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13 Cards in this Set
- Front
- Back
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The time it takes for the plasma concentration of the drug to fall by one half is...
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Half-life of elimination
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What is the equation for half life?
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t1/2= .693 (Vd)/(CL)
If first order, the half-life of a drug is constant and does NOT depend on the dose. |
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What is the four half-life rule?
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a drug is considered to be essentially eliminated from the body after four half-lives:
50% remains after 1t 1/2 25% remains after 2t 1/2 12.5% remains after 3t 1/2 6.25 % remains after 4t 1/2 |
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If we know the half-life of a drug, what can we predict?
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The time it takes for the drug to be eliminated by multiplying the half-life by 4
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We have mentioned a lot of stuff that is NOT dependent on drug dosage, what is something that IS changed when the dose increases?
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The duration of action is DEPENDENT on the dose. In other words, the action of the drug is felt for a longer period of time.
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It is assumed that a drug is effective when its concentration in the body is greater than a MEC; assuming first order, the duration of a drug effects will be proportional to...
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Proportional to its half-life and to the administered dose
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Continuous intravenous (IV) infusion will cause a _____ _____ to be reached. You can measure this with the equation ____________.
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Steady state
D/T = Css x CL This equation is basically can be restated as the rate of infusion = rate of elimination. In other words, the plasma concentration at the steady state is directly proportional to the infusion rate and inversely proportional to clearance of the drug. Css= (D/T)/Cl The previous equation was rearranged to show that steady state plasma concentration (Css) is infact directly related to dosing rate and inversely related to Clearance of the drug |
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What is the equation for steady state?
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D/T = Css x CL
Css is the plasma concentration at the steady state It is important to note that the time to reach the steady state is NOT dependent on dose; time to steady state is ONLY determined by the elimination half-life of the drug |
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The only thing that determines the time it takes to reach the steady state of a drug is...
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The elimination half-life of the drug
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We assume that if following first order kinetics, a drug will disappear from plasma in a time equal to 4 t1/2, if the dosing interval is shorter than 4 half lives, what happens?
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Then drug accumulation will occur. In fact, the amount absorbed from each dose is added to the amount remaining in the body.
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If repeated administration of a drug continues, the total amount of drug in the body will rise until the rate of administration is equal to ...
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rate of administration is equal to the rate of elimination. This would indicate that the steady state plasma concentration is reached.
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If a drug has a half life of 4 hours, which dosing interval will reach the steady state first: 4 hour dosing intervals or 8 hour dosing intervals?
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They will both reach the steady state at the same time; one will not be faster than the other.
*remember steady state is independent from dose and dosing interval, it is SOLELY dependent on the elimination half-life of the drug. |
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Using bioavailability (F), what is the equation to find the mean plasma concentration at the steady state?
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Css= (FxD)/ (ClxT)
F=bioavailability D=dose T=dosing interval Cl= clearance |
