Mr. Peter Green, a 55-year-old male with a history of heart failure (ejection fraction 30%), is initiated on digoxin 0.125 mg daily. He is also taking furosemide 40 mg daily, ramipril 5 mg daily, and carvedilol 12.5 mg twice daily. One week later, he develops a urinary tract infection and is prescribed trimethoprim/sulfamethoxazole (TMP/SMX) DS (160 mg/800 mg) twice daily. Which of the following is the most important monitoring parameter for Mr. Green?

Question: Mr. Peter Green, a 55-year-old male with a history of heart failure (ejection fraction 30%), is initiated on digoxin 0.125 mg daily. He is also taking furosemide 40 mg daily, ramipril 5 mg daily, and carvedilol 12.5 mg twice daily. One week later, he develops a urinary tract infection and is prescribed trimethoprim/sulfamethoxazole (TMP/SMX) DS (160 mg/800 mg) twice daily. Which of the following is the most important monitoring parameter for Mr. Green?

Answer options:

• Serum potassium levels. ✅ Symptoms of digoxin toxicity.
• Blood pressure readings.
• Blood glucose levels.

Correct answer: Symptoms of digoxin toxicity.

Explanation: TMP/SMX can inhibit P-glycoprotein (P-gp) and renal tubular secretion of digoxin, leading to increased serum digoxin concentrations. This can predispose patients to digoxin toxicity. Therefore, monitoring for signs and symptoms of digoxin toxicity (e.g., nausea, vomiting, visual disturbances, arrhythmias) is crucial.