Question

Pharmacology clinical chemistry laboratory case studies

A patient is receiving procainamide for treatment of cardiac arrhythmia. An intravenous loading dose resulted in a peak serum concentration of 6 mu g/mL. Since the half life of procainamide is 4 hours an equivalent dose was administered n intravenous bolus 4 hours after the initial dose. This resulted in a serum peak concentration of procainamide of 7.5 mu g/mL. Based on the half-life, does the serum concentration after the second dose seem appropriate? If not, calculate what the predicted serum concentration should be at this time? Explain your reasoning. What could be an explanation for these results? Should the lab be assaying for another drug in addition to procainamide? If so what drug? If the patient had been given one dose of procainamide resulting in a peak concentration of 10 pg/mL, what would the blood level have been 12 hours later, if no further drug was administered (half-life is 4 hours)?

Answer 1

1). The elimination half-life or t1/2 is the time required to reduce the plasma concentration of the drug by 50%.

The t1/2 of the drug is used to determine the plasma drug concentration at different time intervals, which indicates the duration of drug action.

A drug molecule is having 4 h t1/2 means, the plasma concentration of the drug decreases to 50% by 4 hours. After another 4 h, the plasma concentration decreases by another 50%, i.e. the drug concentration becomes 25%.

So, the expected concentration of procainamide after 4 hours is, 3 ug/ml. An equivalent dose, i.e. another 3 ug is administered at 4 hours interval. Thus, the expected plasma concentration of procainamide is, 6 ug/ml.