BJA Advance Access originally published online on April 2, 2007
British Journal of Anaesthesia 2007 98(5):604-610; doi:10.1093/bja/aem064
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Dose of alfentanil needed to obtain optimal intubation conditions during rapid-sequence induction of anaesthesia with thiopentone and rocuronium
1 Department of Anesthesia, Aker University Hospital, Oslo, Norway
2 Department of Anesthesia and Perioperative Care, UCSF, San Francisco, CA, USA
* Corresponding author: Department of Anesthesia, Aker University Hospital, Trondheimsvn 235, 0514 Oslo, Norway. E-mail: tom.heier{at}medisin.uio.no
Accepted for publication February 9, 2007.
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Abstract |
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Background: The primary aim of the present study was to determine the dose of alfentanil that must be added to a rapid-sequence induction (RSI) regimen using thiopentone and rocuronium to obtain optimal intubation conditions in >95% of the individuals.
Methods: A total of 60 ASA I patients were randomly allocated to five different alfentanil dose groups (0, 15, 30, 45, or 60 µg kg–1). A blinded dose of alfentanil followed by thiopentone 4 mg kg–1 and rocuronium 1 mg kg –1 was administered in rapid succession, and tracheal intubation was attempted 40 s thereafter. The relationship between the alfentanil dose and the probability of optimal intubation conditions was determined by non-linear logistic regression analysis. Blood pressure (BP) changes were recorded continuously using an intra-arterial catheter.
Results: The success rate of optimal intubation conditions increased with increasing doses of alfentanil. The alfentanil dose needed to obtain optimal intubation conditions in >95% of the patients was 36.4 (CI 33.4–39.4) µg kg–1. In 12 patients, the systolic BP declined to <90 mm Hg during the 3 min immediately after intubation.
Conclusion: Adding 36–40 µg kg–1 alfentanil to a regimen of thiopentone and rocuronium during RSI of anaesthesia may significantly increase the success rate of optimal intubation conditions. Significant hypotension requiring vasopressor treatment may occur.
Keywords: analgesics opioid, alfentanil; induction, anaesthesia; intubation, tracheal tube
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Introduction |
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A frequently used drug regimen during rapid-sequence induction (RSI) of anaesthesia is suxamethonium in conjunction with either thiopentone or propofol.1 Optimal intubation conditions have been demonstrated within 60–70 s with either of these drug combinations in 67–92% of hemodynamically stable ASA I or II patients.2 3 Owing to potentially harmful side effects, for example allergic reactions, hyperkalemia, and increased intracranial pressure, it is suggested that suxamethonium needs to be replaced by a non-depolarizing neuromuscular blocking agent.4 In a RSI study, including 349 haemodynamically stable patients, the frequency of optimal intubation conditions was similar with rocuronium 1 mg kg–1 or suxamethonium 1 mg kg–1 (66 vs 74%).2
In certain emergency situations, for example in some ophthalmic and neurosurgical patients, the anaesthesiologist might prefer to use an induction regimen that provides a higher success rate of optimal intubation conditions than can be expected by the traditional RSI technique. Opioid drugs reduce responses to tracheal stimulation,5 and have been used successfully to improve intubation conditions, both during RSI 3 6 and in a non-RSI setting with/without the use of a neuromuscular blocking agent.7 Although the use of the opioids alfentanil and remifentanil has been recommended during RSI, the efficacy of these drugs in this setting has not been studied thoroughly.8 The aim of the present study was to determine the dose of alfentanil needed to obtain optimal intubation during RSI with thiopentone and rocuronium within 40 s after the injection of the neuromuscular blocking agent in >95% of haemodynamically stable patients.
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Methods |
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After approval from the Local Institutional Review Board, written informed consent was obtained from all patients. We studied 60 ASA I adult patients undergoing elective oral surgery procedures. Patients aged >55 yr or <18 yr, having gastro-oesophageal reflux, BMI > 28, suffering from neuromuscular disease, or undergoing treatment with drugs known to interfere with neuromuscular transmission were excluded. All patients had a Mallampati class 1 or 2 airway anatomy and no anticipated difficulty with mask ventilation or tracheal intubation.
Patients were premedicated with midazolam 0.03 mg kg–1 i.v. within 15 min before induction of anaesthesia. In the operating theatre, routine monitoring of ECG and pulse oximetry was initiated, and i.v. access for a continuous infusion of normal saline was established in a vein on the dorsum of the hand or in the lower forearm. Drugs were injected directly into the i.v. catheter through which normal saline was running. A radial artery catheter was inserted in all patients for continuous recording of systolic blood pressure (BP) and heart rate (HR). The patients were placed supine and flat on the operating table. After preoxygenation with 100% oxygen for 3 min, a blinded dose of alfentanil followed by thiopentone 4 mg kg–1 and rocuronium 1 mg kg–1 was administered as fast bolus injections in rapid succession over 15 s. Cricoid pressure was not applied during the induction of anaesthesia.
