Effects of the rate and composition of fluid replacement on the pharmacokinetics and pharmacodynamics of intravenous furosemide

Effects of differences in the rate and composition of intravenous fluid replacement for urine loss on the pharmacokinetics and pharmacodynamics of furosemide were evaluated using the dog as a model animal. Each of six dogs received 8-hr constant intravenous infusion of 20 mg (15 mg used in one dog) of furosemide with 0% replacement (treatment I), 50% replacement (treatment II), and 100% replacement (treatment III) with lactated Ringer's solution, as well as with 100% replacement with 5% dextrose in water (treatment IV). Most pharmacokinetic parameters, such as plasma clearance, steady-state volume of distribution, mean residence time, and terminal half-life, were essentially the same in all four treatments. Renal clearances and urinary excretion rates of the drug in treatments II–IVwere essentially the same, but about 20% higher than those in treatment I.In spite of the similarities in kinetic properties, diuretic and/or natriuretic effects from furosemide were markedly different among the four treatments. For example, mean 10-hr urine outputs were 646, 1046, 3156, and 1976 ml and mean 10-hr sodium excretions were 87.0, 142, 383, and 97.2 mmole for treatments I–IV,respectively. Except for treatment III,diuresis and/or natriuresis were found to be time-dependent, generally decreasing with time until reaching a low plateau during later hours of infusion. The present findings also showed that (1)no fluid replacement and 100% replacement with 5% dextrose solution both produced the same degree of severe acute tolerance in natriuresis, indicating the insignificance of water compensation in tolerance development; (2)in treatment II,where neutral sodium balance was achieved, the development of acute tolerance in diuresis and natriuresis can mainly be attributed to negative water balance under this special condition; (3)at steady state the hourly diuresis and natriuresis could differ up to about ten times between treatments. Some implications for the kinetic/dynamic relationship or modeling, in the clinical use, and in the bioequivalence evaluation of dosage forms are discussed.