Sodium nitroprusside as a hypotensive agent in intracranial aneurysm surgery

Sodium nitroprusside (SNP) was used to induce hypotension during intracranial aneurysm surgery in 67 patients. The effects of SNP infusion (0.1 mg/ml) on blood pressure were rapid and it was easy to adjust blood pressure to desired levels in most patients. When SNP was stopped, the blood pressure returned instantly to the initial level. In eight patients an increase to about 25% or more above prehypotensive level was seen, counteracted in two patients by administration of small doses of halothane. There was a mean increase of 36% in heart rate. Total doses of SNP were 0.05--120 mg (mean: 10.8), corresponding to 0.08--6.8 micrograms/kg/min (mean: 1.9). No metabolic acidosis indicating cyanide intoxication was observed. Tachyphylaxis was seen in three patients, and SNP had to be discontinued in one. It is concluded that SNP gives a rapid and effective hypotension but tachyphylaxis and subsequent danger of cyanide intoxication exist. Therefore, in some cases SNP has to be replaced by or combined with some other hypotensive agent to achieve the desired effect. As there is a risk of impairment of cerebral autoregulation after the use of SNP, it is important to avoid sudden and prolonged blood pressure fluctuations, and to continue with controlled hyperventilation in the postoperative period to reduce the risk of brain oedema and high intracranial pressure.