eLFH - Pharmacokinetics Flashcards

1
Q

Euler’s number ‘e’ description

A

Mathematical constant that describes the process where a variable increases or decreases at a rate that is proportional to its own magnitude

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2
Q

e value

A

2.718

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3
Q

Tear away function definition

A

Exponentially increasing functions

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4
Q

Example of tear away function

A

Bacterial growth

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5
Q

Examples of exponentially decreasing functions

A

Radioactive decay

Elimination of drug from body

Denitrogenation when pre-oxygenating

Water emptying from cylinder of water with tap at bottom

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6
Q

Asymptote

A

With exponentially decreasing functions, end state never quite reaches zero, but the curve gets ever closer to the x axis

This end state is called an asymptote

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7
Q

Use of logarithms in exponential graphs

A

Using logarithms converts curve to straight line graph when same log base as exponential is used

i.e. Natural logarithms are required for natural exponential processes as they relate to ‘e’

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8
Q

Natural logarithm definition

A

Logarithm with base of ‘e’ (2.718)

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9
Q

Exponential function vs Rectangular hyperbolic function

A

Both asymptote the x axis, but rectangular hyperbolic function also asymptotes y axis, whereas exponential function doesn’t

Exponential function models naturally occurring processes
Rectangular hyperbola is a section of a cone

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10
Q

Three constants that can be used to describe the exponential decay function

A

Rate constant (k)

Time constant (tau)

Half life (t1/2)

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11
Q

Rate constant (k) definition

A

Describes relationship between rate of decrease and the magnitude of the variable

I.e. k is ratio of the slope of the graph to its height

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12
Q

Units of rate constant ‘k’

A

In exponential processes that occur over time, units of k are always sec^-1

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13
Q

Time constant (tau) definition - 3 ways to describe it

A

1) Time required for a process to complete if it continued at its initial rate of fall (i.e. tangent to the graph at time = 0)

2) Time taken for the magnitude of the variable to fall to 37% of its initial value

3) Time to fall to 1/e of its original value

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14
Q

Half life definition

A

Time required for the magnitude of the variable to fall to one half of its original value

Half life must always be shorter than time constant by definition

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15
Q

Relationship between the three constants

A

Only need to know one of the constants to describe the shape of the graph

tau = 1 / k
k = 1 / tau
t1/2 = tau x 0.693

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16
Q

How much of process is complete after 3 time constants

A

94.9%

After 1 tau = 63% complete
After 2 tau = 63% + (63% of remaining 37%) = 86.3%
After 3 tau = 86.3% + (63% of remaining 13.7% = 94.9%

17
Q

Single compartment model

A

Drug administered to the body becomes evenly distributed throughout a single hypothetical compartment which is the volume of distribution

Single compartment model is too simple to accurately model the kinetics of most drugs

18
Q

Volume of distribution

A

Volume of theoretical compartments that a drug is distributed into

Used by TCI models

E.g. 100 mg of propofol in Vd of 20L equates to 5 microgram/ml

19
Q

Example of drugs which follow a three compartment model

A

Propofol

Fentanyl

20
Q

Three compartment model

A

Central compartment - Initial volume of distribution that is very small representing initial dispersal in the plasma

Shallow peripheral compartment - second compartment with perfusion rich organs including brain

Deep peripheral compartment - third and largest compartment with mostly fat redistribution of drug

21
Q

Volume of distribution at a steady state (Vdss)

A

When drug is redistributed throughout all body tissues - Vdss is very large

Vdss = sum of all compartments
= 16 + 32 + 202
= 250 L in this example

22
Q

3 definitions of clearance

A

1) Proportion of the compartment volume that flows in unit time

2) Mass of drug eliminated per unit time per unit drug concentration

3) At steady state, the dose rate (flow rate) divided by the steady state blood concentration

23
Q

Conceptual way to think about clearance

A

Virtual flow of the volume of distribution

When a drug is cleared from the body by passage in the blood through an organ (e.g. liver or kidney) the volume that is completely cleared is the clearance

24
Q

Drug clearance equations (3 of them)

A

Clearance = Drug infusion rate / Blood concentration
(in the steady state for this first equation)

or

Cl = k x Vd

or

Cl = Vd / tau

25
Q

Units of clearance

A

L/min

or

L/kg/min