Procainamide is an antiarrhythmic drug used to control ventricular arrhythmia. Calculate the iv loading dose and the iv infusion rate of procainamide required to maintain a steady state plasma conc of 6 mg/L in 45-kg woman and in a 75-kg man (Procainamide Vd is 2 L/kg, and k is 0.2 hr-1 in both patients).
Loading Dose = Cp X Vd
= 6mg/L X 2 L/kg
Infusion rate = Cp X k X Vd
= 6 mg/L X 0.2 hr-1 X 2 L/kg
In this example the available information suggest that the is proportional to the body weight and the (and the ) are constant. This means that the is also proportional to the body weight.
For the 45-kg woman:
Loading dose=6 mg/L x 90 L= 540mg
K0 = 6 mg/L x 0.2hr-1 x 90 L= 108 mg/hr
For the 75-kg man:
Loading dose=6 mg/L x 150 L=900mg
K0 = 6 mg/L x 0.2hr-1 x 150 L= 180 mg/hr
Patients with larger body weight have larger and require larger loading dose to achieve the desired steady state conc.
In this example patients with larger body weight also have higher (because is constant), and require higher infusion rate to achieve the desired steady state conc.
