How is urine output used to assess renal perfusion during anesthesia?

• A. Urine output is not relevant
• B. Oliguria is normal during anesthesia
• C. Oliguria may indicate hypovolemia or reduced renal perfusion; aim for urine output >0.5 mL/kg/hr in adults
• D. Only measure urine color

Answer: (C)

Urine output during anesthesia serves as a practical readout of renal perfusion and intravascular volume. The kidneys rely on adequate renal blood flow, which depends on mean arterial pressure and cardiac output; anesthesia and the surgical state can alter these factors, so observing urine production helps gauge whether the kidneys are being perfused well. When urine output drops, it can signal hypovolemia or reduced renal perfusion, prompting a quick assessment of volume status and perfusion pressures. The commonly used adult target is urine output greater than 0.5 mL/kg/hour, which for a typical adult supports adequate renal perfusion (for example, about 35 mL/hour for a 70 kg patient). If oliguria persists despite appropriate fluid management and stable hemodynamics, it raises concern for ongoing renal hypoperfusion or injury and guides further optimization, such as adjusting fluids, vasoactive support, and ensuring avoidance of nephrotoxins. Urine color is not a reliable surrogate for renal perfusion, so relying on color alone can be misleading.