Alterations in regional kidney oxygenation during expansion of extracellular fluid volume in conscious healthy sheep.

Expansion of extracellular fluid volume with crystalloid solutions is a common medical intervention, but its effects on renal cortical and medullary oxygenation are poorly understood. Therefore, we instrumented sheep under general anesthesia to enable continuous measurement of systemic and renal hemodynamics, global renal oxygen delivery and consumption, and intra-renal tissue perfusion and oxygen tension (pO2 ) in conscious animals (n=7). The effects of three sequential intermittent infusions of 500 mL of compound sodium lactate solution, administered at hourly intervals, were determined. Volume expansion induced transient increases in mean arterial pressure (+7±2%), central venous pressure (+50±19%) and cardiac output (+15±3%). There were sustained increases in renal medullary tissue pO2 (+35±10%) despite increases in global renal oxygen consumption (+66±18%) and renal oxygen extraction (+64±8%). Volume expansion did not significantly alter renal blood flow, renal oxygen delivery or medullary perfusion. The sustained increase in medullary pO2 was paralleled by increased bladder urine pO2 (34 ± 4%). Cortical perfusion and pO2 did not change significantly. Our findings indicate that extracellular fluid volume expansion can increase renal medullary oxygenation, providing a potential mechanistic basis for its use as prophylaxis against iatrogenic acute kidney injury. They also indicate that continuous measurement of bladder urine pO2 could be used to monitor the effects of volume expansion on medullary oxygenation. However, the mechanisms mediating increased medullary oxygenation during volume expansion remain to be determined.