Undesirable effects of muscle relaxants include anaphylactoid reactions, residual postanaesthesia muscle weakness and awareness during general anaesthesia.1–3 Tracheal intubation without muscle relaxants is possible with varying doses of remifentanil in combination with propofol or thiopental, and optimal conditions for direct laryngoscopic intubation have been investigated.4–7 The optimal dose of remifentanil when used with thiopental for lightwand intubation without muscle relaxants has not yet been evaluated.
Remifentanil has the advantage of rapid return of spontaneous ventilation, avoiding the side effects of succinylcholine for rapid sequence induction. In clinical practice, the rapid return of spontaneous ventilation and rapid emergence makes remifentanil a practical opioid choice in ambulatory anaesthesia.
This study was designed to evaluate the dose requirements of remifentanil coadministered with thiopental for lightwand intubation without muscle relaxants that provided clinically acceptable intubating conditions with rapid return of spontaneous ventilation.
This study was approved by the institutional review board of Seoul National University Bundang Hospital, Seongnam, South Korea on 9 February 2010 (Chairperson: Professor Kyoung-Chan Park, number: B-0912–090–009).
After obtaining informed and written consent, we prospectively studied patients (American Society of Anesthesiologists’ physical status class I-II, ages 18 to 65 years) who required general anaesthesia for elective surgery. Exclusion criteria included history of asthma, hypertension, concurrent use of sedative medication, cardiovascular disease and anticipated difficult intubation. The patients were assigned to one of three groups using random number tables by a nurse who was otherwise not involved in this study, and the assignments were concealed in sealed envelopes until immediately before induction.
During the procedure, cardiac rate and rhythm, noninvasive blood pressure, oxygen saturation and end-expired CO2 were monitored. We recorded heart rate (HR), mean arterial pressure (MAP) and peripheral arterial oxygen saturation values.
Midazolam 0.03 mg kg−1 was given, intravenously (i.v.), 15 min before induction of anaesthesia. Remifentanil was diluted to a volume of 10 ml with 0.9% saline. Remifentanil 1 μg kg−1 (group R1), remifentanil 2 μg kg−1 (group R2) or remifentanil 3 μg kg−1 (group R3) was administered as a slow bolus infusion over 60 s by a nurse who was blinded to this study. When the bolus dose of remifentanil was given, thiopental 5 mg kg−1 was administered over 30 s. The anaesthetic circuit was saturated with 2% sevoflurane prior to mask ventilation. After induction, mask ventilation was maintained for 60 s with 2% sevoflurane in 50% nitrous oxide in oxygen at a total flow of 4 l min−1 in all groups. Forty-five seconds after drug administration, postinduction vital signs were recorded. Lightwand (Surch-Lite, Aaron Medical Industries, St. Petersburg, Florida, USA) intubation was attempted 90 s after the remifentanil administration. After tracheal intubation and cuff inflation, postintubation vital signs were recorded. Lightwand intubation was attempted by three anaesthesiologists with previous experience of more than 100 lightwand intubations.
Intubating conditions were assessed as excellent, good or poor on the basis of the extent of jaw relaxation and patient response to intubation and cuff inflation. Intubating conditions were assessed as excellent if jaw relaxation was easy and there was no patient movement or cough, good if jaw relaxation was fair and patients showed slight movement or cough once or twice after tracheal intubation and poor if jaw relaxation was difficult or persistent movement or cough occurred after tracheal intubation. Overall intubation conditions were regarded as clinically acceptable if they were considered excellent or good and clinically unacceptable if considered poor.
Intubation time was defined as the time from insertion of the lightwand into the oral cavity to the confirmation of position by capnography. As described in a previous report,7 duration of apnoea was defined as the period from its onset to the return of spontaneous ventilation. Mechanical ventilation was started at 30 ml kg−1 min−1 after intubation. The restoration of spontaneous respiration was observed at 1-min intervals after intubation. Spontaneous respiration was considered as eight or more breaths per minute. If spontaneous respiration had not resumed within 15 min, this time was regarded as the duration of apnoea.
If intubation failed on the first attempt, rocuronium 0.6 mg kg−1 was given to facilitate tracheal intubation. If the HR was less than 50 beats min−1, atropine 0.5 mg was administered. If the MAP decreased by over 25%, ephedrine 5 mg was administered.
In this study, the primary outcome measurement was intubating conditions and the secondary outcome measurement was the duration of apnoea.
The sample size calculation was based on the results of a previous study using a combination of thiopental and remifentanil for tracheal intubation without the use of neuromuscular blocking agents, which yielded excellent intubating conditions in 50% of cases.4 A sample size of 32 patients per group was required to detect a difference of 30%, with a power of 80% and an α of 0.05. One-way analysis of variance or the χ2-test was performed to assess differences in patient characteristics, clinical signs and the duration of apnoea among the three groups. χ2-test was performed to assess differences in intubating conditions among the three groups. Repeated measures analysis of variance was used to examine how continuous variables (MAP, HR) differed over the study period among the groups, with the groups compared for between-participant (group) differences and analysed over time as the within-participant (group) factor and for interaction effects between differences in groups by time. A value of P less than 0.05 was considered statistically significant.