The doses of alfentanil, 0 (control), 15, 30, 45, and 60 µg kg–1, were diluted in normal saline to a total volume of 10 ml in all cases, and the patients were randomly allocated to the dose group. At 40 s after injection of rocuronium, laryngoscopy was attempted, with the aim of having the tracheal tube passed through the patient's vocal cords and the cuff inflated after 15 s (i.e. completed tracheal intubation within 70 s after start of alfentanil administration). Assisted ventilation was not used before tracheal intubation. Systolic BP and HR were recorded for 3 min after tracheal intubation. During this period, no i.v. drugs or inhaled anaesthetics were administered, and only minor manipulation of the patients was allowed. Systolic BP < 70 mm Hg was treated with i.v. phenylephrine 1 µg kg–1. The study was complete 3 min after tracheal intubation, and anaesthesia thereafter was maintained with either inhalation or i.v. anaesthetics, at the attending anaesthesiologist's discretion.
All intubations were performed by one of the authors, and the intubation conditions were graded according to the criteria outlined in Table 1.9 Excellent scores in all five categories in Table 1 were needed for a grading of optimal intubation conditions (intubation recorded as a success); all other scores were considered failures (intubation recorded as a failure). The success rate of optimal intubation conditions in each alfentanil dose group was recorded. The investigator performing the intubation and assessing the conditions was blinded to the dose of alfentanil administered.
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During the 3 min post-intubation period, maximum and minimum percentage changes in systolic BP and HR from pre-intubation value (baseline), number of patients within each alfentanil dose group with systolic BP <90 mm Hg and <70 mm Hg, number of patients within each alfentanil dose group with HR <50 min–1, and success ratios within each alfentanil dose group in preventing systolic BP or HR from increasing >10% compared to pre-intubation value were recorded.
Logistic regression analysis10 was used to analyse the success rate of obtaining optimal intubation conditions as described previously.11 12 The equation used for the logistic regression was
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In order to estimate confidence intervals for the D50, D90, and D95 variables, Monte Carlo simulations were run,13 using the estimates for the model parameters and their probability distribution (Crystal Ball, Decisonioeering Inc., Denver, CO, USA). The 1000 simulations from the Monte Carlo analysis were then subjected to Bootstrap analysis by the multiple-simulation method within Crystal Ball, using resampling with replacement from the simulations dataset. Thus, estimates for the percentiles 2.5 and 97.5 for the variables D50, D90, and D95 were calculated.
ANOVA and Dunnett's test were used to compare the effects of alfentanil dose on the magnitude of BP and HR responses to tracheal intubation. Least squares linear regression was used to analyse the correlation between the lowest recorded systolic BP and simultaneously measured HR in patients with hypotension (systolic BP < 90 mm Hg) post-intubation. Patients' physical characteristics in the five study groups were compared by ANOVA, or Chi-square test, as appropriate. A P-value <0.05 was considered statistically significant.
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Results |
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The five study groups did not differ with respect to age, weight, or gender distribution (Table 2). Before leaving the operating theatre, the patients were routinely asked if they had slept well during surgery. None reported awareness.
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All patients were intubated within 70 s after alfentanil or saline injection. Intubation conditions within each alfentanil dose group are shown in Figure 1. The success rates of optimal intubation conditions used for the logistic regression analysis are shown in Table 3. The logistic regression curve for the alfentanil dose vs probability of success (optimal intubation conditions) is shown in Figure 2. The best estimates of D50, D90, and D95 of alfentanil and their confidence limits (expressed as 2.5–97.5 percentiles) were 21.5 (19.0–23.8) µg kg–1, 33.1 (30.1–35.8) µg kg–1, and 36.4 (33.4–39.4) µg kg–1, respectively.
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The maximum and minimum percentage changes in BPs from baseline during the 3 min period immediately after tracheal intubation are shown in Table 4. The maximum change in the systolic BP in the control group was significantly higher than all the alfentanil dose groups (P < 0.05). The maximum change in systolic BP values in the alfentanil 45 and 60 µg kg–1 groups were significantly lower than in the other two alfentanil dose groups (P < 0.05). The minimum change in systolic BP in the control group was significantly higher than in any of the alfentanil dose groups (P < 0.05). The minimum change in systolic BP in the alfentanil 45 µg kg–1 group was significantly lower than in the alfentanil 15 and 30 µg kg–1 groups (P < 0.05), and similar to that in the 60 µg kg–1 group. When the maximum and minimum changes in systolic BPs after intubation within each group were compared, a significant difference was found in the control and alfentanil 15 µg kg–1 dose groups (P < 0.05), but not in the three higher alfentanil dose groups.
