Tavalin Equations Flashcards
Dosing Rate Equation
Point of the equation
Intravenous Infusion Equation
Dosing Rate = CL x Css
where CL = Clearance Css = steady state concentration of the drug
The clinician wants to maintain a steady-state concentration of a drug within the therapeutic window, so he’ll administer the drug at the same rate that it is eliminated.
Css = infusion rate
CL<sub>total</sub>
Clearance Equation
CL = rate of elimination
concentration
You’ll want to add the clearance rates of different organs together in order to get the total clearance from the body
Do most drugs obey first order or zero order kinetics?
What does your choice for answer one look like on a graph where both the X (time) and Y (concentration of drug) axes are linear?
What about on a graph where Y axis is log concentration of drug and X axis is linear time?
Most drugs obey first order kinetics
A first order reaction is an exponential decay function on a linear graph
A straight line is obtained when the y-axis is log and the x-axis is linear
What does the slope of a first order reaction plotted on a log/linear graph represent?
It is the elimination rate of the drug. Because the elimination rate of the drug is equal to the slope of a linear curve, we can infer that a **CONSTANT FRACTION of the drug is eliminated per unit of time. **This also means that there is a half-life associated with first order reactions.
Describe elimination of a drug that follows first order kinetics
BUZZ WORD: **Fraction. **A CONSTANT FRACTION of the drug is eliminated per unit of time, so the absolute amount of drug removed will be concentration-dependent, and there will be a half-life associated with the elimination. There will be a plateau.
Half life in a first order reaction
The time it takes for the plasma concentration or the amount of drug in the body to be reduced by 50%. So on a graph, if your total administered concentration was 8 CCs (or whatever), find the time at which the concentration becomes 4 CCs in the blood, and that will be your half life.
Volume of distribution: sometimes it describes a primary fluid compartment, and other times it has no relationship to how the drug is distributed in the body. If there were a correlation, though, list the main compartments and their volume
Plasma Volume: 4 Liters
Extracellular Fluid Volume: 12 Liters
Total Body Water: 40 Liters
Half Life Equation
T1/2 = (0.7)(Vd)
CL<sub>total</sub>
T1/2 = (0.7)
Ke
Dosing rate equation that accounts for an oral medication’s bioavailability
Explain the subtle change in the equation when a doctor is giving maintenance doses after administering a loading dose
F x Dosing Rate = CL x Css
Where:
F = bioavailability
CL = Clearance
Css = Steady State
Follow the same equation, except the question stem might say something like “target steady state” instead of just steady state.
Equation that determines steady state concentrations after various dosing schedules AND accounts for bioavailability
Css = (F x Dose)
CL x T
Where:
T = the dosing interval
F = bioavailability
Also notice that this equation is just a slight rearrangement from the equation that accounts for bioavailability, but not for a dosing schedule
What is the bioavailability of a drug administered by IV?
100%
How many half lives does it take to reach a steady state?
5 half lives
The time to steady state is strictly dependent on half life. It does not matter how much medicine the doctor gives or how frequently he gives it.
Loading Dose Equation and the reason for giving a loading dose
LD = (Css x Vd)
F
Loading doses are given when the doctor does not have time to wait for 5 half lives to achieve a therapeutic range (e.g. heart attacks). Loading doses can be dangerous due to the high concentrations that are achieved. The same equation is used regardless of whether you have a zero order or first order kinetic drug.
Zero Order Kinetics
Zero order kinetics work like a Michaelis-Menten equation in that you’ll have saturation of the enzyme metabolizing the drug, as well as Km, which is the dose that produces 50% of the maximal elimination rate. Vm is the maximum rate of the process. Example drugs are ethanol, aspirin, and heparin.
BUZZ WORDS: Amount. The same AMOUNT of drug is metabolized regardless of the level of the drug. In addition, NO PLATEAU IS OBSERVED.
Volume of distribution equation
Vd = amt admin
plasma conc