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Flashcards in Volume of Distribution Deck (19)
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1
Q

What is the apparent volume of distribution?

A

The relationship between concentration and the amount of a drug in the body

2
Q

What are the units for amount, volume and concentration?

A

mg L mg/L

3
Q

How can the loading dose be predicted?

A

If the target concentration and the drug apparent volume distribution are known

4
Q

What are the three physical compartments of the body?

A

Vascular- blood 5L plasma 2.5 L
Extracellular- 18 L
Total body water- 35L

5
Q

What is the only drug that undergoes tissue binding?

A

Digoxin (Na/K ATPase)

6
Q

Why does the measured amount of digoxin lower than if it were evenly distributed throughout the body? (sponge)

A

The apparent volume must be larger than the physical volume. The apparent volume of distribution will be large when there is extensive binding to tissue proteins

7
Q

How does fat affect volume of distribution?

A

Lipophilic drugs increase apparent volume of distribution

Hydrophilic drugs have no effect

8
Q

How does bone affect volume of distribution?

A

Some drugs are absorbed by bone and this will increase their apparent volume of distribution (tetracycline and bisphosphonates)

9
Q

How does plasma protein binding affect volume of distribution? (red herring)

A

Leads to a smaller apparent volume

Bind to proteins such as albumin and alpha acid glycoprotein

10
Q

What is the idea way to measure drug concentration?

A

Unbound but the loading dose calculated will be the same as long as the target concentration type matches the apparent volume type

11
Q

Why are plasma proteins insignificant?

A

Negligible effect on total unbound amount in the body if plasma protein binding is changed

12
Q

Why does warfarin have a very small apparent volume?

A

It binds extensively to plasma proteins

13
Q

Why does gentamicin have a small apparent volume?

A

It is highly ionised and does not cross cell membranes easily so its apparent volume is close to the physical volume of ECF

14
Q

What is the apparent volume of distribution of theophylline?

A

35L-on par with total body fluid as it crosses cell membranes

15
Q

What is the apparent volume of distribution of digoxin?

A

500 L due to tissue binding

16
Q

What are the two pharmokinetic compartments and why do they matter?

A

Drugs travel from plasma to extracellular space and finally to tissues so is in different places at different times. Apparent central compartment volume (ECF volume) and apparent tissue compartment volume

17
Q

How does drug concentration change in a single compartment model?

A

With elimination from the beaker drug concentration drops

18
Q

How does drug concentration change in a double compartment model?

A

Concentration remains constant even with elimination

19
Q

What are the differing distribution half lives between drugs?

A

Minutes- thiopentone
Hours- digoxin
Days- lithium