Background and objective: The aim of the present study was to determine the dosage of remifentanil that must be added to thiopental 5 mg kg(-1) to provide excellent intubating conditions in 95% of patients.
Methods: Sixty-eight premedicated female American Society of Anesthesiologists class 1 and 2 patients were randomly allocated to four remifentanil dose groups (3, 4, 5 or 6 microg kg(-1)). Induction of anaesthesia was performed with a blinded dose of remifentanil infused over 60 s, simultaneously coadministered with thiopental 5 mg kg(-1) infused over 20 s; and tracheal intubation was attempted 150 s after the beginning of induction. The intubating conditions were assessed with the Copenhagen score, according to jaw relaxation, resistance to laryngoscope, vocal cord position and movements and reaction to cuff inflation. A probit analysis was performed to calculate the intubating efficient doses (IEDs) of remifentanil in 50 and 95% of patients (IED50 and IED95). Haemodynamic tolerance was also analysed.
Results: The IED95 of remifentanil was 7.8 microg kg(-1) (95% confidence interval 5.9-10.9). Although the maximum decrease in heart rate and mean arterial pressure was less than 30% during induction of anaesthesia, the haemodynamic tolerance of such a relaxant-free protocol for anaesthesia providing excellent intubating conditions in 95% of patients when using remifentanil and thiopental could not be assessed, as the IED95 of remifentanil was more than 6 microg kg(-1).
Conclusion: The dosage of remifentanil providing excellent intubating conditions in 95% of patients when coadministered with thiopental 5 mg kg(-1) in healthy premedicated female patients undergoing elective surgery was more than 7 microg kg(-1). Our results question the feasibility of using thiopental for a relaxant-free protocol for anaesthesia.