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© 1999 International Anesthesia Research Society
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Abstract |
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To determine the target plasma concentration of propofol required to place either a laryngeal mask airway (LMA) or a cuffed oropharyngeal airway (COPA), we started a continuous target-controlled infusion of propofol in 60 ASA physical status I or II unpremedicated patients scheduled for minor orthopedic surgery with peripheral nerve block. The target plasma concentration of propofol was initially set at 2 µg/mL. When the effect-site calculated concentration of propofol was equal to the plasma concentration according to the computer simulation, the target plasma concentration was increased by 0.5-µg/mL steps until successful placement of either the LMA (n = 30) or the COPA (n = 30). The mean target plasma concentration of propofol required to place a LMA was 4.3 ± 0.8 µg/mL compared with 3.2 ± 0.6 µg/mL to place a COPA (P < 0.001). To successfully place the airways in 95% of patients, the target plasma concentration of propofol had to be increased up to 4 µg/mL for the COPA and 6 µg/mL for the LMA. We conclude that placing a LMA in healthy, unpremedicated patients requires target plasma concentrations of propofol higher than those required for placing a COPA.
Implications: We evaluated the use of target-controlled infusion of propofol to place extratracheal airways in this prospective, randomized study and demonstrated that the target plasma concentration of propofol required to successfully place a laryngeal mask in >95% of healthy, unpremedicated patients is 6 µg/mL, compared with 4 µg/mL to place a cuffed oropharyngeal airway.
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Introduction |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Extratracheal airways have become increasingly popular for spontaneously breathing patients undergoing minor surgical procedures (1). Although the laryngeal mask (LMA) has been largely evaluated for both conventional and nonconventional uses (2,3), clinical experience with the cuffed oropharyngeal airway (COPA; a modified Guedel airway with an inflatable distal cuff and proximal 15-mm connector for attachment to the anesthetic breathing system) is still limited.
Nakata et al. (4) recently demonstrated that the duration of exposure to sevoflurane required to achieve acceptable conditions for COPA placement was significantly shorter than that required to place the LMA. However, propofol is more frequently used to induce anesthesia when inserting extratracheal airways (5,6), and target-controlled infusion (TCI) systems have recently become available to physicians, allowing them to accurately achieve and maintain a desired propofol concentration without performing the complex calculation required when using a manual scheme (7). The purpose of this clinical investigation was to determine the target plasma concentration of propofol required to place either a LMA or COPA.
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Methods |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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After local ethics committee approval and informed consent had been received, ASA physical status I or II patients aged 18–65 yr scheduled for elective minor orthopedic surgery involving the lower limb (ankle and foot procedures) were considered for the study. Patients with a history of allergic reactions to propofol or local anesthetics; pregnant women; and patients with gastroesophageal reflux, neurologic or psychiatric disease, and suspected difficult airway (Mallampati classification >2) were excluded.
Standard monitoring, including noninvasive arterial blood pressure, heart rate, and pulse oximetry, was used throughout the study. After a combined sciatic-femoral nerve block had been performed as previously described (8,9), a continuous IV infusion of propofol was started using a TCI infusion system in 60 unpremedicated patients. The pharmacokinetic data set applied by the Diprifusor® (Fresenius Spa, Modena, Italy) TCI system includes a three-compartment pharmacokinetic model with the pharmacokinetic variables introduced by Marsh et al. (10) to improve the predictive performance (11). This TCI system also includes a more recent modification of the software, displaying the calculated effect-site concentration of propofol. The propofol infusion was set by an independent observer at a target plasma concentration (Cpt) of 2 µg/mL, and then increased by 0.5 µg/mL steps until the patient showed loss of reactions to a light painful stimulus (squeezing the trapezius) (12). Because the site of drug effect for propofol is the brain, and there is a delay between achieving a set target concentration in the plasma and this equilibrating with its effect site (7), the Cpt of propofol was increased only after the equilibrium between the plasma and effect-site calculated concentrations had been achieved according to the calculated values displayed by the pump. Using sealed envelopes, patients were then randomly allocated to receive either a LMA (Group LMA, n = 30) or a COPA (Group COPA, n = 30). All extratracheal airways were placed by the same physician (AC), who was unaware of the actual plasma concentration of propofol achieved when placing the considered device. The insertion, choice of size, and cuff inflation volume of both extratracheal airways were performed using the prescribed technique (5,6,13). After no reactions were observed when squeezing the trapezius, the placement of the designated airway was carefully attempted. If jaw relaxation was inadequate or if the patient coughed and swallowed during airway insertion, the maneuver was stopped, and the Cpt of propofol was increased by further 0.5 µg/mL. After the new equilibrium between the plasma and effect-site calculated concentrations of propofol was achieved, a new attempt at airway insertion was performed. This sequence was repeated until successful placement of the designated airway.
