This study demonstrated that EC50 ± SD of remifentanil for OT and NT intubation was 5.58 ± 0.75 ng/ml and 6.08 ± 0.75 ng/ml, respectively, in adults during propofol TCI at an effect-site concentration of 5.0 μg/ml without neuromuscular blocking agent, and no statistical difference was observed between the groups.
Acceptable NT intubation without neuromuscular blockade during propofol anaesthesia has been reported, although no previous report has evaluated NT intubation conditions using propofol and remifentanil TCI. Andel et al (Andel,2000). demonstrated the minimal propofol dose in combination with fentanyl required for NT intubation. They reported that intubation of all patients without neuromuscular blockade is possible, and that median value of the required propofol dose in combination with 3 μg/kg of fentanyl was 2.74 mg/kg.
Another previous study in adults has demonstrated acceptable NT intubation in 83% of patients receiving propofol 2.5 mg/kg and alfentanil 20 μg/kg (Coghlan,1993). In our study using isotonic regression estimated from the PAVA response rate, EC50 and EC95 of remifentanil for NT intubation during propofol TCI was 5.75 ng/ml and 7.42 ng/ml, respectively.
Because the tracheal tube has to be inserted through the nasal passage, NT intubation is a more time-consuming and more invasive airway procedure than OT intubation. As a result, NT intubation can cause greater haemodynamic response than OT intubation.
Findings from a previous study have demonstrated that the hypertensive response after NT intubation is significantly greater than that after OT intubation under comparable conditions (Smith, 1991; Fassoulaki, 1990). In our study, we assumed that NT intubation may be more stimulating, and that the dose of remifentanil required for NT intubation without neuromuscular blockade may be increased. However, our results have shown that EC50 of remifentanil for OT and NT intubation was 5.58 ± 0.75 ng/ml and 6.08 ± 0.75 ng/ml, respectively, during propofol TCI, and that there was no strong evidence that the target
A possible explanation for our results may be that tracheal intubation was the most potent stimulus of the airway during nasotracheal intubation (Singh, 2003), which typically comprises three distinct stages: (i) nasopharyngeal intubation; (ii) direct laryngoscopy for identification of the vocal cords; and (iii) passage of the tracheal tube into the trachea (tracheal intubation). Singh and Smith (Singh, 2003) suggested that the third stage produced a marked pressor response that was significantly greater than that of the first two stages. This implies that NT intubation resulted in a comparable magnitude of airway manipulation to the OT intubation, and thus required a similar depth of anaesthesia. And, this finding concurs with evidence that tracheal stimulus, not stimulus to the oropharyngeal structure, is the primary cause of the stress response to OT intubation (Takahashi,2002;
Hirabayashi, 1998). Takahashi et al.(Takahashi, 2002) reported that the magnitude of haemodynamic changes associated with tracheal intubation using the lightwand device (Trachlight) is almost the same as that which occurs with the direct laryngoscope. And, they suggested that haemodynamic changes are likely to occur as a result of direct tracheal irritation rather than direct stimulation of the larynx. Our haemodynamic results are consistent with these previous studies showing that cardiovascular responses to tracheal intubation with lightwand did not differ from those with Macintosh laryngoscope (Takahashi, 2002; Hirabayashi, 1998).
Propofol TCI and adjuvant remifentanil have been shown to provide acceptable intubation conditions for OT intubation in adults without the use of a neuromuscular blocking agent (Troy, 2002; Ithnin, 2009). Troy et al.(Troy, 2002) suggested that an effect-site concentration of remifentanil 8 ng ml-1 along with an effect-site concentration of propofol 3 μg/ml could provide acceptable conditions for OT intubation in 95% of adult patients. In addition, another previous study reported that EC50 and EC95 of remifentanil
min after propofol TCI. Johansen et al. (Johansen, 2000) suggested that anaesthetic maintenance at BIS values between 50 and 65 was associated with shortened emergence and recovery from general anaesthesia.
We utilized isotonic regression to estimate EC50 and EC95 values. However, when small samples, such as the present one, are evaluated, accuracy of the parameter estimates has been questioned. Despite this criticism, isotonic regression has been suggested as a more favorable backup analysis for obtaining EC50 with a smaller bias and tighter CIs compared to standard probit or logit regression, which is likely to produce biased estimators (Pace, 2007). And, the EC50 calculated with the isotonic regression analysis from the up-and-down data in each group was similar to EC50 values obtained by crossover points in our study.
In conclusion, the required effect-site concentration of remifentanil at which acceptable NT intubation is possible in 50% of adults was 6.08 ng/ml during a TCI of 5.0 μg/ml propofol without neuromuscular blockade, using the modified Dixon’s up-and-down method.
Concentration of remifentanil for acceptable intubating conditions might not differ according to the route of intubation.
Table 1
Assessment of intubation conditions
Intubating conditions
Acceptable unacceptable
Variables Excellent Good Poor
Ease of laryngoscopy
(jaw relaxation) Easy Fair Difficult
Vocal cord position Abducted Intermediate Closed
Vocal cord movement None Moving Closing
Airway
reaction(coughing) None Diaphragm Sustained
Movement of the limbs None Slight Vigorous
Excellent: all criteria are excellent. Good: all criteria are either excellent or good. Poor:
presence of a single criterion listed under ‘Poor’
Table 2
OT, patients with orotracheal intubation; NT, patients with nasotracheal intubation; LOC, loss of consciousness.
Table 3
Intubation conditions and causes of unacceptable intubating conditions OT
(n = 24)
NT (n = 23)
Excellent 4 4
Good 8 7
Poor 12 12
Causes of unacceptable intubation
Poor jaw relaxation 2 3
Closed vocal cord position 3 4
Closing vocal cord movement 0 2
Sustained coughing 7 3
Vigorous limb movement 3 4
Values represent the number of patients.
Table 4
Values represent mean ± SD.
No significant differences between the groups were noted.
MAP, mean arterial blood pressure; HR, heart rate.
Haemodynamic and bispectral index (BIS) data from patients with acceptable intubation during anaesthesia induction