To evaluate whether furosemide administration before cardiopulmonary bypass (CPB) is associated with altered intraoperative urinary oxygen tension (PuO₂) and early postoperative renal outcomes in patients undergoing elective on-pump coronary artery bypass grafting (CABG).

Prospective, single-center, observational pilot study.

Tertiary cardiac surgery center.

A total of 122 adult patients undergoing elective isolated on-pump CABG with preserved baseline renal function.

Patients were categorized according to routine perioperative practice into a furosemide group (n = 66), receiving 20 mg intravenous furosemide 10-15 minutes before CPB, and a no-furosemide group (n = 56), receiving no loop diuretic before CPB.

PuO₂ was measured from anaerobically collected urine samples using a blood gas analyzer at four predefined time points: after induction of anesthesia, before CPB initiation, 20 minutes after CPB initiation, and before transfer to the intensive care unit. Postoperative renal outcomes assessed during the index hospitalization included serum creatinine, urine output, and acute kidney injury according to KDIGO criteria. Baseline PuO₂ values were comparable between groups. After CPB initiation, PuO₂ was significantly lower in the furosemide group at 20 minutes after CPB initiation (28.7 ± 6.7 v 35.4 ± 6.3 mmHg; p < .001) and before intensive care unit transfer (31.0 ± 7.1 v 39.1 ± 6.8 mmHg; p < .001). Furosemide administration remained independently associated with lower PuO₂ at 20 minutes after CPB initiation after adjustment for age, estimated glomerular filtration rate, mean arterial pressure, and oxygen delivery during CPB. Postoperative creatinine, urine output, acute kidney injury incidence, and in-hospital mortality did not differ significantly between groups.

Pre-CPB furosemide administration was associated with a greater intraoperative reduction in PuO₂ during elective on-pump CABG, without significant differences in early postoperative renal outcomes. These findings suggest altered urine oxygen tension dynamics and should be considered hypothesis-generating.