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None of the patients had HR <50 min–1 before intubation. The maximum and minimum percentage changes in HRs from baseline during the 3-min study period after tracheal intubation are shown in Table 4. The maximum change in HR in the control group was significantly higher than that in the three higher alfentanil dose groups (P < 0.05), but there was no difference between any of the alfentanil dose groups. The minimum change in HR in the control group was significantly higher than that in any of the three higher alfentanil dose groups (P < 0.05), and similar in all alfentanil groups. When the maximum and minimum changes in HRs after intubation within each group were compared, no significant difference was found in any of the groups.
In 12 patients, the lowest recorded systolic BP was <90 mm Hg (Table 4) and 1 patient had systolic BP <70 mm Hg (60 mm Hg in the alfentanil 15 µg kg–1 group).
The success ratio in preventing BP or HR from increasing >10% after intubation increased with increasing doses of alfentanil is shown in Table 3. However, the distribution of the dose–response data did not allow the determination of a valid logistic regression curve.
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Discussion |
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Our study shows that optimal conditions for laryngoscopy and tracheal intubation in RSI commencing at 40 s after thiopentone and rocuronium 1 mg kg–1 can be obtained by adding alfentanil. The logistic regression analysis suggests that approximately 36 µg kg–1 of alfentanil is needed to obtain such intubation conditions in 95% of patients. Our data further suggest that this drug regimen provides a higher success rate of optimal intubation conditions during RSI than the standard technique using thiopentone and suxamethonium (>95 vs 74%).2 Therefore, the technique presented in this study represents a satisfactory alternative to a regimen involving the use of suxamethonium.
Because optimal intubation conditions were the primary goal, we chose drugs with a fast onset time, and timed their administration so that tracheal intubation was performed when drug concentrations at the effect site were close to their maximum. Alfentanil is the opioid with the highest plasma-effect site equilibration rate constant, with peak drug effect occurring approximately 60 s after a fast bolus injection.14 Thiopentone reaches the maximum brain concentration in approximately 45–50 s after a rapid bolus injection.15 16 Therefore, in this study alfentanil was administered before thiopentone. With this technique, laryngoscopy and tracheal intubation could be performed during 55–70 s after alfentanil administration.
Ideally, rocuronium should have been administered as the first drug in the RSI sequence, because maximum effect of a 1 mg kg–1 dose at the laryngeal muscles occurs in approximately 60 s in most patients.17 However, traditionally, the neuromuscular blocking agent has been administered as the final drug during RSI to ensure unconsciousness before inducing neuromuscular block,1 and no study has been conducted showing that the administration of the neuromuscular blocking agent 10 s earlier is advantageous. The onset time of rocuronium may be reduced by prior administration of a drug that increases cardiac output (e.g. ephedrine),18 but the effect of such drugs in a RSI setting has not been investigated.
Intubation conditions are not solely dependent on the effect of a neuromuscular blocking agent. Tracheal intubation can be performed with a sedative alone or with a combination of a sedative and an opioid, when used in high doses.19 A main advantage of adding a neuromuscular blocking agent is that the dose of the sedative may be reduced, therefore potentially decreasing the incidence of hypotension during induction of anaesthesia. Our study shows that moderate doses of thiopentone and alfentanil in combination with a high dose of rocuronium create optimal conditions for laryngoscopy and tracheal intubation within approximately 60 s, a time frame considered adequate for RSI.20 However, our results may be valid only during RSI in the setting applied in the study. If patients other than ASA I are included, or if different anaesthetic drug combinations are administered, different doses of alfentanil may be needed to obtain optimal intubation conditions.
Although propofol is considered superior to thiopentone with respect to muscle relaxation,21 we chose thiopentone for two reasons. First, thiopentone has the faster onset of action. The estimated equilibration half-time between blood and brain for thiopentone is 1.2 min,16 while that of propofol is 3 min.22 Second, propofol has a higher potential for hypotension and bradycardia,21 probably because of pronounced peripheral vasodilation.23
Alfentanil and remifentanil have similar onset times,14 and therefore should be equally appropriate in a RSI setting. The organ-independent metabolism of remifentanil is an obvious advantage, and the administration of alfentanil 36 µg kg–1 might carry a risk of prolonged respiratory depression. However, after a bolus injection, alfentanil distributes rapidly, and the plasma concentration falls below that in cerebral cortex within 1–2 min.24 Therefore, the plasma concentration of alfentanil should decline below the threshold for spontaneous ventilation within 20–30 min after a bolus dose of this magnitude.25 An alfentanil bolus dose of 30–40 µg kg–1 during induction of anaesthesia is consequently suitable even for short surgical procedures. Alfentanil was therefore chosen in the present study because it represents a much less expensive alternative than remifentanil.
