Equations (1)

Question 1

What is the equation for Volume of Distribution?

Answer 1

where C = concentration of drug in blood/plasma

Amount of drug in body = Dose administered

Question 2

What is the equation for Clearance? What is the unit most commonly used for Clearance?

Answer 2

where C = concentration of drug in blood/plasma

Units for Clearance: mL/min

Clearance is the amount of plasma cleared of the drug per unit time.

Question 3

What is the equation for Rate of Elimination?

Answer 3

where CL = Clearance and C = concentration of drug in blood/plasma

Question 4

What is the equation for Half-life?

Answer 4

where V = volume of distibution and CL = clearance

Question 5

What is the equation for Hepatic Extraction Ratio?

Answer 5

where ER = extraction ratio, Q = hepatic blood flow

Question 6

Keeping in mind the equation for the rate of Elimination, what is the equation for the Maintenance Dose in an IV infusion?

Answer 6

where ss = steady state, CL = clearance, TC = target concentration

Question 7

If you switched from an IV infusion to an oral dosing regimen, how would you alter the “dosing rate” that you would use in your equations?

Answer 7

where Dosing Rate = the rate used for IV infusions, F = bioavailability

Question 8

If a patient is on an IV infusion, and you halved the target concentration, how would this affect the infusion rate?

Answer 8

This would simply halve the infusion rate. Re-arranging the equation, it is:

Target Concentration = “Rate in” / Clearance

Question 9

Patients A and B are both receiving an IV infusion of lidocaine, and both require the same target concentration at steady state. However, Patient B has renal disease, and so the clearance is half that of Patient A. How should these two patients’ infusion rates differ?

Answer 9

Remember the equation:

Target Concentration = “Rate in” / Clearance

So Patient B’s infusion rate should be half that of Patient A.