NCM 106 PHARMACOLOGY
READINGS
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PHARMACOKINETICS
is the process of drug movement to achieve drug
action.
The four processes are absorption, distribution,
metabolism (or biotransformation), and excretion
(or elimination).
The nurse applies knowledge of pharmacokinetics
when assessing the patient for possible adverse drug
effects. The nurse communicates assessment
findings to members of the health care team in a
timely manner to promote safe and effective drug
therapy for the patient.
Absorption
is the movement of drug particles from the GI tract to
body fluids by passive absorption, active absorption,
or pinocytosis.
Most oral drugs are absorbed into the surface area
of the small intestine through the action of the
extensive mucosal villi. Absorption is reduced if the
villi are decreased in number because of disease,
drug effect, or the removal of small intestine.
Protein-based drugs such as insulin and growth
hormones are destroyed in the small intestine by
digestive enzymes.
o Passive absorption occurs mostly by diffusion
(movement from higher concentration to lower
concentration). With the process of diffusion,
the drug does not require energy to move
across the membrane.
o Active absorption requires a carrier such as an
enzyme or protein to move the drug against a
concentration gradient. Energy is required for
active absorption.
o Pinocytosis is a process by which cells carry a
drug across their membrane by engulfing the
drug particles.
Remember, drugs that are lipid soluble and nonionized
are absorbed faster than water-soluble and ionized
drugs. Drugs given IM are absorbed faster in muscles that
have more blood vessels (e.g., deltoids) than in those that
have fewer blood vessels (e.g., gluteals). Subcutaneous
tissue has fewer blood vessels, so absorption is slower in
such tissue.
The process in which the drug passes to the liver first is
called the first-pass effect, or hepatic first pass.
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BIOAVAILABILITY
is a subcategory of absorption.
It is the percentage of the administered drug dose
that reaches the systemic circulation. For the oral
route of drug administration, bioavailability occurs
after absorption and first-pass metabolism. The
percentage of bioavailability for the oral route is
always less than 100%, but for the IV route it is
100%.
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Distribution
is the process by which the drug becomes available
to body fluids and body tissues.
Drug distribution is influenced by blood flow, the
drug’s affinity to the tissue, and the protein-binding
effect.
Metabolism
is the process by which the body inactivates or
biotransforms drugs.
Drugs can be metabolized in several organs;
however, the liver is the primary site of
metabolism. Most drugs are inactivated by liver
enzymes and are then converted or transformed by
hepatic enzymes to inactive metabolites or watersoluble substances for excretion.
The half-life (t 1/2) of a drug is the time it takes for one
half of the drug concentration to be eliminated.
Metabolism and elimination affect the half-life of a
drug.
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Elimination
The main route of drug elimination is through the
kidney (urine). Other routes include bile, feces,
lungs, saliva, sweat, and breast milk. The kidneys
filter free unbound drugs, water-soluble drugs, and
drugs that are unchanged. The lungs eliminate
volatile drug substances and products metabolized
to carbon dioxide (CO2) and water (H2O).
PHARMACODYNAMICS
is the study of the way drugs affect the body.
Drug response can cause a primary or secondary
physiologic effect or both. The primary effect is
desirable, and the secondary effect may be desirable
or undesirable.
An example of a drug with a primary and secondary
effect is diphenhydramine (Benadryl), an
antihistamine.
The
primary
effect
of
diphenhydramine is to treat the symptoms of allergy,
and the secondary effect is a central nervous system
depression that causes drowsiness. The secondary
effect is undesirable when the patient drives a car,
but at bedtime it could be desirable because it
causes mild sedation.
PARACETAMOL is a generic name. Its common brand
names are Panadol, Tylenol, and Bioflu.