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20 Cards in this Set
- Front
- Back
What does it mean that a drug follows first order kinetics/elimination? |
It means that the rate of the elimination of the drug depends on the plasma concentration of the drug. As the plasma concentration of the drug decreases the rate of elimination decreases. |
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What makes a drug follow first order kinetics rather than zero order kinetics? |
If the elimination mechanisms of the drug are not saturated at therapeutic concentrations the drug follows first order kinetics. |
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What does it mean that a drug follows zero order kinetics/elimination? |
It means that the rate of the elimination of the drug is constant and doesn't depend on the plasma concentration of the drug. |
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What makes a drug follow zero order kinetics rather than first order kinetics? |
If the elimination mechanisms of the drug are saturated at therapeutic concentrations the drug follows zero order kinetics. |
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Do most drugs follow zero order or first order kinetics? |
First order kinetics |
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Name some drugs which follow zero order elimination. |
Ethanol, salicylates |
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Describe the elimination kinetics of phenytoin |
Phenytoin follows so-called non-linear or mixed order pharmacokinetics. The elimination of phenytoin is saturated if the daily dose exceeds 300 mg. This means that phenytoin follows zero order kinetics when given in high doses and first order kinetics when given in lower doses. |
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Why does ethanol follow zero order kinetics? |
Because alcohol dehydrogenase uses NAD+ as a cofactor, and NAD+ is quickly depleted. The elimination of alcohol is therefore limited to the rate at which the liver can regenerate NAD+ from NADH, which occurs at a constant rate |
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What is the half-life of a drug? |
The half-life of a drug is the time it takes for the plasma concentration of the drug to decrease by 50%
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Why is the half-life of a drug important for a clinician? |
- The half-life indicates how long it will take before the drug is completely eliminated from the body - The half-life is related to the duration of action of the drug - The half-life related to the time needed to reach the steady state - The half-life influences the size of fluctuation of the plasma concentration around the steady state concentration |
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After how many half-lives is a drug practially eliminated from the body? |
After 4 - 5 half-lives |
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If you double the dose of drug administered, how much does the duration of action increase by? |
One half-life |
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What is the total body clearance? |
Total body clearance is the sum of the clearance of the kidney, liver and other organs |
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What is the steady state? |
The steady state refers to a state where the concentration of a drug in the plasma is relatively constant |
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What is the dose rate? |
The dose rate is the amount of drug which needs to be administered every unit of time to achieve steady state |
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What does the dose rate depend on?
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The dose rate depends on the total body clearence |
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How long does it take to reach the steady state if a loading dose is not given? |
Without a loading dose the steady state is reached after 4 – 5 half-lives |
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What is a loading dose? |
A loading dose is an initial large dose of drug which is given to immediately reach steady state |
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What does the size of the loading dose depend on?
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The size of the loading dose depends on the volume of distribution, the steady state concentration and the oral bioavailability |
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What is a maintenance dose?
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Maintenance doses are much smaller drug doses which are given after the loading dose to maintain the steady state |