A probability approach, based on success rates, was used in this study to define the relationship between conditions for tracheal intubation and the dose of alfentanil. This approach has been used successfully in dose–response studies previously.11 12 We believe our estimated alfentanil dose to obtain optimal intubation conditions in 95% of the individuals (36 µg kg–1) is reliable.26 A basic requirement for the regression analysis is that the success rate of the binary variable should increase gradually.13 This was the case with our data. Also, the confidence limits obtained were small, suggesting that an appropriate number of individuals were included in the study (Fig. 2).
Our study design deviated from standard RSI in two different ways. First, we administered a small dose of midazolam as a premedication. This was required by our institutional Review Board, because elective surgical patients were included. Although midazolam might influence intubation conditions and autonomic responses during RSI differently, the fact that the BP increase in our control group was similar to that of previous studies investigating autonomic responses in the absence of opioids, suggests that the premedication administered did not significantly influence the estimation of the optimal dose of alfentanil.27 Second, we did not apply cricoid pressure. Although generally recommended during RSI, cricoid pressure may reduce the visibility of the larynx and therefore the speed of tracheal intubation. Sometimes the pressure may even have to be released to visualize the larynx.28 We therefore decided not to apply cricoid pressure, to secure uniform intubation conditions in all participants.
As expected, the systolic BP and HR increased by >10% after intubation in approximately 75% of the patients in the control group (Table 3). On an average the maximum change in BP after intubation was >40% in the control group (Table 4), which is consistent with previous results.27 The BP response declined gradually with increasing alfentanil doses, but was significant even in the 30 µg kg–1 group (Table 4). A similar change in HR was not observed, and the HR decreased to <50 min–1 in only two patients (Table 4). A logistic regression analysis was performed on the BP data, but the data distribution did not allow the determination of a valid dose–response curve. The use of a different study design, including more alfentanil dose groups and smaller dose intervals, may provide an adequate prediction of the dose–response curve for haemodynamic variables. However, our data suggest that the administration of alfentanil 30–60 µg kg–1 will blunt most of the autonomic responses during RSI. Our RSI technique may therefore be suitable during emergency care of patients who do not tolerate BP or HR increases well.
In our study, both the maximum increases and decreases in BP and HR responses to tracheal intubation were recorded, because some groups of patients may not tolerate hypotension and bradycardia well. Individual data show that 20% of our patients had systolic BP < 90 mm Hg after intubation (Table 4). Even higher numbers should be expected if alfentanil 30–40 µg kg–1 is administered to patients with hypovolemia or reduced cardiac output, because our results are applicable only to haemodynamically stable patients.29 In head injury patients, systolic BP <90 mm Hg is associated with worsened outcome.30 Therefore, if alfentanil is used during RSI in this particular group of patients, the anaesthesiologist must be prepared to administer a vasopressor drug to maintain adequate BP.
During the 3 min period after intubation, a significant difference between maximum and minimum changes in BPs was recorded in the control and alfentanil 15 µg kg–1 groups, but not in the higher alfentanil dose groups (Table 4). This finding is to be expected as when a low dose of alfentanil is used, the response to tracheal intubation is evident, and because the response is short-lived,29 a significantly lower BP is recorded later during the 3 min period. When a high dose of alfentanil is administered, the autonomic response is blunted mostly, and no significant difference between the maximum and minimum BPs in the 3 min period is observed. A similar interpretation of HR data (Table 4) is not possible, probably due to larger variability in responses.
In our study, we recorded BP and HR for only 3 min post-intubation for two reasons. First, we focused on the immediate autonomic responses to tracheal intubation, which occur within the first 3 min after the application of the stimulus.29 Second, after 3 min, the effect of thiopentone may start to wear off in some patients. This necessitates administration of additional anaesthetics, potentially confounding the relationship between autonomic response and the alfentanil dose.
In conclusion, this study suggests that alfentanil in a dose of 36 µg kg–1, when used in conjunction with thiopentone 4 mg kg–1and rocuronium 1 mg kg–1, will secure optimal intubation conditions during RSI in >95% of haemodynamically stable patients. We consider this anaesthesia induction technique as a valuable alternative to a regimen involving the use of suxamethonium.
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