This study was designed to investigate the influence of anaesthesia induced and maintained with propofol on the haemodynamic effects and the dose requirements of SNP during the course of induced hypotension.

Twenty-four adult ASA physical status I patients undergoing middle ear surgery were randomly assigned to receive anaesthesia with either morphine, thiopentone, d-tubocurarine, halothane 0.6% end-tidal and N₂O 70% in oxygen (group I n=12), or morphine, propofol, d-tubocurarine, propofol infusion 108 μg. kg⁻¹. min⁻¹ and N₂O in oxygen (group 2 n = 12). Mean arterial blood pressure (MAP) was reduced to 60-65 mmHg in all patients using a continuous infusion of sodium nitroprusside (SNP) 0.01%. Propofol produced a significant (17%) reduction in the MAP before institution of SNP infusion. This was related to a 24% reduction in the systemic vascular resistance index (SVRI). In the halothane group SVRI was significantly reduced during SNP infusion. Halothane anaesthesia was associated with significant reflex tachycardia in response to SNP induced hypotension. Eight patients in the halothane group (66%) required propranolol 0.5-3 mg to control tachycardia. Propofol anaesthesia attenuated significantly the reflex tachycardia in response to SNP induced hypotension. Two patients in the propofol group (16%) required 0.5 mg propranolol to control reflex tachycardia. The mean SNP dose requirements were 7.25 ± 1.6 and 2.1 ± 1.4 μg. kg⁻¹.min⁻¹ in the halothane and propofol groups, respectively (P < 0.0001). None of the patients in the two groups developed rebound hypertension following SNP withdrawal. It is concluded that propofol anaesthesia reduces significantly the dose requirements for SNP through attenuation of the homeostatic reflex increase in the heart rate and systemic vascular resistance.