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Pharmicokinetics Flashcards

1
Q

What does ‘half-life’ mean?

A

The half-life of a drug is the time it takes for one-half of the drug to be eliminated from the body.

Katzung BG, Masters, SB, & Trevor AJ. Basic and Clinical Pharmacology. 12th ed. New York: McGraw-Hill; 2012: 42.

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2
Q

What is ‘clearance’?

A

Clearance refers to the ability of the body to eliminate a drug.

Katzung BG, Masters, SB, & Trevor AJ. Basic and Clinical Pharmacology. 11th ed. New York: McGraw-Hill; 2009: 37.

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3
Q

What is volume of distribution?

A

The volume of distribution is a number that describes the amount of drug in the body compared to how much is actually in the bloodstream. It is calculated as Vd = Amount of drug in body/Plasma concentation of the drug. It actually describes an apparent volume. If a large amount of a drug has been administered and only a small amount of the drug appears in the plasma, the drug is said to have a large volume of distribution.

Katzung BG, Masters, SB, & Trevor AJ. Basic and Clinical Pharmacology. 12th ed. New York: McGraw-Hill; 2012: 38.

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4
Q

What does the term bioavailability mean?

A

Bioavailability refers to the amount of the drug that is able to reach its target receptor after it is introduced into the circulatory system.

Nagelhout JJ, Plaus KL. Nurse Anesthesia. 5th ed. St. Louis, MO: Elsevier Saunders Company; 2014: 66.

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5
Q

What does the term ‘steady-state’ mean?

A

Steady-state refers to a state in which all of the tissues in the body have had time to reach equilibrium. The concentration of drug differs between organs, but no longer varies from moment- to-moment.

Nagelhout JJ, Plaus KL. Nurse Anesthesia. 5th ed. St. Louis, MO: Elsevier Saunders Company; 2014: 70.

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6
Q

What pharmacokinetic measurement predicts the time it takes for half of a drug to be eliminated when a continuous infusion of the drug is discontinued?

A

The context-sensitive half-time. It should be noted that context sensitive half-times are computer models that only predict the time for 50% of the drug in the central compartment to be eliminated. It does not predict the time for recovery from the drug.

Nagelhout JJ, Plaus KL. Nurse Anesthesia. 5th ed. St. Louis, MO: Elsevier Saunders Company; 2014: 72-73.

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7
Q

What does ‘extent of absorption’ mean?

A

Extent of absorption refers to the degree a drug reaches the blood stream from the site of administration. For example, only about 70% of a dose of digoxin reaches the bloodstream when it is administered orally.

Katzung BG, Masters, SB, & Trevor AJ. Basic and Clinical Pharmacology. 12th ed. New York: McGraw-Hill; 2012: 43.

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8
Q

How does molecule size affect a drugs ability to cross a biologic membrane?

A

The smaller the molecule is, the easier it is for it to cross a biologic membrane. Usually, molecules heavier than 100-200 Daltons do not cross cell membranes.

Nagelhout JJ, Plaus KL. Nurse Anesthesia. 5th ed. St. Louis, MO: Elsevier Saunders Company; 2014: 5944.

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9
Q

To what does the term ‘stereochemistry refer?

A

It refers to the ability for a chemical to have more than one possible structural arrangement. It is based on the presence of a structural component called a chiral carbon. The chiral carbon acts as a point in which a portion of the chemical can be rotated into another position. The two chemicals are isomers of each other and often have substantial differences in their pharmacologic effect.

Nagelhout JJ, Plaus KL. Nurse Anesthesia. 5th ed. St. Louis, MO: Elsevier Saunders Company; 2014: 69.

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10
Q

How do albumin, alpha-1 acid glycoprotein, and beta- globulin differ in their ability to bind with drugs?

A

Although albumin is able to bind to drugs that are acidic, neutral, or basic, it binds more readily to acidic compounds. Alpha-1 acid glycoprotein and beta-globulin bind more readily to basic compounds.

Nagelhout JJ, Plaus KL. Nurse Anesthesia. 5th ed. St. Louis, MO: Elsevier Saunders Company; 2014: 64.

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11
Q

What factors affect the extent of absorption of a drug?

A

Factors that affect the ability of the drug to cross from the GI tract to the bloodstream affect the extent of absorption (e.g. the drug is too hydrophilic or too lipophilic, transport mechanisms may tend to pump the drug back into the gastrointestinal system, etc).

Katzung BG, Masters, SB, & Trevor AJ. Basic and Clinical Pharmacology. 12th ed. New York: McGraw-Hill; 2012: 43.

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12
Q

What is the most prevalent protein in the bloodstream?

