The Renal Effects of Sodium Nitroprusside in Postoperative Cardiac Surgical Patients

Abstract

Sodium nitroprusside (SNP) is frequently used to control hypertension and/or improve systemic blood flow following cardiac operations. Although SNP causes renal vasodilation when infused into isolated kidneys, the reported effects of SNP on renal vascular resistance and blood flow in intact animals and humans varied. To define the effects of SNP in postoperative cardiac surgical patients, renal clearances and hemodynamics were measured in 7 patients within 24 h of coronary bypass grafting. Studies were delayed until patients were stabilized and had rewarmed following operation. Following baseline measurements (off SNP), SNP infusion was used to lower mean arterial pressure to 85 torr. Pulmonary wedge pressure was maintained by appropriate fluid therapy, and the measurements repeated 1 h later. SNP administration resulted in equivalent decreases in renal (-31%), pulmonic (-29%) and systemic (-33%) vascular resistance. Notwithstanding the decrease in arterial pressure (109 .+-. 14 to 91 .+-. 9 torr, P < 0.01), renal blood flow increased by 20% (653 .+-. 193 to 792 .+-. 210 ml/min per 1.73 m2, P < 0.02) in direct proportion to the increase in cardiac index (2.5 .+-. 0.4 to 3.0 .+-. 0.3 l/min per m2, P < 0.01). In postoperative cardiac surgical patients, SNP administration can be expected to improve renal blood flow, if left atrial hypotension is avoided, and the decline in systemic arterial pressure is not excessive. The improvement in renal blood flow achievable with SNP may be critical for patients with severely depressed left ventricular function in whom severe depression of renal blood flow may occur as an antecedent to acute renal failure.