Ninety-six patients enrolled in the study were allocated to the three groups. The personal characteristics of the three groups were similar (Table 1). The mean duration of apnoea showed a dose-related increase (2.9, 4.6, 10.2 min, P < 0.01). The intubation time was shorter for group R3 than for group R1 (22 ± 8 vs. 33 ± 18 s, P < 0.01). There was no significant difference between groups R2 and R3 (Table 1).
Intubation in all patients was initially successful except for two patients in group R1. Significantly, more patients in group R1 had unacceptable intubating conditions compared with groups R2 and R3 [12 (37%) vs. one (3%) and zero (0%), respectively; P < 0.01, Fig. 1]. Excellent intubating conditions were present in seven (22%), 21 (66%) and 27 (84%) patients in groups R1, R2 and R3, respectively. There was a significant difference in presence of excellent intubating conditions between group R1 and the other groups (P < 0.01), but not between groups R2 and R3.
The baseline values in the MAP and HR were similar among the groups. There were decreases in the MAP and HR after induction, but these was not statistically significant. No patient required ephedrine or atropine. There were no significant differences in MAP between groups at any time (Table 2).
The present study has shown that i.v. remifentanil 2 or 3 μg kg−1 administered simultaneously with i.v. thiopental 5 mg kg−1 produces acceptable intubating condition for lightwand intubation without muscle relaxants, with similar haemodynamic changes and intubation time. Remifentanil 2 μg kg−1 was more likely to yield rapid return of spontaneous ventilation than remifentanil 3 μg kg−1.
Lightwand intubation is considered useful in difficult intubation and failed laryngoscopic intubation,8–10 and can also be advantageous in known difficult intubation wherein neuromuscular blockade is undesirable or is not required for the planned surgical procedure.
We note from our clinical experience that remifentanil has a short duration of apnoea. However, the duration of apnoea in patients using thiopental and remifentanil has not been assessed. In the present study, apnoea was prolonged in a dose-related fashion. The mean duration of apnoea was less than 5 min in groups R1 and R2, but more than 10 min in group R3. The former combination may be useful for procedures which require rapid recovery of spontaneous ventilation and rapid emergence.
Few previous studies have demonstrated that tracheal intubation using a lightwand is possible without the use of muscle relaxants.11–13 Masso et al.13 showed that lightwand intubation had a high failure rate (50%) on the first attempt when muscle relaxants were not used. We suspect that the small dose of remifentanil (1 μg kg−1, then 0.3 μg kg−1 min−1) administered in that study provided inadequate relaxation of the larynx for lightwand intubation. Our results show that remifentanil 1 μg kg−1 failed to provide clinically acceptable intubating conditions. Furthermore, Masso et al. administered remifentanil before propofol, and lightwand intubation was attempted 3.5 min after remifentanil administration. Timing of injection of both the anaesthetic agent and remifentanil can influence intubating conditions.14 The peak effects of remifentanil, propofol and thiopental are respectively achieved approximately 1.5, 1.6 and 1.75 min after i.v. administration.15–17 The timing of tracheal intubation should coincide with the peak effects of both the anaesthetic agents and remifentanil to achieve the best possible intubating conditions. Thus, we performed intubation 90 s after remifentanil administration.
In our study, more patients showed excellent intubation conditions with remifentanil 3 μg kg−1 than with remifentanil 2 μg kg−1, although the difference was not statistically significant. A larger dose of remifentanil may have provided excellent conditions for lightwand intubation in more patients. However, increasing the dose would be inappropriate due to the associated prolonged duration of apnoea when rapid recovery of spontaneous ventilation is desired. In addition, hypotension is common with a large dose of remifentanil.6
We attempted intubation once, and stopped the attempt if cough or patient movements occurred. Cough and movements of the patient can increase difficulties of tracheal intubation and cause serious complications, such as laryngospasm, laryngeal injury or mucosal trauma.
One potential limitation of our study is that three anaesthesiologists performed lightwand intubation, and their intubation skills may have varied. However, all of the anaesthesiologists had previous experience of more than 100 lightwand intubations. Our data may not apply to the inexperienced user. This study was performed in healthy individuals. This technique may not be appropriate in patients with significant cerebrovascular or cardiovascular disease. In this study, remifentanil was not continuously infused following i.v.-bolus administration because continuously infused remifentanil could have influenced our results, especially apnoea time.
In summary, our results suggest that remifentanil 2 or 3 μg kg−1 coadministered with thiopental 5 mg kg−1 is likely to provide clinically acceptable lightwand intubation conditions without muscle relaxants. Remifentanil 2 μg kg−1 seemed to give more rapid return of spontaneous ventilation than remifentanil 3 μg kg−1.
Assistance with the article: none declared.
Financial support and sponsorship: support was provided solely by the Department of Anesthesiology, Seoul National University Hospital, Seoul, Korea.
Conflicts of interest: none declared.
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