The Basics of Pharmacology

Pharmacology is perceived as one of emergency medicine’s most intimidating topics, especially for EMTs transitioning to paramedics. We will cover the basics of pharmacology, so you have a better understanding of how individual medications work. Hopefully, by the end of this article, big fancy words like the mechanism of action, bioavailability, pharmacokinetics, and pharmacodynamics won’t be as scary!

Let’s start with the definition of pharmacology, according to Webster.

pharmacology

1: the science of drugs including their origin, composition, pharmacokinetics, therapeutic use, and toxicology,

2: the properties and reactions of drugs especially with relation to their therapeutic value

Desired Effects

There is a lot of miscommunication about desired effects, side effects, and adverse effects of medications. Desired effects seem obvious. It’s what we want the drug to do, but even this can be a bit confusing because there are medications that do several things.

Let’s use epinephrine as an example. It is one of the medications we use in cardiac arrest for the heart. It increases cardiac contractility and hopefully increases the heart rate. But epinephrine also opens up the airway, so we use it for anaphylaxis. Opening the airway during cardiac arrest isn’t the intended purpose, but it comes with it.

Side Effects

Side effects are secondary effects of a drug or medical treatment that are typically undesirable. For example, we use Albuterol for bronchodilation during asthma attacks or COPD. The intended purpose is to make breathing easier, but a side effect is that it will stimulate the heart to start beating faster. Something important to remember about side effects is that we expect them to happen. It’s important to know what side effects medications can have so you are not confused when they occur in the field.

Adverse Effects

Pharmacology aims to get just the desired effect, limit the side effects, and avoid adverse effects. Adverse effects are reactions we don’t expect, but they do occur.

Let’s take, for example, a drug designed to get you high, but it also makes you vomit, and sometimes it kills 1% of the population. So the desired effect is getting you high, the side effect is making you vomit, and the adverse effect is killing you. The big difference between the three is expectations vs. outcomes. Adverse effects are always going to have negative effects on the body.

Therapeutic Index

With every medication, there is an amount that won’t produce the desired effect because the amount is too low and an amount that won’t have the desired effect because the amount is too high. The in-between phase is the therapeutic index. Generally, the wider the therapeutic index, the safer a drug is. It’s also important to remember that every medication outside of its therapeutic index, generally on the far side, can be a toxin. We can’t stress enough that too much of a good thing is bad, especially in pharmacology.

Increasing the dose of a medication does not always increase the desired effects, side effects, or adverse effects. There is a limit to the body’s response to drugs, and more isn’t always better. There is a therapeutic index that we target, and there are certain levels that we want to reach to achieve the desired effects. Remember, going beyond those levels won’t produce any additional benefit, but it can increase side effects and adverse effects.

The Confusion with the Therapeutic Index
Here’s a quick story to illustrate why this can be confusing. I was on a run and bumped into a guy taking fentanyl recreationally, and he told me he was really struggling with the drug use. He said it didn’t matter if he spent $15 or $100; he would take all of the medication he could get because he wanted to get as high as he could. It doesn’t work that way, though. You can’t just “get higher” than the maximum euphoric effect your body can experience. You can, however, significantly increase the chances of side effects and adverse effects by taking an increased dose.

To learn how therapeutic indexes are determined, check out this article, “The determination and interpretation of the therapeutic index in drug development.“

Efficacy versus Potency

Efficacy is the ability of a medication to produce a desired or intended result. Potency is the amount of drugs needed to achieve the desired effect. An example of efficacy is certain antibiotics’ ability to treat specific infections. The potency of an antibiotic doesn’t matter if it is not efficacious. An additional point to consider is that even if you select an efficacious medication, it needs to be taken six times a day because it isn’t very potent; it may not be the best choice for the patient’s lifestyle. Therefore, you may consider a medication with less efficacy but higher potency.

These are general concepts, but when EMS providers are out in the field, the protocols will drive what medications and doses are administered. Our goal is to help providers better understand the protocols and why some medicines are not recommended for specific conditions.

Routes of Medications

The route of a medication is how it is administered. There are seven main routes.

