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Zorluk: OrtaPharmacokinetic and Pharmacodynamic Principles

A 55-year-old female patient is hospitalized for the management of severe pneumonia complicated by sepsis. Her current body weight is 80 kg80 \text{ kg}. The medical team decides to initiate therapy with an intravenous antimicrobial agent. This medication has a volume of distribution (VdV_d) of 0.5 L/kg0.5 \text{ L/kg} and follows first-order elimination kinetics with a half-life (t1/2t_{1/2}) of 6.93 hours6.93 \text{ hours}. The drug is prescribed to be administered intermittently as an intravenous infusion every 12 hours12 \text{ hours}. To achieve optimal bactericidal activity, the target average steady-state plasma concentration (Css,avgC_{ss,avg}) is 10 mg/L10 \text{ mg/L}. Assuming 100%100\% bioavailability for the intravenous route, what maintenance dose should be administered every 12 hours12 \text{ hours}?

Cevap: 480 mg

Cevap

480
The maintenance dose (MDMD) is the amount of drug required to replace what is eliminated over a dosing interval (τ\tau) to maintain a target average steady-state concentration (Css,avgC_{ss,avg}). The formula is MD=Css,avg×CL×τFMD = \frac{C_{ss,avg} \times CL \times \tau}{F}. First, clearance (CLCL) must be determined. Clearance is the product of the elimination rate constant (kek_e) and the volume of distribution (VdV_d). The patient's total VdV_d is 0.5 L/kg×80 kg=40 L0.5 \text{ L/kg} \times 80 \text{ kg} = 40 \text{ L}. The elimination rate constant is ke=0.693t1/2=0.6936.93 hours=0.1 hr−1k_e = \frac{0.693}{t_{1/2}} = \frac{0.693}{6.93 \text{ hours}} = 0.1 \text{ hr}^{-1}. Therefore, CL=0.1 hr−1×40 L=4 L/hrCL = 0.1 \text{ hr}^{-1} \times 40 \text{ L} = 4 \text{ L/hr}. For an intravenous drug (F=1F = 1), the maintenance dose administered every 12 hours12 \text{ hours} is MD=10 mg/L×4 L/hr×12 hours=480 mgMD = 10 \text{ mg/L} \times 4 \text{ L/hr} \times 12 \text{ hours} = 480 \text{ mg}.

Adım Adım Çözüm

1
Calculate the patient's total volume of distribution (VdV_d).
Vd=0.5 L/kg×80 kg=40 LV_d = 0.5 \text{ L/kg} \times 80 \text{ kg} = 40 \text{ L}
Total volume of distribution is required to calculate the total body clearance of the drug.
2
Calculate the elimination rate constant (kek_e) and total body clearance (CLCL).
ke=0.6936.93 hr=0.1 hr−1k_e = \frac{0.693}{6.93 \text{ hr}} = 0.1 \text{ hr}^{-1}. Clearance (CLCL) = ke×Vd=0.1 hr−1×40 L=4 L/hrk_e \times V_d = 0.1 \text{ hr}^{-1} \times 40 \text{ L} = 4 \text{ L/hr}.
Clearance determines the rate at which the drug must be replaced to maintain steady-state concentrations.
3
Calculate the maintenance dose (MDMD) for the specified dosing interval.
MD=Css,avg×CL×τ=10 mg/L×4 L/hr×12 hr=480 mgMD = C_{ss,avg} \times CL \times \tau = 10 \text{ mg/L} \times 4 \text{ L/hr} \times 12 \text{ hr} = 480 \text{ mg}
The maintenance dose replaces the amount of drug cleared over one dosing interval (τ\tau) to maintain the target steady-state concentration.

Anahtar Kavram

Calculation of maintenance dose requires determining clearance from half-life and volume of distribution, then multiplying by the target steady-state concentration and dosing interval.
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