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Original Article

Comparison of the anaesthetic requirement with target-controlled infusion of propofol to insert the laryngeal tube vs. the laryngeal mask

Richebé, P.1; Rivalan, B.1; Baudouin, L.1; Sesay, M.1; Sztark, F.2; Cros, A.-M.3; Maurette, P.1

Author Information
European Journal of Anaesthesiology: November 2005 - Volume 22 - Issue 11 - p 858-863
doi: 10.1017/S0265021505001456

Abstract

Introduction

The laryngeal tube (LT; VBM Medizintechnik GmbH, Sulz, Germany) is a recently introduced device [1,2] used to secure a patent airway during anaesthesia, similar to the laryngeal mask airway (LMA) [3,4]. Compared with the face mask both of these devices are easy to use. They reduce the minimal dead space ventilation and gastric inflation observed with the face mask [3,5] and permit mechanical ventilation during general anaesthesia [6-8]. The frequency of additional manoeuvres to obtain adequate ventilation during anaesthesia is also lower with LT or LMA than with the Guedel airway or face mask. LT and LMA have different designs and insertion methods, but share the same indications for spontaneous breathing or controlled ventilation during anaesthesia [5,8-10] or resuscitation [11-16]. Recent studies compared both LT and LMA in terms of insertion success rate, gas leak pressure and incidence of gastric insufflations, and concluded that LT provided better oropharyngeal seal than LMA [7,14]. Other studies comparing ease of insertion and postoperative morbidity with LT and LMA, found similar results for both devices [17] as well as their respective derivatives such as Laryngeal Tube Suction and ProSeal LMA [18]. The target effect-site concentration (Cet) of propofol (used alone for induction of anaesthesia) to insert the LMA, in non-premedicated patients, was recently reported to be about 5 μg mL−1 [19]. But no comparison was made between LT and LMA with regard to propofol requirement for insertion. The purpose of this prospective, randomized, plasmatic propofol concentration-blinded, clinical investigation was to determine the Cet with target-controlled infusion (TCI) of propofol used alone to place either LMA or LT in adults scheduled for general anaesthesia with bispectral index (BIS) monitoring.

Material and methods

All patients were scheduled for gynaecological or radiological procedures under general anaesthesia. After local Ethics Committee approval and informed consent had been received, 40 non-premedicated patients, ASA I-II, were randomized into two groups: LT or LMA. Inclusion criteria were: age 18-75 yr, ASA I-II, peripheral surgery <2 h, free access to both the head and the device used. Exclusion criteria were: difficult intubation (Mallampati Class > 2, thyro-mental length ≤ 6 cm, mouth opening ≤ 3 cm, or airway abnormality); history of major neurological, psychiatric, cardiac, pulmonary, renal and hepatic major abnormalities; aspiration risk (full stomach, obesity); allergy or hypersensitivity to any of the drugs used; pregnancy.

Standard cardiorespiratory monitoring was used (electrocardiogram (ECG), non-invasive arterial blood pressure every minute, pulse oximetry, respiratory rate, capnography and inspired oxygen concentration), associated with the anaesthesia depth monitoring using the BIS (A-1050, Version 3.4; Aspect Medical Systems, Newton, MA, USA).

No pre-medication was given before surgery. On arrival in the operating room, patients were randomized into LT or LMA group following a randomization grid, then a 18- or 20-G catheter was placed in one of the patient's forearm veins, and each patient received 500 mL of intravenous (i.v.) crystalloid solution 30 min before the induction of anaesthesia. Ephedrine was prepared for injection in case the mean arterial blood pressure (MAP) dropped below 70 mmHg during the induction period and before insertion of the device. All patients inhaled 100% oxygen through a face mask for 5 min before induction of anaesthesia. Anaesthesia was induced by an anaesthetist who did not insert the device and was not involved in the evaluation of the patients' reactions. No local anaesthetic was injected i.v. before the induction of anaesthesia with propofol. Propofol was used as the exclusive anaesthetic induction agent, and its target plasma concentration on Diprifusor (AstraZeneca, Rueil-Malmaison, France) was adjusted according to the concentration used for insertion in the previous patient as indicated by the Dixon's up-and-down statistical method (Fig. 1). If the insertion was a success in this previous patient, the target plasma concentration was decreased by a step of 0.5 μg mL−1. If it was a failure, the target-concentration was increased by the same dose. In each group, the target plasma concentration of propofol of the first patient was 5 μg mL−1, with regard to results obtained in previous studies. A single experienced and trained anaesthetist, blinded for the target concentration of propofol used, performed all insertions of LT and LMA. Sizes of the LT or LMA were chosen according to the patient's height and the manufacturer's guidelines (VBM Medizintechnik GmbH, Sulz, Germany). The LMA was inserted using the technique described by Brain [4]. The LT was placed as follows: both cuffs of LT deflated, head in neutral position, simple mouth opening; the LT was then inserted until distinct resistance was felt or until the second bold black line on the tube was located between the upper and the lower teeth. The cuffs of the LMA or the LT were inflated using a cuff inflator (VBM Medizintechnik GmbH) to an intra-cuff pressure of 60 cmH2O. This pressure was maintained stable throughout the procedure by continuous monitoring with a manometer. The device was inserted 5 min after the equilibrium between plasma and Cet was obtained as shown on the screen of the Diprifusor. Insertion was evaluated in this study only once for each patient. If it was a failure, anaesthesia was deepened and insertion was once more attempted before tracheal intubation was decided.