A

Albumin

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13
Q

How does protein binding affect the ability of a drug to exert its pharmacologic effects?

A

The more protein bound a drug is, the less of the drug is available to exert its effects and vice versa.

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14
Q

What is a reversible antagonist?

A

A reversible antagonist will compete with the agonist for receptor binding locations and will decrease the ability of the agonist to activate the receptor. If the relative concentration of the agonist increases, it can ‘dislodge’ the antagonist from the receptor and the pharmacologic effects of the antagonist will diminish.

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15
Q

How is receptor affinity for a drug related to the potency of a drug?

A

Receptor affinity refers to the degree of attraction that exists between a drug and the receptor to which it attaches. Drugs with a high affinity for a receptor bind to it easily. Drugs with a low affinity for a receptor do not tend to bind with it as quickly.

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16
Q

How do ionized and nonionized forms of a drug compare in their ability to cross cell membranes?

A

Ionized forms of a drug are charged and are water-soluble. Nonionized forms of a drug do not have an electrical charge and are lipid-soluble. Because cell membranes are composed of lipids, only lipid-soluble drugs can pass through them easily. Lipid membranes tend to repel ionized drugs because of their electrical charge.

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17
Q

What is the general chemical composition of most drugs?

A

Most are salts of either a weak base or a weak acid.

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18
Q

How does hypoalbuminemia affect the action of a highly protein-bound drug like warfarin?

A

About 98% of warfarin is protein-bound. As the plasma protein level decreases, there is less of the protein available for the drug to bind with, and more of the free form of the drug is available to exert its pharmacologic action. In summary, hypoalbuminemia intensifies the action of highly protein-bound drugs.

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19
Q

What percentage of the body weight does plasma constitute?

A

5%

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20
Q

What does the term ‘maximum effect’ mean?

A

All drugs have a maximum effect. This represents the maximum response that can be clinically seen. Once this point is reached, the administration of more of the drug will not increase the response any further.

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21
Q

What percentage of the body weight does adipose tissue constitute?

A

20%

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22
Q

What percentage of the body weight does transcellular fluid constitute?

A

2%

23
Q

What percentage of the body weight does interstitial fluid constitute?

A

16%

24
Q

How does enzyme induction affect the plasma half- life of agents that undergo biotransformation via the same metabolic pathway?

A

Enzyme induction reduces the plasma half-life of agents that are metabolized by that particular metabolic pathway.

25
Q

In what circumstance can the volume of distribution for a drug be greater than the total volume of the body?

A

Drugs that leave the plasma and accumulate in fat or other tissues can actually exhibit a calculated volume of distribution that is greater than the volume of the body.

26
Q

How does the plasma half-life of a drug relate to its rate of clearance?

A

The plasma half-life is inversely proportional to its rate of clearance.

27
Q

How does the term metabolism differ from the term biotransformation and what do they mean?

A

Metabolism and biotransformation are synonyms that refer to enzyme-catalyzed changes in the structure of a drug. The change in the chemical structure also changes the pharmacologic effect of the drug.

28
Q

Where in the body does biotransformation occur?

A

Most drugs are metabolized in the liver, but they can also undergo biotransformation in the bloodstream, kidneys, lungs, heart, brain, gastrointestinal tract, and skin.

29
Q

How does gender influence the pharmacokinetics of anesthesia drugs?

A

It has been demonstrated that female patients exhibit a 20-30% increase in sensitivity to vecuronium, rocuronium, and pancuronium. Males also show an increased sensitivity to propofol with some studies proposing reduced dosing requirements for male patients. A study evaluating the emergence from general anesthesia with propofol, alfentanil, and nitrous oxide has been shown to be quicker for females than males with females being 3 times more likely to experience recall under anesthesia.

30
Q

How does temperature affect the metabolism of intravenous anesthetics?

A

Temperature affects tissue blood flow and metabolism. Studies have demonstrated that increased patient temperature increases the elimination of propofol.

31
Q

What is the most important factor in determining the rate of diffusion of a drug across a cell membrane?

A

According to the Fick equation, the most important factor in determining the rate of diffusion of a drug across a membrane is the concentration gradient of the drug. Membrane thickness is an important factor in the rate of diffusion as is molecular weight, which affects the diffusion coefficient (p) in the equation, but neither of these variables are as important as the concentration gradient.

32
Q

How does the plasma half-life of a drug relate to its volume of distribution?

A

The plasma half-life of a drug is directly proportional to its volume of distribution.

33
Q

What percentage of the body weight does intracellular fluid constitute?

A

35%

34
Q

What is the primary function of a receptor antagonist?

A

A receptor antagonist binds to the receptor and prevents an agonist, which is capable of activating the receptor, from binding with it.

