Prospective randomised comparison of Marsh and Schnider pharmacokinetic models for propofol during induction of anaesthesia in elective cardiac surgery

CONTEXT: Haemodynamic stability during induction is a cornerstone of cardiac anaesthesia. The evaluation of pharmacokinetic models for propofol during induction is lacking. OBJECTIVE: To compare haemodynamics during cardiac anaesthesia induction with two pharmacokinetic models. DESIGN: Randomised controlled trial. SETTING: Department of Cardiothoracic Surgery, São João Hospital; July to December 2010. PATIENTS: Ninety consecutive elective adult cardiac surgical patients. INTERVENTION: Random assignment to effect-site target-controlled infusion by Marsh (n = 45) or Schnider (n = 45) pharmacokinetic models with an equilibration constant of 1.2 min(-1) adapted to Marsh's model. Invasive blood pressure measurements, propofol dose, and bispectral index (BIS) were recorded. After an initial target concentration of 1.5 μg ml(-1), concentrations were upward-titrated in 0.5 μg ml(-1) increments until the BIS was <50. RESULTS: No differences were observed between Marsh and Schnider models in required propofol dose (0.99 ± 0.26 vs. 0.93 ± 0.31 mg kg(-1), P = 0.322), decrease in mean blood pressure (25 ± 13 vs. 22 ± 14%, P = 0.192) or the need for vasopressors (20 vs. 24%, P = 0.800), but the use of the Marsh model resulted in a lower predicted effect-site concentration (2.3 ± 0.4 vs. 2.7 ± 0.6 μg ml(-1), P = 0.006) and shorter time to induction (296 ± 59 vs. 338 ± 87 s, P = 0.024). There was a greater decrease in mean blood pressure in older patients (>60 years; 29 ± 10 vs. 22 ± 11%, P = 0.004) irrespective of model, but obese (BMI ≥30 kg m(-2)) and nonobese patients did not differ. MAIN OUTCOME MEASURES: Decrease in mean blood pressure. CONCLUSION: In effect-site targeting with a 1.2 min(-1) equilibration constant, Marsh's model is comparable to Schnider's during induction of anaesthesia.