• Oral
• Sublingual/ Buccal
• Rectal
• Topical
• Transdermal
• Inhalation route/ pulmonary
• Injection

In the field, there are many factors to consider when choosing the route, which include: convenience, the state of the patient, the desired onset of action, the patient’s cooperation, the nature of the drug, the age of the patient, effect of gastric pH, digestive enzymes and first-pass metabolism.

Circumstances and the type of medication often dictate the route a provider needs to take. For example, if the patient has a compromised airway they are unable to take oral medications. Additionally, if you want the medication to act quickly, for example, with someone in excruciating pain, then you might select an IV route where the drug is 100% bioavailable instantly in the bloodstream, but if you want the medication to act slowly over time, then you may give it orally as it will be digested over a longer period.

This is also why EMS providers give lower concentrations of medications into an IV because it is 100% bioavailable. All of it is available to the body immediately. Contrast this to a medication that is given as an intramuscular injection. In this case, a higher concentration will be needed because it has to pass through the muscle, and it will lose concentration before it arrives at the vessels.

When we are discussing absorption into the body, some medications have to go through the liver and the kidney to be changed in a way so they can be effective. Additionally, some medications can’t be given through an IV because in that form, they won’t be effective. In fact, some may even be toxic through an IV.

The First Pass Effect

The first pass effect in pharmacology is a series of events in which a drug passes through the liver and kidneys and gets metabolized at a specific location in the body that results in a reduced concentration of the active drug upon reaching its site of action. This circles back to the concepts of dosing and knowing that not all of the medication will reach the final destination at the original concentration.

This is where loading and maintenance dosing come into play. Providers may give a lot more medication in the field to get an influx of medication into the system, and when the patient arrives at the hospital, they will be given maintenance doses to sustain the medication level.

Pharmacokinetics and Pharmacodynamics

Pharmacokinetics is the study of the movement of medications through the body from absorption through excretion. It is how the body processes the drug. Pharmacodynamics is what the drug is doing to the body. It describes the intensity of a drug effect in relation to its concentration in a body fluid, usually at the site of drug action.

In pharmacology, there are three main ways drugs have their effects on the body. It will bind to a receptor, it will bind to an enzyme and trigger a cascade of events to happen in the body, or it won’t do either of those things. If it does neither, then it is a non-specific drug interaction.

Activated charcoal is an example of a non-specific drug. It is typically given orally and has pores that can trap chemicals in the stomach and then it is excreted. It doesn’t act on a receptor or enzyme so it falls into the non-specific category.

Pharmacology Agonist and Antagonist

An agonist is a drug that binds to the receptor, producing a similar response to the intended chemical and receptor. Conversely, an antagonist is a drug that binds to the receptor and stops the receptor from producing a response. The best comparison for agonist and antagonist is a light switch turning the receptor on and off.

An example of an agonist is Albuterol. It amps up the dilation effect in the airway, prevents bronchospasm in patients with asthma, bronchitis, emphysema, and other lung diseases, and prevents bronchospasm caused by exercise. Atrovent is an antagonist. It prevents the constriction of the airway. It is used to prevent bronchospasm, or narrowing airways in the lungs, in people with COPD, including chronic bronchitis and emphysema. So both are acting through opposite mechanisms, but they’re doing the same thing. The same desired effect of opening the airway.

Additionally, antagonists can be competitive or non-competitive. A competitive antagonist binds to the same site as the agonist but does not activate it. Instead, it blocks the agonist’s action. A non-competitive antagonist binds to an allosteric (non-agonist) site on the receptor to prevent receptor activation.

There are also reversible and irreversible medications. A reversible drug is non-covalent and can be knocked off the receptor. An irreversible antagonist binds covalently to the receptor and has to be used up.

In emergency medicine, using drugs that turn receptors on and off offer the benefit of having a quicker effect on the body vs triggering an enzyme reaction.

In emergency medicine, using drugs that turn receptors on and off offer the benefit of having a quicker effect on the body vs triggering an enzyme reaction.

Pharmacology Flashcards

If you made it to the end of the article, congratulations! We have a special gift for you. Here is a set of pharmacology flashcards for free. These are included as part of our Paramedic NREMT Test Prep Course.