The nurse must have adequate understanding of the phases of drug action: they are
pharmaceutic, pharmacokinetic, and pharmacodynamic.
In the Pharmaceutic phase, drugs become a solution.
This phase takes place in the gastrointestinal, or GI, tract; thus it is absent with parenteral administration.
The four parts of the pharmacokinetics phase achieve drug action. These phases are
absorption, distribution, metabolism or biotransformation, and excretion or elimination.
Absorption is the movement of the drug to the body fluids. It takes place by
passive absorption, active absorption, and pinocytosis.
Passive absorption takes place
without energy, by diffusion.
Active absorption requires
energy to move a drug against a concentration gradient.
Pinocytosis is accomplished by
cells engulfing a drug particle to carry the drug across the membrane.
Drug absorption can be altered by
blood flow, pain, stress, hunger, fasting, food, pH, and method of administration.
The term distribution refers to
the drug becoming available to body fluids and body tissues. Distribution is influenced by blood flow, affinity to the tissue, drug dosage, and drug concentration in the body.
Metabolism or biotransformation may occur in either
the GI tract or the liver. Most often it takes place in the liver.
Drug elimination is most often via the
kidney, although elimination may also take place via the liver, bile, feces, lungs, saliva, sweat, and breast milk.
Decreased kidney function will
alter drug elimination.
The term pharmacodynamics refers to
the effects of drug concentration on the body systems.
To understand the pharmacodynamics of a drug, the nurse must
understand the different aspects of drug action.
One important aspect is the
onset, peak, and duration of action of a drug.
The onset of action is
the time necessary for the drug to reach minimum effectiveness.
The peak action
occurs when the drug reaches the highest blood concentration.
The duration of action is
the length of time the drug has pharmacologic effects.
Drugs that produce a response are called
Drugs that block responses are called
The term nonspecific drug effects refers to
the lack of specific or selective effects of many drugs. While these drugs work on only one type of receptor, they may affect these receptors in all locations in which the receptor is found. This causes the drug to alter function dependent on where the receptor is located.
Nonselective drugs are
drugs that activate many different types of receptors, again causing a variety of responses from the different receptor sites.
There are four categories of
Drugs that are stimulators
increase the rate of cell activity or gland secretion.
Drugs that are depressants
have the opposite effect on cell or gland activity.
Replacement drugs replace
essential body compounds no longer existing in the body.
Drugs that inhibit or kill organisms
interfere with cell growth.
Drugs that are considered irritants cause
irritation, which affects the desired action.
The half-life of a drug, or the time it takes for one half of the drug concentration to be eliminated, can
be used to guide drug dosage intervals.
The safety of drug administration is maintained by
using the therapeutic index and therapeutic range of drugs.
A low therapeutic index refers to
a drug with a narrow safe level. Patients taking these drugs need to have their serum drug levels monitored closely.
Drugs with a high therapeutic index are
considered to have a wide safe level and do not require routine serum drug level monitoring.
The therapeutic range refers to
the range of serum drug level that allows the drug to be effective but remain below the level that would cause toxic effect.
Monitoring peak and trough drug levels is the method used to
measure serum drug levels for those drugs with narrow therapeutic indexes.
The peak level is
the highest serum concentration, and it measures the rate of absorption of the drug.
The trough is
the lowest serum concentration, and it measures the rate of drug elimination.
Loading doses are used to
achieve rapid minimum serum drug concentrations.
Drug side effects are
effects that are other than the effects intended. All drugs have side effects, some of which are undesirable.
Adverse reactions are
more severe than side effects and are always undesirable. They may be mild or severe.
Toxicity or toxic effects are identified by
monitoring the serum therapeutic ranges of drugs. When a drug exceeds the therapeutic range, toxic effects of the drug occur.
Drug tolerance refers to
decreased effectiveness of a drug over the course of therapy.