A 45-year-old man weighing 80 \text{ kg} is admitted to the intensive care unit with sever… — USMLE Step 3

A 45-year-old man weighing $80 \text{ kg}$ is admitted to the intensive care unit with severe sepsis. He requires immediate intravenous administration of an antibiotic. The target peak plasma concentration ( $C_p$ ) for this drug is $12 \text{ mg/L}$ . Pharmacokinetic data indicates that the volume of distribution ( $V_d$ ) for this antibiotic is $0.5 \text{ L/kg}$ . What is the appropriate intravenous loading dose (in mg) required to achieve the desired target plasma concentration for this patient?

Answer: 480 mg

Answer

480

The appropriate loading dose is calculated by the formula:

LD = (C_p \times V_d) / F

. First, calculate the total volume of distribution:

0.5 \text{ L/kg} \times 80 \text{ kg} = 40 \text{ L}

. Since the drug is given intravenously, bioavailability (

F

) is 1. Multiplying the target concentration of

12 \text{ mg/L}

by the total

V_d

40 \text{ L}

yields

480 \text{ mg}

Step-by-Step Solution

Calculate the total volume of distribution (

V_d

) for the patient.

V_d = 0.5 \text{ L/kg} \times 80 \text{ kg} = 40 \text{ L}

The provided volume of distribution is per kilogram of body weight, so it must be scaled to the patient's total weight.

Calculate the loading dose using the standard pharmacokinetic formula.

LD = 12 \text{ mg/L} \times 40 \text{ L} = 480 \text{ mg}

The loading dose formula is Target Concentration

\times

Volume of Distribution. Bioavailability is 100% for intravenous administration.

Key Concept

Calculation of an intravenous loading dose using target concentration and volume of distribution.

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