Tobramycin-furosemide interaction

A 70-year-old, 75-kg white female with a history of congestive heart failure was admitted with a two-week episode of progressive shortness of breath, increased abdominal distension, and ankle edema. Sputum Gram stains revealed gram-negative bacilli, and the patient was started on a loading dose of tobramycin 150 mg (2 mg/kg) over 30 minutes. The peak tobramycin level was 3.7 micrograms/ml drawn 45 minutes after the infusion was completed. Seven hours after the loading dose infusion, a trough level of 1.5 micrograms/ml was obtained. Based on the Sawchuk-Zaske method, the patient was started on a regimen of tobramycin 180 mg iv q8h. The following day, the patient received furosemide 120 mg iv. The trough and peak levels drawn 30 minutes before and after the fourth dose of tobramycin were 5.3 micrograms/ml and 16.2 micrograms/ml, respectively. The half-life of tobramycin remained relatively constant, while the volume of distribution decreased 40 percent after the administration of furosemide. This case illustrates that moderate doses of furosemide, administered to edematous patients receiving tobramycin, may cause an increase in both peak and trough levels, thus increasing the chances of ototoxicity and nephrotoxicity.