Medication Action: 4 Steps of Pharmacokinetics is the action of the body on a particular drug that has been administered. The 4 steps of pharmacokinetics follows the movement of drug molecules throughout the body.
Medications are given to treat an acute or chronic medical condition, to relieve the symptoms of an illness or disease. Also, medications can be given to prevent or even cure a disease. It is important that nursing students understand the science of pharmacokinetics and medication action.
Medication action is related to the mechanisms of absorption, distribution, metabolism, and excretion of the drug. In this article, we will look at these mechanisms.
Absorption
Absorption begins with medication administration. From the site of entry, the mediation enters the bloodstream. There are several factors that influence absorption.
The factors that influence the absorption of medications are the (1) route of administration, (2) how well the medications are dissolved, (3) the lipid solubility of the medication administered, (4) the condition of the administration site, and (5) the blood flow to the site of administration.
Route of administration
Firstly, we will look at the route of administration. There are many different routes of medication administration. And, each route has a different rate of absorption.
For example, medication applied to the skin will have a different rate of absorption than medication given intravenously.
As a general rule medication given orally will take the longest to absorb. That is because these medications have to pass through the gastrointestinal tract.
When thinking about the rate of absorption of medications, think about the number of blood vessels available at the site of entry. Tissues that contain more blood vessels will have a faster rate of absorption.
Tissues such as the mucus membrane and the respiratory tract will have faster absorption because they are rich in blood vessels.
Medications administered intravenously go immediately into the bloodstream. Therefore, these medications go directly into the systemic circulation. They actually, do not have to go through the process of being absorbed.
Some medications have very little absorption. These medications produce their action directly at the site of administration.
Ability of the Medication to Dissolve
When thinking about the ability of the medication to dissolve, we are considering mostly oral medications. Because, oral medications have to go through the gastrointestinal tract, the form of the medication makes a difference.
Some oral medications may be dispensed as an enteric-coated pill. Other oral medications may be dispensed in a liquid or suspension. The liquid or suspension is already essentially dissolved and is ready to be absorbed upon administration.
This means some medications will be absorbed in the stomach while others may not be absorbed until they reach the gastrointestinal tract.
Drugs that are acidic absorb well in the stomach. While drugs that are considered basic or alkaline are not absorbed until they reach the small intestine.
Condition of the Administration Site
The condition of the administration site is also very important.
One example is the skin. When administering medication topically, the larger the area you cover the greater the absorption. A patient having some problem with their skin, for instance, a lot of scar tissue will have a slower rate of absorption.
When a medication is administered orally, the presents of food in the stomach will affect the rate of absorption. Some medications need to be given in between meals because food can sometimes impair absorption.
Enteric-coated pills are designed to have delayed absorption. They are absorbed in the small intestine and not the stomach. Patients that have an impaired gastrointestinal tract will have a different rate of absorption than a patient with a normal gastrointestinal tract.
This is why it important to know your patient’s history and diagnosis to safely administer medications. Also, this is why assessment is included in the rights of medication administration.
Blood Flow to the Site of Administration
As mentioned earlier, tissues that contain more blood vessels will have a faster rate of absorption. If a site of administration has an abundant blood supply, the absorption will be enhanced.
If there is an impaired systemic circulation or if the circulation to a site of administration is impaired the absorption is decreased.
Distribution
After a medication is administered and absorbed into the bloodstream, then distribution begins. The medication is sent throughout the body and becomes available to body tissues and organs. At this point, the medication is being carried to the site of action.
Like absorptions, distribution depends upon blood flow to the tissue and organs. It also depends upon the ability of the medication to leave the bloodstream and enter the target cell.
When looking at distribution we are still considering the properties of the medication and the condition of the patient. The medical history and any acute or chronic conditions of the patient receiving the medication will affect distribution.
The chemical makeup of the medication and the individual patient will also affect how fast the medication will reach the tissue and organs. Any problem that limits or inhibits blood flow will affect the distribution of the medication.
Lipid-Solubility or Cell Membrane Permeability
In order for a medication to reach its site of action, it must cross the cell membrane (also called plasma membrane). Cell membranes are composed mostly of lipids and proteins.
Tip: Lipids are mainly responsible for the integrity of the cell membrane.
