral medications, confirm your client is able to swallow and assess their bowel function for problems like diarrhea or constipation.

For intravenous medications, be sure to assess your client's circulation, as well as their intravenous access site, checking that it's patent and without redness, swelling, or pain.

For intramuscular and subcutaneous injections, assess the degree of circulation to the injection site, as well as your client's muscle size and amount of subcutaneous tissue, to verify there's enough tissue to absorb the volume of medication to be injected.

If a transdermal, buckle or sublingual medication is prescribed, be sure to assess your client's skin and mucous membranes for impaired skin integrity or dry mucus membranes, which can affect medication absorption.

Also remember to consider other client characteristics that could affect medication absorption. For example, the skin of infants and children is much more permeable compared to an adult skin, which results in increased absorption of topical medications.

On the flip side, older adults experience gastrointestinal changes that can affect medication absorption, including increased gastric pH as well as decreased gastrointestinal blood flow and motility.

Next, you'll need to teach your client about how to optimize the absorption of their prescribed medication. For clients taking oral medications, be sure to let them know if their medications should be taken on an empty stomach without regard to food, or with food.

If a medication can be taken with food, teach your client if there are any foods to avoid. For example, the absorption of tetracycline is reduced when taken with dairy products.

For enteric-coated medications, teach your client that taking the medication with a high-fat meal can decrease how fast the medication is absorbed.

Next, review all your clients' medications and supplements, and check for medications that can interact and affect the absorption.

Like metoclopramide, that decreases absorption of other medications by causing them to move through the gastrointestinal tract faster.

Antaids can increase or decrease the rate at which other medications are absorbed, as well as how much is absorbed by altering gastric pH or forming non-absorbable complexes with other medications.

Now, for subcutaneous medications like insulin, teach your client about the importance of rotating injection sites to prevent lipid dystrophy, which is the abnormal accumulation of fat tissue that can form with repeated injections.

That can cause an unpredictable rate of insulin absorption. If your client is prescribed a transdermal medication, like a patch for example, teach them to apply their patch to skin that is clean, dry and intact, and to avoid applying it to parts of the body where absorption can be altered, like where the skin is broken, irritated, very sensitive, or very oily.

Also, remind them how the absorption of their topical medication can be greatly increased if the application site is exposed to heat, like a warmed blanket, heater, or other heat source.

Lastly, if your client is using a metered dose inhaler for an inhaled medication, teach them how to use a spacer to optimize absorption of the medication.

Once your client has started their medication, continue to assess for factors that influence medication absorption, and use this information to evaluate the therapeutic response to their medication.

Alright, as a quick recap, pharmacokinetics refers to the movement and modification of a medication inside the body, and involves absorption, distribution, metabolism, and elimination.

The rate of absorption depends on factors such as pH in the surface area of the absorption site, as well as the route and pharmaceutical preparation of the medication.

Nursing considerations for medication absorption include reviewing the client's history, current medications, and considering client-specific factors that can affect medication absorption.

Client teaching focuses on how to safely and effectively administer their prescribed medication.