Sodium Nitroprusside-Induced Hypotension in Dogs

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Abstract
The effects of small circulating blood volume alterations on the SNP [sodium nitroprusside] dosage required to maintain the mean arterial pressure at 70 mm Hg were studied in 6 anesthetized dogs. The conditions were normovolemia, hypovolemia by arterial bleeding 7 ml/kg and hypervolemia by infusing 7 ml/kg whole blood. The animals became extremely SNP-sensitive during hypovolemia and resistant during hypervolemia (SNP dosage A: 25.8, B: 14.7, C: 70.0 .mu.g/kg per min); these changes were accompanied by significant changes in cardiac output (A: 193, B: 142, C: 249 ml/kg per min). The i.v. administration of SNP with the aim of inducing hypotension generally causes a balanced pre- and afterload reduction. This balance is rather delicate and easily disturbed by small intravascular fluid changes leading to cardiac output and SNP sensitivity changes. Slowly developing tachyphylaxis can be explained by such intercompartmental fluid shifts. SNP tachyphylaxis or resistance might be treated by preload reducing measures like phlebotomy or nitroglycerin administration.