The Cpt of propofol required to abolish patient reactions when squeezing the trapezius and to allow successful placement of the extratracheal airway and the occurrence of any side effect during the study (including gastric regurgitation, bronchospasm, laryngospasm, or any other unanticipated event) were recorded. Patients were questioned about the presence of sore throat before discharge from the recovery area and 24 h after surgery.
Statistical analysis was performed by using Stat-View 3.0 (Abacus Concepts, Berkeley, CA). Demographic data and the Cpt of propofol required to place the extratracheal airways were analyzed by using an unpaired t-test. Contingency table analysis with Fisher’s exact test were used to analyze dichotomous variables, such as the incidence of postoperative sore throats or any other unanticipated untoward event. A value of P < 0.05 was considered significant. Continuous variables are presented as mean ± SD; ordinal data are presented as number (percentage).
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Results |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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No differences in the anthropometric variables and incidence of side effects were observed between the two groups (Table 1). Sore throat was more frequent after using the LMA than after using the COPA both before discharging the patients from the recovery area and 24 h after surgery, but these differences failed to reach statistical significance (Table 1).
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No differences in the Cpt of propofol required to abolish patient’s reaction when squeezing the trapezius were observed in the two groups (2.6 ± 0.5 µg/mL in Group COPA and 2.7 ± 0.5 µg/mL in Group LMA). The mean Cpt of propofol required to place a LMA was 4.3 ± 0.8 µg/mL, compared with 3.2 ± 0.6 µg/mL to place a COPA (P < 0.001). To successfully place the airways in >95% of patients, the Cpt of propofol had to be increased up to 4 µg/mL in Group COPA and 6 µg/mL in Group LMA (Figure 1).
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) or the cuffed oropharyngeal airway (
) with increasing concentrations of propofol. Cpt95 = the target plasma concentration required to successfully place the airways in 95% of patients.No differences in hemodynamic variables were observed between the two groups throughout the study (data not shown); the percent changes from baseline values of mean arterial blood pressure and heart rate measured immediately before successful placement of the airway were -10% ± 17% and -9% ± 14% in Group LMA and -11% ± 9% and -9%± 14% in Group COPA (not significantly different).
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Discussion |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Propofol without a muscle relaxant is routinely used as an induction drug when placing extratracheal airways such as the LMA because of its profound inhibitory effect on pharyngeal and laryngeal reactivity (14,15). The reported dose of propofol required to obtain adequate conditions for LMA placement in adult patients ranges between 2.5 and 3 mg/kg (16,17), and similar doses have been used for placing the COPA (5,6). However, we are unaware of previous studies evaluating the Cpt of propofol required to place either a LMA or a COPA. Results of the present investigation demonstrated that the Cpt of propofol required to successfully place a LMA in 95% of patients is approximately 1.5-fold greater than that required to place a COPA. In agreement with our results, Nakata et al. (4) reported that, when using inhaled induction, the duration of anesthetic exposure associated with acceptable conditions for COPA placement was significantly shorter than that for LMA, with 50% and 95% effective dose values of 90 s and 145 s or 164 s and 261 s, respectively. The difference in both the placement maneuver and the final position of the two extratracheal airways may explain this finding. In fact, inserting the LMA requires that a finger be introduced into the patient’s mouth, whereas the COPA is inserted like a Guedel cannula. Moreover, once the airway is successfully placed, the LMA requires positioning behind the laryngeal opening, whereas the COPA is placed at the base of the tongue, where the reactivity to tissue stimulation is lower than that of laryngeal structures (18). Greensberg et al. (5) suggested that the COPA may produce less pharyngeal trauma than the LMA because the incidence of sore throat was reduced with the COPA. However, contrasting results have been reported by Brimacombe et al. (6). In the present investigation sore throat was more frequent in the Group LMA, but statistical analysis failed to demonstrate significant differences between the two groups. However, because of the small size of the two treatment groups, we cannot exclude a type II error when evaluating the incidence of sore throat.
Even if the plasma concentrations of propofol reached in Group LMA were higher than those required to successfully place the COPA, no differences in hemodynamic variables were observed between the two groups. This was probably due to both the progressive and slow increase in the plasma concentration of propofol according to the study design, and the minimal risk of overdosing the drug because of the computerized control of propofol infusion.
The use of computer-controlled infusions of propofol has been developed to overcome drawbacks of manual schemes of infusion, allowing an easier and more accurate achievement and maintenance of a desired blood concentration of propofol (19). Coetzee et al. (20) demonstrated that different pharmacokinetic sets may affect the accuracy of TCI devices when different pharmacokinetic variable sets are used. The Diprifusor® has been demonstrated to provide a median performance error of 5.7% (11), which is acceptable for clinical practice; moreover, it also displays the calculated effect-site concentration of propofol, allowing the anesthesiologist to optimize the time required to achieve the equilibration between the Cpt set on the pump and the effect-site concentration calculated by the model.
