Pharmacokinetics and Drug Calculations Flashcards

A pharmacist is calculating a loading dose for lidocaine. The target plasma concentration is 2 mg/L, the Vd is 1.1 L/kg, and the patient weighs 70 kg. What is the loading dose?

Answer: 154 mg

Vd (total) = 1.1 L/kg × 70 kg = 77 L. Loading dose = Target Cp × Vd = 2 mg/L × 77 L = 154 mg. This is the standard loading dose calculation. For IV lidocaine, this is typically given as 1–1.5 mg/kg (1 mg/kg × 70 = 70–100 mg, reflecting the fact that Vd decreases during distribution phase for lidocaine). The formula result is 154 mg.

A maintenance infusion is required to maintain a steady-state plasma concentration of 15 mg/L for a drug with a clearance of 8 L/hr. What infusion rate (Ro) is needed?

Answer: 120 mg/hr

At steady state: Infusion rate (Ro) = Target Css × Clearance = 15 mg/L × 8 L/hr = 120 mg/hr. This fundamental equation shows that steady-state concentration is determined by the ratio of infusion rate to clearance. Doubling clearance (e.g., enzyme induction) would require doubling the infusion rate to maintain the same Css.

An IV infusion of 500 mg drug is administered over 2 hours. The drug has a clearance of 10 L/hr and a half-life of 6 hours. What is the Vd?

Answer: 86.6 L

Vd = (CL × t½) / 0.693 = (10 L/hr × 6 hr) / 0.693 = 60 / 0.693 = 86.6 L. This derives from the relationship: t½ = (0.693 × Vd) / CL, rearranged to Vd = (CL × t½) / 0.693.

A pharmacist needs to calculate the AUC (area under the concentration-time curve) for a drug given at 200 mg IV bolus. The clearance is 20 L/hr. What is the AUC?

Answer: 10 mg·hr/L

AUC = Dose / Clearance = 200 mg / 20 L/hr = 10 mg·hr/L. The AUC represents total systemic drug exposure. Clearance is calculated from AUC: CL = Dose/AUC. This relationship is used in bioequivalence studies where AUC is measured and compared between formulations.

A patient receives gentamicin and a peak concentration of 8 mg/L is measured 1 hour after a 30-minute IV infusion. The trough is 1 mg/L at 8 hours post-infusion. What is the approximate half-life?

Answer: 2.3 hours

From peak to trough: 8 mg/L → 1 mg/L over a 7-hour period. This represents 3 half-lives (8→4→2→1 mg/L). Half-life = 7 hours / 3 = approximately 2.33 hours. Normal gentamicin t½ is 2–3 hours in patients with normal renal function.

What does a two-compartment pharmacokinetic model describe that a one-compartment model does not?

Answer: A rapid distribution phase (alpha phase) as drug distributes from blood to tissues, followed by a slower elimination phase (beta phase)

In a two-compartment model, after IV bolus there is an initial rapid decline (alpha/distribution phase) as drug rapidly distributes into peripheral tissues, followed by a slower monoexponential decline (beta/elimination phase) as drug is cleared. This creates a bi-exponential plasma concentration curve. Many drugs (digoxin, aminoglycosides) follow two-compartment kinetics.