Figure 1.
Figure 1.:
Dixon's up-and-down method: assessment of failure or success for the device insertion. Success or failure for LMA™ or LT™insertion is defined in the text. The black crosses represent the mean propofol concentration when crossing from a failure to a success for insertion. The average of the crossings is represented by the horizontal black dashed lines.

The success of insertion of the LT or LMA was assessed using a grading system modified from Muzi [20] associated with variations in haemodynamic data (MAP, heart rate (HR)) and anaesthesia depth (BIS). Insertion was considered acceptable if the Muzi's score was 2 or less for either of the two scales: jaw mobility and coughing/movement. The Muzi's score was assessed by the anaesthetist and the nurse anaesthetist in charge of the patient. The occurrence of spontaneous breathing before placement, breath holding, laryngospasm or tearing after the placement was also recorded. Insertion failure was defined as an increase >20% of the baseline values (baseline assessed during the last minute before insertion of MAP, HR, or BIS). These parameters were noted every minute during induction and for 3 min afterwards. After insertion had been evaluated, remifentanil was introduced and anaesthesia was adapted for the device's use and maintained with the two drugs until the end of surgery. Leak pressure during surgery was evaluated whenever it was possible. In all the included patients, we also evaluated the ease of assisted, controlled and spontaneous ventilation during surgery or when the patient was awake (scores were easy or difficult, difficult meaning the necessity of simple additional manoeuvres to achieve adequate ventilation). After removal, both the device and the patient were examined in search of blood stains on the device and oropharyngeal trauma. The presence or absence of sore throat was also noted in the recovery room.

Statistical analysis

The Dixon's up-and-down method (staircase method) was used to determine the mean and standard deviation of propofol Cet for acceptable LT or LMA insertion. This was calculated from the mid-points of pairs of the propofol concentration in successive patients, in which a failure of insertion was followed by a success [21,22]. Six pairs failure-success are necessary for each group for the statistical analysis. Data are expressed as mean and 95% confidence interval (95% CI ± 2 SD). Data were also analysed using a logistic regression model to determine the effective target concentration of propofol needed to insert the device under acceptable conditions in 50% and 95% of subjects (ED50 and ED95, respectively). In this analysis, the logit transformation of the probability of successful insertion (P), ln(P/1 - P), was expressed as a linear function of the target concentration of propofol. Data were plotted and analysed using SigmaPlot 8.0 and Systat 10.0 (SPSS Inc., Chicago, IL, USA). Non-parametric data were analysed by using the χ2-test, and parametric data were analysed by using analysis of variance. A P-value <0.05 was considered as statistically significant.

Results

No significant difference in patient characteristics variables was noticed between the groups (Table 1). In both groups the patient's height and the manufacturer's guidelines permitted the predominate use of size 4 of LT (13/20) or LMA (16/20) more often than size 3 or 5. The incidence of sore throat was 10% in both groups. No case of trauma was observed during insertion and removal of the devices (Table 2).

Table 1
Table 1:
Patient characteristics data.
Table 2
Table 2:
Airway reactivity.

The Cet of propofol required to place the LT was 6.3 ± 0.3 μg mL−1 (Dixon's method) and 6.1 μg mL−1 (5.9-6.4) (ED50; logistic regression method). The corresponding values for LMA were 7.3 ± 0.2 μg mL−1 and 7.3 μg mL−1 (7.1-7.5), respectively (P < 0.05). The ED95 evaluated with logistic regression was 6.8 μg mL−1 (5.9-7.6) for the LT and 7.7 μg mL−1 (7.3-8.0) for the LMA (P < 0.05).

After induction of anaesthesia and before insertion of the device, we observed a decrease in MAP >20% in 11/20 (55%) of the LMA group vs. 6/20 (30%) in LT group (6/20) (P < 0.05). Hypotensive events (one in LT group and six in group LMA) were treated with i.v. boluses of ephedrine 3-6 mg.

Assisted, controlled and spontaneous ventilation were easily performed in 95% of cases for LT and for LMA (Table 3). Additional manoeuvres were necessary to obtain a clear airway and adequate ventilation in 10% of patients in each group.