Further studies should be performed to evaluate the influence of various factors, such as age and concomitant administration of hypnotic and/or opioids, on the Cpt of propofol required to successfully place extratracheal airways such as LMA and COPA. However, our results demonstrated that placing a LMA in healthy, unpremedicated patients requires Cpt values of propofol higher than those required to place a COPA.
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Acknowledgments |
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This study was supported by the IRCCS San Raffaele Hospital.
We thank Mallinckrodt Medical (Italy) for the COPA. We also thank Dr. G. Crispigni and the staff of anesthesia nurses (University Department of Anesthesiology, IRCCS San Raffaele Hospital), without whose help and cooperation this study would not have been possible.
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References |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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- Rego SA, Watcha MF, White PF. The changing role of monitored anesthesia care. Anesth Analg 1997;85:1020–36.[Web of Science][Medline]
- Brimacombe J. The advantages of the LMA over the tracheal tube or face mask: a meta-analysis. Can J Anaesth 1995;42:1017–23.[Web of Science][Medline]
- Verghese C, Brimacombe JR. Survey of laryngeal mask airway usage in 11,910 patients: safety and efficacy for conventional and nonconventional usage. Anesth Analg 1996;82:129–33.[Abstract]
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Nakata Y, Goto T, Saito H, et al. The placement of the cuffed oropharyngeal airway with sevoflurane in adults: a comparison with the laryngeal mask airway. Analg 1998;87:143–6.
[Abstract/Free Full Text] - Greensberg RS, Brimacombe J, Berry A, et al. A randomized controlled trial comparing the cuffed oropharyngeal airway and the laryngeal mask airway in spontaneously breathing anesthetized adults. Anesthesiology 1998;88:970–7.[Web of Science][Medline]
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Brimacombe JR, Brimacombe JC, Berry AM, et al. A comparison of the laryngeal mask airway and cuffed oropharyngeal airway in anesthetized adult patients. Anesth Analg 1998;87:147–52.
[Abstract/Free Full Text] - Glass PSA, Jacobs JR, Reves JG. Intravenous anesthetic delivery. In Miller RD, ed. Anesthesia. 3rd ed. New York:Churchill Livingston, 1990:367–88.
- Fanelli G. Peripheral nerve block with electric neurostimulation. Anestesiol 1992;58:1025–6.
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Fanelli G, Casati A, Beccaria P, et al. A double-blind comparison of ropivacaine, bupivacaine and mepivacaine during sciatic and femoral nerve blockade. Anesth Analg 1998;87:597–600.
[Abstract/Free Full Text] - Marsh B, White M, Morton N, Kenny GNC. Pharmacokinetic model driven infusion of propofol in children. Anaesth 1991;67:41–8.
- Glen JB. The development of ‘Diprifusor’: a TCI system for propofol. Anaesthesia 1998;58 (1):13–21.
- Chernik DA, Gilling SD, Laine H, et al. Validity and reliability of the Observer’s Assessment of Alertness/Sedation scale: study with intravenous midazolam. J Clin Psychopharmacol 1990;10:244–51.[Web of Science][Medline]
- Brimacombe J, Brain AIJ, Berry A. The laryngeal mask airway instruction manual. 3rd ed. Henley-on-Thames, UK:Intavent Research Limited, 1996.
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Barker P, Langton JA, Wilson IG, Smith G. Movements of the vocal cords on induction of anaesthesia with thiopentone or propofol. Br J Anaesth 1992;69:23–5.
[Abstract/Free Full Text] - Brown GW, Patel N, Ellis FR. Comparison of propofol and thiopentone for laryngeal airway insertion. Anaesthesia 1991;46:771–2.[Web of Science][Medline]
- Blake DW, Dawson P, Donnan G, Bjorksten A. Propofol induction for laryngeal mask airway insertion: dose requirement and cardiorespiratory effects. Intensive Care 1992;20:479–83.
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- Tagaito Y, Isono S, Nishino T. Upper airway reflexes during a combination of propofol and fentanyl anesthesia. Anesthesiology 1998;88:1459–66.[Web of Science][Medline]
- White M, Kenny GMC. Intravenous propofol anaesthesia using a computerised infusion system. Anaesthesia 1990;45;204–9.[Web of Science][Medline]
- Coetzee JF, Glen JB, Wiumm CA, Boshoff L. Pharmacokinetic model selection for target-controlled infusions of propofol. Assessment of three parameter sets. Anesthesiology 1995;82:1328–45.[Web of Science][Medline]
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