35
Q

What is first-pass metabolism?

A

When a drug is absorbed through the gastrointestinal wall, the blood into which it passes drains through the portal vein and into the liver. The liver can extract a portion of the drug and eliminate before it reaches the systemic circulation. For example, the extent of absorption of orally administered morphine is almost 100%. The hepatic extraction ratio, however is about 0.67, which means that the liver metabolizes about 67% of the drug before it reaches the systemic circulation.

36
Q

What is a receptor antagonist?

A

A receptor antagonist binds to the receptor but does not activate it.

37
Q

What is a ligand?

A

A ligand is any chemical that can bind with a protein to form another complex. Typically, it is a signal triggering molecule that binds with a target protein.

38
Q

What are receptors composed of?

A

Most receptors are composed of proteins. Their polypeptide structure provides the diversity, electrical charge, and morphology to bind with ligands.

39
Q

What characteristics of a receptor determine the affinity it has with a specific drug?

A

The size, shape, and electrical charge of a receptor help determine the affinity it will have for a specific drug. Slight changes in the chemical composition or structure of a drug can cause vast changes in how it interacts with a receptor.

40
Q

What is ion trapping?

A

Ion trapping occurs when a drug is in a lipid-soluble form and crosses the cell membrane. If the pH on the inside of the cell is different from the pH on the outside of the cell, the drug can begin to become ionized. Because it is ionized, it cannot pass back through the cell membrane and leave the cell. As a result, the ionized form of the drug becomes trapped inside the cell. Ion trapping can also occur between a mother and her fetus. Drugs administered to the mother can cross the placenta in their lipid-soluble form. The bloodstream of the fetus, however, is slightly more acidotic and the drug becomes ionized. Because it cannot cross the placental barrier back to the mother, it becomes trapped in the fetus.

41
Q

What is active transport?

A

Active transport is a process that requires energy and is usually faster than passive transport. It often involves the combination of the drug with a carrier molecule that is capable of transporting the drug across a membane even if it is against a concentration gradient.

42
Q

What is passive transport?

A

Passive transport does not require energy and is simply the diffusion of a drug from an area of higher concentration to an area of lower concentration.

43
Q

What is an irreversible antagonist?

A

An irreversible antagonist will form such a strong bond with the receptor that it cannot be dislodged at all.

44
Q

What determines the duration of action of an irreversible antagonist?

A

Because the drug binds irreversibly with the receptor, the duration of action of the drug is not as closely related to its metabolism as it is to the speed at which new receptors can be produced.

45
Q

What determines the duration of action of a reversible antagonist?

A

Reversible antagonists can be metabolized and eliminated by the body. An increase in the concentration of the agonist may also shorten the duration of action of a reversible antagonist.

46
Q

What is zero-order kinetics?

A

Zero-order kinetics refers to metabolism in which the amount of the drug eliminated over time is constant. Alcohol is an example of a drug that exhibits zero-order kinetics.

47
Q

What is first order kinetics?

A

First-order kinetics refers to drugs that are eliminated by a constant percentage over time. For example, if the half-life of a drug is one hour, then half of the drug will be eliminated over 60 minutes. Half of the remaining amount will be eliminated in the second hour and so on.

48
Q

What is enzyme induction?

A

When the body is exposed to certain drugs over an extended period of time, it will increase the amount of enzymes that metabolize it. When this occurs, other agents that are metabolized by the same enzymes will undergo biotransformation much more rapidly.

49
Q

What is a compartment model?

A

A compartment model is a way of describing the body in different sections into which a drug is distributed.

50
Q

What is meant by a one-compartment model? A two- compartment model?

A

For many drugs, a single-compartment model, which represents the distribution of the drug throughout the entire body, sufficiently explains the pharmacokinetics of the drug. For lipid- soluble drugs, however, a two-compartment model better illustrates how the drug is distributed. In the two-compartment model, there is a central compartment, which consists of the intravascular fluid and highly perfused tissues such as the
heart, lungs, brain, kidneys, and liver. The peripheral compartment consists of the muscles, adipose tissue, and bone.

51
Q

What is another term for the central compartment?

A

The vessel-rich group

52
Q

What portion of the body’s mass does the central compartment comprise? What portion of the cardiac output does it receive? How does this compare to the peripheral compartment?

A

The central compartment comprises only 10% of the body’s mass, but receives 75% of the cardiac output. In contrast, the peripheral compartment comprises the remaining 90% of the body’s mass, but only receives 25% of the cardiac output.

53
Q

What are the two ways in which drugs leave the central compartment?

A

They are either distributed into the tissues or metabolized and excreted.

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