Effect of intravenous furosemide on serum theophylline concentration.

The effect of furosemide on the serum theophylline concentration (STC) was investigated in 10 patients at steady state on a continuous i.v. aminophylline infusion. A baseline STC was obtained 48 hours after the initiation of the maintenance infusion. The patients were monitored closely for the last 24 hours of the maintenance period with hourly flow records to assure the same continuous rate. Urine output was measured over four hours, then, a 40-mg dose of furosemide was administered i.v. over two minutes, and the aminophylline infusion was continued at the same rate. Urine output was measured for four hours after the furosemide dose, and then a second STC was obtained. The 40-mg furosemide i.v. bolus injection caused an average increase of 2.9 micrograms/ml in STC, with a range of 0.5--5.5 micrograms/ml. The urine volumes increased by an average of 995 ml above the baseline, with a range of 630--1377 ml. Both of the increases were significant at the p less than 0.005 level. Patients stabilized on a theophylline dosage regimen should be monitored closely during the addition or discontinuation of furosemide therapy. Furosemide has been shown to be another factor that can alter STC.U