Cell membranes are a barrier to the passage of medication. An example would be the central nervous system (CNS) and the blood-brain barrier.
The blood-brain barrier is very selective as to what passes. This is to protect the CNS. In order for a medication to pass the blood-brain barrier, it must be lipid-soluble or have some type of transport system to cross.
Therefore, when a medication is more lipid-soluble it passes through the cell membrane easier. And, the medication will be absorbed faster.
Metabolism
Metabolism is also known as biotransformation. During metabolism the medication becomes inactive or it is biotransformed to a new form by the body. The process breaks down and removes active chemicals in the medication.
The primary site of medication metabolism is the liver. However, the intestine, lungs, kidneys can metabolize medication. Most medications are changed to an inactive metabolite in order to prepare it for excretion.
Excretion
After metabolism, the next step is excretion. Excretion is the process in which medications and their metabolites exit the body. The organs of excretion include the lungs, skin, gastrointestinal tract, and kidneys. The chemical makeup of a specific medication will determine which organ is used for excretion.
The lungs mainly excrete medications that are gaseous and any anesthetics that have been inhaled. The skin excretes medication via the sweat glands. Many drugs are excreted through the gastrointestinal tract in the feces. However, the kidneys are the main organ of medication excretion.
So, effective secretion is dependent upon the circulatory system, and the functioning of the organs that are used. (Lungs, Skin, Gastrointestinal Tract, and Kidneys).
Medication Dose-Response and Serum Drug Levels
A serum blood level is a procedure in which blood is drawn to measure the amount of mediation in a patient’s bloodstream at a particular time.
When a medication is administered, you desire it to reach a concentration in which the medication can exert its pharmacological action in the body. This is determined by the amount of medication in the bloodstream.
If the concentration is too low the medication may not be effective. If the concentration is too high toxicity may occur. The goal is for the medication concentration to be within a therapeutic range. The therapeutic range is the concentration of medication in the blood serum to produce the desired effect.
Therapeutic Ranges
Within the therapeutic range are different levels.
The first is the peak level. The peak is the highest serum concentration of a medication after absorption is complete. Therefore factors that affect the absorption of a medication can affect the peak level. When a medication is administered intravenously, the peak concentration happens quickly.
The second is the trough level. The trough is when the medication has reached its lowest concentration. This trough level gives an idea of the rate of elimination. Some medications are administered based on the results of peak and trough levels. Trough serum levels are drawn 30 minutes before the next dose is given.
Tip: Although we say peak and trough, the trough is drawn before the peak. We draw the serum trough level first. This is because we want the drug at its lowest concentration to get a baseline. Therefore, the prescribing provider can make sure the next dose is sufficient to maintain serum blood levels in a therapeutic range. And, also to make sure that the serum concentration does not fall into a toxic range.
IMPORTANT: That’s why you always call the healthcare provider with the results of the trough before administering the next dose.
There are a few more terms to be familiar with. The first term is duration. Duration is the time in which the medication is present in a concentration that is high enough to produce a response.
The second term is the plateau. The plateau is the serum concentration of a medication that is reached and maintained after a patient has received a dose of a medication repeatedly.
Finally, there is the serum half-life. The serum half-life is the amount of time it takes for the serum concentration of a drug to decrease by one-half or by 50%. This is dependent upon metabolism and excretion. So, the serum half-life can be affected by the same conditions or circumstances that affect metabolism and excretion.
Tips: Drugs with a shorter half-life will have to be administered more often than a drug with a longer half-life.
Conclusion
Safe medication administration requires knowledge of pharmacokinetics and medication actions in the body. An understanding of the things which alter or impair the process of pharmacokinetics can alleviate potential problems from prescribed medications. It can also, make sure the therapeutic effect is reached with the desired outcome for the patient.
Sources
Taylor, PhD MSN, RN, Carol, et al. Fundamentals of Nursing: The Art and Science of Person-Centered Nursing Care. 8th ed., Philadelphia, Wolters Kluwer Health/Lippincott Williams & Wilkins, 2015.
Potter RN, MSN, PhD, FAAN, Patricia A., and Anne G. Perry RN, EdD, FAAN. Fundamentals of Nursing. 9th ed., St. Louis, Mosby Elsevier, 2017.
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