Question

5-6. A 74-year-old retired mechanic is admitted with a myocardial infarction and a severe acute cardiac arrhythmia. You decide to give lidocaine to correct the arrhythmia.

5. A continuous intravenous infusion of lidocaine, 1.92 mg/min, is started at 8 AM. The average pharmacokinetic parameters of lidocaine are: Vd 77 L; clearance 640 mL/min; half-life 1.4 h.

What is the expected steady-state plasma concentration?

(A) 40 mg/L

(B) 3.0 mg/L

(C) 0.025 mg/L

(D) 7.2 mg/L

(E) 3.46 mg/L

6. Your patient has been receiving lidocaine for 8 h, and the arrhythmia is suppressed. However, there are some signs of toxicity. You decide to obtain a plasma concentration measurement. When the results come back, the plasma level is exactly twice what you expected. How should the infusion rate be modified?

(A) Changed to 0.48 mg/min

(B) Changed to 0.96 mg/min

(C) Halted for 1.4 h and then restarted at 0.96 mg/min

(D) Halted for 1.4 h and then restarted at 1.92 mg/min

(E) No change but the plasma level should be measured again

Answer 1

5. (B) 3.0 mg/L

This is the expected steady-state plasma concentration. The drug is being administered continuously.

6. (C) Halted for 1.4 h and then restarted at 0.96 mg/min

This is the correct answer because if the half-life is 1.4 h, the plasma concentration should approach steady-state after 8 h (more than 4 half-lives). The steady-state concentration is a function of dosage and clearance, not Vd. If the plasma level is twice that predicted, the clearance in this patient must be half the average value. To reduce the risk of toxicity, the infusion should be halted until the concentration diminishes (1 half-life) and then restarted at half of the previous rate.