Uptake & Distribution of IV Agents Flashcards
What is pharmacokinetics?
what your body does to the drug
What are the four dimensions pharmacokinetics?
things you can measure:
absorption
distribution
metabolism
excretion
Absorption is _____.
transportation of unmetabolized drug from site of admin to circulation
Distribution is ______.
volume distribution - when the drug is brought to the body compartments
mathematical expression of the sum of the volumes of the compartments
Metabolism is _______.
convert pharmacologically active lipid soluble drugs into water soluble and inactive metabolites.
Excretion is ______.
process of removing drug and metabolites from body via kidneys, liver, GI.
What is Pharmacodynamics?
what the drug does to the body
3 dimensions of pharmacodynamics?
mechanism of effect
sensitivity
responsiveness
Mechanism of effect is ______.
the processes that produce the desired effect of the drug
Sensitivity is ______.
the lowest input (smallest dose) required to produce a given degree of output (normal response)
Responsiveness is _______.
degree of effect that drug does to body
Measured parameters of injected drugs are (4)
elimination half time
bioavailability
clearance
volume of distribution (Vd)
Elimination half time (E1/2t) is ______.
time necessary for [PLASMA] of drug to decline 50% during elimination, independent of drug dose
Describe relationship between elimination half time and Volume of distribution.
directly proportional,
as Vd increases, elimination increases
Bioavailibility is ______.
amount of drug that enters systemic circulation after a given dose, and the ability of the formulation of the drug to deliver it to the site of drug action
Clearance is ______.
capacity for drug removal by various organs defined as volume/unit time.
Volume of distribution is ______.
volume into which a drug appears to have been dissolved after administration; sum of all volumes of compartments
What are compartment models?
division of body into compartments that represent theoretical spaces
What are the two compartments in 2 compartment model?
central compartment
peripheral compartment
Describe central compartment.
where drugs go first, rapid equilibration
highly perfused tissues: kidney, liver, lungs, heart, brain (vital organs)
receive 75% of CO
only 10% of body mass
What does “arm to brain circulation” mean?
drug reaches brain rapidly, 30-60 sec
Describe the pathway of the drug when administered?
Drug accumulates in the tissues because the plasma concentration (central) is initially greater than the tissue concentration (peripheral).
However, eventually the plasma concentration falls to such an extent that the net drug movement is from tissues (peripheral) back into blood (central) to be excreted.
What is the peripheral compartment?
the tissues that are less perfused: muscle, skin, fat, nails, hair
drug equilibrates slower
no metabolism occurs here
large volume of distribution
What is rate of transfer? In what population is rate of transfer decreased? What’s the significance?
distribution of drug between compartments
old people
greater drug plasma concentration, ex thiopental
Describe what a drug reservoir is. Give an example.
tissues that accumulate drug
affects drug availability by maintaining plasma concentration when released from storage to bloodstream
permits sustained drug release over time
ex. adipose tissue, bone, transcellular (ion trapping)
What can occur with large, repeated doses of drug?
reservoir tissues become saturated after multiple concurrent doses, prolonging the duration of the drug
What are the components of 3 compartment model?
vessel rich
vessel poor
muscle
What component of 4 compartment model?
vessel rich
muscle
fat
vessel poor
What is the body mass percent for each of the four compartments for a 70kg?
vessel rich 10
muscle 50
fat 20
vessel poor 20
What is the blood flow percent of cardiac output in a 70kg person in each of the four compartments?
vessel rich 75
muscle 19
fat 6
vessel poor <1
What type of drugs are preferred and why?
lipophilic d/t faster action
Describe lung uptake. Give examples of drugs that do this.
lung uptakes basic, lipophilic drugs (amines, pKa > 8) and acts as a reservoir to release back into systemic circulation.
lidocaine, fentanyl, demerol
How much of fentanyl goes to lung?
70%
What is the blood brain barrier (BBB)?
functional and structural protective barrier to prevent ionized, water soluble drugs from crossing into brain circulation
How can the BBB be overcome (3)? Example of type of drug that is deadly if large enough dose crosses BBB?
large doses of drug,
hypoxemia,
head injury
lidocaine - seizures, cardiac collapse
What types of drugs have an easier time crossing BBB?
lipophillic drugs
Where do drugs exert their biological effect?
biophase or effect site (milleau) AKA membrane, receptors, enzymes
What is KE0?
The rate constant of drug elimination from effect site. Different for every drug.
What is the formula Vd?
Vd = dose of IV drug/ [plasma} BEFORE elimination
aka at time = 0
What 3 factors influence Vd? And their relationships?
lipid soluble (highest influence) - direct
binding to plasma proteins - inverse
molecular size - inverse
E1/2t is _______ proportional to its Vd.
directly (independent of dose)
Define elimination half life.
Time necessary to eliminate 50% of drug from BODY
What occurs if dosing intervals are less than elimination half life?
drug accumulation
In anesthesia world, does elimination half time and elimination half life ever be equal? Why?
No. Most IV meds/anesthetics are lipophillic so they will be different.
(but PO meds that are water soluble will cause both to be equal to each other sometimes)
How many half lives before most of the drug is eliminated?
4-6 half lives
93.8 - 98.4%
Describe graph of half life plasma concentration of drug.
x: time
y: log plasma concentration
initial concentration high at time 0
distribution phase (alpha) E1/2t alpha - sharp decline in concentration - peripheral compartment
elimination phase (beta) - more gradual decline in plasma concentration - back in central compartment
When does context sensitive half time apply?
refers to discontinuing an infusion
What factors affect context sensitive half time? (3)
distribution and excretion, properties of drug, and length of infusion
What is context sensitive half time?
time required for 50% of plasma concentration to fall after discontinuing an infusion
Systemic absorption depends on ________ regardless of route of drug.
drug solubility AKA lipophillicity
What are advantages of oral route for drugs? (2)
most convenient
most economic
What are disadvantages of disadvantages? (3)
emesis,
irregular absorption w/ food or other drugs
destruction by enzymes or gastric fluid
What is the first pass effect?
drug into GI system
goes to portal venous blood and passes through liver where it is metabolized
before it enters systemic circulation and arrives at tissue receptors
What routes have the first pass effect?
oral
proximal (deep) rectum
What is the advantage of sublingual or transmucosal route?
rapid onset d/t bypassing liver preventing first pass effect and membrane being so vascular and thin
Describe transdermal route of drug administration.
sustained therapeutic [plasma}
absorption through sweat ducts and hair follicles (penetrates several layers of human skin - diffusion shunt)
What are some factors that affect transdermal route? (2)
stratum corneum of epidermis - thickness (chest skin thin)
thickness
blood flow to that area of the skin