Urinary Oxygenation as a Surrogate Measure of Medullary Oxygenation during Angiotensin II Therapy in Septic Acute Kidney Injury

Objectives: Angiotensin II is an emerging therapy for septic acute kidney injury, but it is unknown if its vasoconstrictor action induces renal hypoxia. We therefore examined the effects of angiotensin II on intrarenal Po₂ in ovine sepsis. We also assessed the validity of urinary Po₂ as a surrogate measure of medullary Po₂. Design: Interventional study. Setting: Research Institute. Subjects: Sixteen adult Merino ewes (n = 8/group). Interventions: Sheep were instrumented with fiber-optic probes in the renal cortex, medulla, and within a bladder catheter to measure Po₂. Conscious sheep were infused with Escherichia coli for 32 hours. At 24-30 hours, angiotensin II (0.5-33.0 ng/kg/min) or saline vehicle was infused. Measurements and Main Results: Septic acute kidney injury was characterized by hypotension and a 60% ± 6% decrease in creatinine clearance. During sepsis, medullary Po₂ decreased from 36 ± 1 to 30 ± 3 mm Hg after 1 hour and to 20 ± 2 mm Hg after 24 hours; at these times, urinary Po₂ was 42 ± 2, 34 ± 2, and 23 ± 2 mm Hg. Increases in urinary neutrophil gelatinase-associated lipocalin (12% ± 3%) and serum creatinine (60% ± 23%) were only detected at 8 and 24 hours, respectively. IV infusion of angiotensin II, at 24 hours of sepsis, restored arterial pressure and improved creatinine clearance, while not exacerbating medullary or urinary hypoxia. Conclusions: In septic acute kidney injury, renal medullary and urinary hypoxia developed several hours before increases in currently used biomarkers. Angiotensin II transiently improved renal function without worsening medullary hypoxia. In septic acute kidney injury, angiotensin II appears to be a safe, effective therapy, and urinary Po₂ may be used to detect medullary hypoxia.