Additive effects of dihydralazine during enflurane or isoflurane hypotensive anaesthesia for spinal fusion

Sixteen patients (13-38 yr) undergoing spinal fusion for scoliosis under controlled hypotension were studied to determine the haemodynamic and neuroendocrine responses to IV dihydralazine (1.0 mg.kg-1) followed by 0.5 and 1 MAC of enflurane or isoflurane. Twenty minutes after dihydralazine administration mean arterial pressure (-20 per cent) and systemic vascular resistance (-50 per cent) decreased, and cardiac index (+57 per cent), heart rate (+37 per cent) and intrapulmonary shunt increased. Plasma Renin activity and aldosterone and norepinephrine levels increased. Further decreases in mean arterial pressure and in systemic vascular resistance were observed when 0.5 MAC enflurane or isoflurane were added. With 1 MAC anaesthetic levels a further decrease in mean arterial pressure was observed in both groups, but pressure fell to a lower level with isoflurane than with enflurane (p less than 0.01). The reduction of arterial blood pressure to a level of 50-60 mmHg for three to four hours was easy to control and was free of complications. The preliminary IV administration of dihydralazine allowed a reduced volatile agent concentration which attenuated undesirable haemodynamic effects, in spite of Renin and norepinephrine release, and permitted a rapid intraoperative awakening.