Digoxin Pharmacokinetics and Pharmacodynamics

Questions provided by Nicholas C. Schwier, Pharm.D.

Question 1. XO is a 70 year-old male patient with a past medical history of CHF and Hypertension, who presents to the cardiac intensive care unit with worsening cough and a fever. He is currently taking metoprolol succinate 50 mg once daily, lisinopril 40 mg once daily, furosemide 40 mg twice daily and digoxin 0.125 mg once daily. Sputum cultures come back and the team decides to start the patient on clarithromycin 500 mg PO twice daily. Four days later, the patient is experiencing visual disturbances and is vomiting. He is complaining of nausea and feels so weak that he cannot get out of bed to use the bathroom. The patient's pertinent vitals, demographics, and laboratory values are listed below:

Height: 74 inches BUN: 18 mg/dL Mg²⁺: 3.9 mEq/L

Weight: 218 lb Na: 138 mEq/L Heart rate: 50 bpm

Css digoxin level
at steady state: 3.04 ng/mL Cl: 99 mEq/L

SerumCr: 1.2 mg/dL K: 5.6 mEq/L

Your attending would like to use DigiFAB^® and asks for you recommendation on how much of the drug to give. Based on your calculations, you should recommend administering how much of DigiFAB^®?

Question 2. It has been two days and the patient has fully recovered from digoxin toxicity and has been off the clarithromycin. You draw a level 10 hours after the next dose was supposed to given and it comes back at 2.4 ng/mL. Your attending would like to know how long it will take for the digoxin concentration to get to 1.59 ng/mL. [Assume all the DigiFAB^® has been eliminated) and ke (days^-1) = 0.158 + 0.00276 (CLcr)]

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Some Equations:

Equation 27.2.1 IBW for Male Patients

Equation 27.2.2 IBW for Female Patients

Creatine Clearance C-G Equation
Equation 27.2.3 Creatinine Clearance (Cockcroft-Gault Equation)

Steady State Concentration Equation
Equation 27.2.4 Steady State Concentration

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Height: 74 inches	BUN: 18 mg/dL	Mg²⁺: 3.9 mEq/L
Weight: 218 lb	Na: 138 mEq/L	Heart rate: 50 bpm
Css digoxin level at steady state: 3.04 ng/mL	Cl: 99 mEq/L
SerumCr: 1.2 mg/dL	K: 5.6 mEq/L