Changing renal blood flow following sodium nitroprusside in patients undergoing nephrolithotomy.

During nephrolithotomy in 23 adult patients under sodium thiopental-N2O-O2-fentanyl anesthesia, renal blood flow to one kidney was studied. Muscle relaxation was obtained with pancuronium, and renal blood flow (RBF) was measured with an electromagnetic flow probe. Percutaneous arterial pressures were also recorded. Administration of sodium nitroprusside caused a decrease in RBF in all patients when mean arterial pressure (MAP) was reduced approximately 44 percent below baseline. Patients whose initial RBF was below 300 ml/min showed greater falls in RBF with hypotension. Changes in renal vascular resistance (MAP/RBF) was found to be markedly higher in the "low-flow" group. As expected, the low-flow group started with a higher renal vascular resistance. From these observations, it is felt that more concern should be exercised when planning deliberate hypotension with nitroprusside, especially in patients with decreased renal function.