Objective: The commonly used continuous intravenous (i.v.) furosemide dosing schedule after cardiac surgery in children is largely empirical and may not be optimal. This may even be more marked in children after cardiac surgery who are haemodynamically unstable, and in whom transient renal insufficiency may occur. A study was performed to obtain an impression regarding which clinically applicable measures may be used to design a rational scheme for continuous i.v. furosemide therapy in children after cardiac surgery.
Subjects and methods: Twelve paediatric patients (5F/7 M, age 0-33 weeks) post-cardiac surgery, who were to receive 3 days of continuous i.v. furosemide treatment, were included in an open study. Blood and urine samples were taken for furosemide, creatinine, and electrolyte levels, and fractionated urinary output was measured. Furosemide in blood and urine was measured using high performance liquid chromatography (HPLC).
Results: The mean starting dose of continuous i.v. furosemide was 0.093 (+/- 0.016) mg/kg per hour. The mean dose was increased to 0.175 (+/- 0.045) mg/kg per hour per hour on day 2, and changed to 0.150 (+/- 0.052) mg/kg per hour on day 3. Infusion rates were increased from day 1 to day 2 in ten cases, and decreased from day 2 to day 3 in three cases. Serum furosemide levels never exceeded ototoxic levels. The urinary furosemide excretion rate was inversely related to serum creatinine levels.
Conclusions: This study extends the observation of the beneficial effects of continuous i.v. furosemide also to those children who are haemodynamically unstable after cardiac surgery. However, as the effects of furosemide are dependent on renal function, it can be hypothesised that the dosing schedule may be optimised. Contrary to the currently used dosage schedule in which the dose of furosemide is gradually increased over time, it may be more rational to start with a higher dose and adapt this dose (downward) guided by the observed effect (urine output). Because the infusion rate was increased to 0.2 mg/kg per hour in nine out of 12 patients on day 2 and was never increased further, this suggests that a starting rate of 0.2 mg/kg per hour may be optimal.