pharm

Pharmacology Basics

When a drug is taken or administered, it goes through 3 phases, a pharmaceutics phase, a pharmacokinetics phase, and a pharmacodynamics phase.

• Pharmaceutics- refers to how the body uses different “forms” of meds. The meds have to be in their simplest form; this phase occurs primarily when a drug is taken by mouth (po). If in solid form, the med goes thru a disintegration phase where it is broken down into smaller particles. Then it goes thru a dissolution phase where it is dissolved into a liquid state that can be absorbed and sent into the circulatory system. This phase is important in deciding what order to administer meds in. Those that are already in “user friendly form” can be given last, where those that have to go through disintegration and dissolution need to be given first.

• Pharmacokinetics- is basically what the body does to the drug. This phase is subdivided into 4 subclasses: absorption, distribution, metabolism, and excretion.

o Absorption is how the drug gets into the body fluids. This can be done by passively (from areas of high to low concentration and no energy source is needed, sort of like floating on a lazy river); active absorption needs a carrier, some sort of enzyme or protein to transfer the drug; and pinocytosis, where the cells engulf the drug particles and “drag” them across the cell membranes, like PacManJ. Most drugs must be lipid soluble to cross the membranes. Absorption is affected by many factors such as stress, food, blood flow, the status of the digestive tract, age, immaturity of organs, etc.

▪ The term hepatic first-pass means that the drug absorbed from the stomach or small intestine, sent to the liver via the portal vein where it is metabolized or broken down before going into the circ. system. A large portion of initial dosage is decreased this way. VERY important if the liver is not working correctly because this would cause the drug to accumulate in the body, possibly causing toxicity!

▪ Since the drugs are broken down, not all of the drug will reach the circulation. Bioavailability means how much of the actual drug reaches the circulation. Only drugs given intravenously are 100% bioavailable.

o Distribution is how the drug becomes available to the body. Drugs must be protein bound to move, and they will attach to a protein receptor site, which is found all over the body. The higher the protein binding, the better the drug works and the longer it stays in the system. There are 2 parts to the drug, the free drug (active drug) and the protein bound part (inactive drug). I like to refer to the protein bound portion as a protective shell that must be “eaten” thru before reaching the free drug. What does this mean? If a person has low protein, these drugs will work faster because they have nothing to bind to; therefore they (patients) will be more at risk for drug reactions, etc. On the other hand, those drugs bound to protein structures in individuals who are “nutritionally sound” will stay in the body longer, but be safer.

o Metabolism is how the drug is broken down by the body. The liver is mostly responsible for this. The term half-life refers to the amount of time it takes for half of the drug to be eliminated from the body. Drugs with a long half life will be given less often i.e.: if a drug has a half life of 12 hours, it will be administered approx. 2x daily. Monitor AST & ALT for liver function.

o Excretion is how the drug leaves the body. The main route of excretion is the kidneys. So what happens with someone with kidney malfunction? Yes, the drug will stay in the body longer, building up and possibly putting the person in drug toxicity. Other routes are any way body fluids leave your body, such as respirations, sweating, bowels, etc. Monitor blood levels of Creatinine & BUN (blood, urea, nitrogen) for overall kidney function and GFR & urine creatinine clearance for glomerular function.

• Pharmacodynamics is what the drug does to the body after the other 2 phases have been completed. Drugs will have either a primary effect (intended or therapeutic effect) or secondary effect (can be desirable or undesirable). A good example of these effects are with the drug Benadryl. Benadryl does a good job of drying up itchy, watery eyes but can also cause drowsiness- this is ok if you have time for a nap but undesirable if you’re trying to study for an exam!

Know the definition of the terms onset (the time it takes for a drug to start producing minimal therapeutic effects), peak (when a drug is producing maximum therapeutic effects) and duration (how long a drug is able to provide a therapeutic effect). All drugs “land” or attach to one or more receptors. Because of this, drugs can also have nonspecific or nonselective effects. Nonspecific effects means the drug attaches to ONE receptor and whatever bodily functions that ONE receptor controls, the function will be affected by the drug. NONSELECTIVE effects means the drug lands on MORE THAN ONE RECEPTOR! SO whatever functions these control, will be affected. This can be BAD for instance, if it lands on Alpha and Beta receptors. It may increase the heart rate BUT cause bronchoconstriction, causing the person to have difficulty breathing!

Therapeutic Index is the safety margin of a drug. On drugs that have a narrow therapeutic index, drug levels must be done at specified times or under special criteria because the patient is at high risk for drug toxicity. Other terms are peak (maximum amount of drug working-see above), trough (lowest amount of drug working)- both of these are important with potent antibiotics and must be monitored for drug toxicity, loading dose-giving extra dose to rapidly achieve a higher blood level (example would be when you get an antibiotic from the dr and label says to take 2 now then one every 6 hrs until gone- the initial 2 would be a loading dose), side effect (undesirable effect but does not change the overall intended result- example would be constipation caused from pain pills), adverse effect (more severe than side effects; BAD effects-can’t take drug anymore), and toxic effect (either too high of a dose or organ failure causing the dosage to accumulate in the body).

Body Surface Area (BSA) is a measurement of the total surface area of the human body. It is typically expressed in square meters (m2). It is based on an individual’s body weight and height and used to estimate the extent of burns, drug dose adjustments, fluid and nutritional management. A common formula used to calculate BSA is:

​​ √(height in cm x weight in kg) / 3600 

Patient Safety is ALWAYS the goal of drug administration! Know the rights of drug administration- patient, drug, dose, route, time, and documentation . Orders need a date, time, drug name, dose, route, frequency, & duration. They must be signed by the ordering health care provider.

In addition to the above rights, you also need the right documentation (date, time, medication, dose, route, initials/name & title, and site of application/administration of the medication if applicable); right assessment (ex. if med is for hypertension, need current BP); right education for the client (remember informed consent-include benefits and risks, side effects, therapeutic effects, etc when teaching); right evaluation (therapeutic effectiveness); and finally, the client’s right to refuse. You need to notify the health care provider when the refusal poses a threat to the overall well-being of the client (ie- you would notify MD immediately if the client refused dose of anticoagulant if has a clot but not necessarily so if a client refuses a laxative after having a bowel movement prior to taking it).

All routes of medication administration will be covered in NUR 1023.

Frandsen, G. & Pennington, S. Abrams' Clinical drug therapy: Rationales for nursing practice.

(13 th ed.). E-Book on Lippincott CoursePoint for Nursing Concepts