Changes in plasma concentration of adrenaline and noradrenaline in anaesthetized patients during sodium nitroprusside-induced hypotension.

Changes in plasma concentrations of adrenaline and noradrenaline were measured when sodium nitroprusside (SNP) was used for the induction of hypotension in healthy adults undergoing middle ear surgery (group 1) and patients undergoing neurosurgery for subarachnoid haemorrhage (group 2). The mean catecholamine concentrations before the infusion of SNP were four to five times greater in group 2 than in group 1. Group 1 showed significant increases in plasma catecholamines during hypotension induced with SNP, the increase correlating significantly with the degree of hypotension and the rate of infusion of SNP. Group 1 showed an increase in heart rate of about 10%, which could not be related to the catecholamine concentrations. In group 2, changes in plasma catecholamines following infusion of SNP were inconsistent. This may have been because the sympatho--adrenal medullary system was already maximally active and was unable to respond further to induced hypotension.