Table 3
Table 3:
Ventilation modes and side-effects.

Discussion

Our study is the first to evaluate the Cet of propofol, without any premedication or analgesic drugs, to insert the LT. No local anaesthetic was used topically or i.v., despite the use of i.v. propofol injection, to avoid its potential interaction with the insertion of the device [23]. This device shares the same indications as the LMA to secure the airway during anaesthesia under spontaneous, assisted or controlled ventilation as described in recent studies [7]. TCI of propofol is routinely used to insert such devices because of its profound inhibitory effect on pharyngeal and laryngeal reactivity [24].

Our main result demonstrated that the Cet with TCI (Diprifusor) of propofol was significantly lower to insert LT than the LMA without any patient's reaction. In our study, the concentration used to insert the LMA and LT without any patient's reaction could be considered as high when compared with a recent study that reported a Cet of propofol for LMA insertion around 3.14 ± 0.33 μg mL−1 [25]. But in that previous study, the use of midazolam premedication and i.v. lidocaine injection (1 mg kg−1) could explain why their anaesthetic requirement for insertion of the LMA was lower than our study in which no premedication or local anaesthetic was used [23]. Nevertheless, such local anaesthetic i.v. injection and midazolam premedication cannot explain 50% difference between that previous study and our results for LMA insertion. Another complementary explanation is that we used an association of different fine criteria to evaluate the success of insertion: not only Muzi's score evaluating throat and motor reactions of the patient, but also BIS and haemodynamic variations. Previous studies only evaluated rough throat reactions or movements of the patient. Using this association of fine criteria of local and general patient's reactions inevitably leads to an increase of anaesthesic requirement to place the device without inducing any variation of each criterion.

In our study, criteria chosen for patient's reactivity included not only oropharyngeal, respiratory and motor reaction criteria (Muzi's score [20]), but also haemodynamic data (hypertension and tachycardia) and depth of anaesthesia BIS. Success was defined as a Muzi's score ≤2 with no increase of more than 20% the baseline values in MAP, HR or BIS within the 3 min following insertion. BIS monitoring was an interesting tool to evaluate patient's reactivity when associated with haemodynamic data and Muzi's score. Nonetheless a single and isolated increase of the BIS value, without any variation of haemodynamic data or Muzi's score, was never considered in our study as an insertion failure. It could be argued that a 20% increase in the BIS value should not be considered as abnormal, or a weak parameter to evaluate patients' responsiveness to insertion, if the absolute BIS value remained in the standard range (20-60). But in all patients with insertion failure, we noticed either a Muzi's score ≤2 or significant haemodynamic variations, sometimes associated with a variation of over 20% in BIS.

As intracuff pressure was maintained stable (60 cm of water), we assume that this parameter did not play a significant role to explain the difference in anaesthesia requirement to insert the LMA vs. the LT. Moreover, insertion of the devices was done by the same trained anaesthetist thereby avoiding the risk of operator related differences between the groups.

The decrease in Cet of propofol to insert the LT vs. LMA could also be explained by the insertion method used in this study. The LMA was inserted as described by Brain [4] and this method needs to introduce the finger and the device into the mouth, leading to a potential increase in reactivity of the oropharyngeal area. In the case of the LT, the insertion method seems to be easier with less oropharyngeal stimulation and reactivity since it needs just a simple mouth opening for its placement.

This increase of the Cet of propofol, administered without premedication or any other anaesthetic and analgesic agent, from 6.3 ± 0.3 μg mL−1 for the LT to 7.3 ± 0.2 μg mL−1 for the LMA, and the same difference noticed for the ED95 (LT: 6.8 μg mL−1 vs. LMA: 7.7 μg mL−1), induced more hypotensive events during the induction phase and before any attempt of insertion: 55% in LMA group vs. 30% in LT group (P < 0.05). Not all of the hypotensive events were considered as unsafe and i.v. ephedrine boluses to correct the hypotension was necessary only in one case for LT and six cases for LMA. Nevertheless, we could argue that the insertion of LT induced less haemodynamic side-effects than the LMA, with regard to its lower anaesthetic requirement.

Trauma defined as blood on the cuffs after removal or mucosal trauma clinically observed in the oropharynx did not exist in the LT group whereas two of the 20 LMA patients sustained this incident. We did not search for dysphonia, dysphagia and sore throat on postoperative day one because most of the patients were scheduled for ambulatory anaesthesia or short hospitalization. However, the similarity of the two devices in our study, concerning oropharyngeal trauma, sore throat and postoperative morbidity corroborate other studies [17,18]. But we cannot at this point conclude on the post operative morbidity because of our small patient population.

Our study has a few limitations: the wide age range (19-75 yr) and the fact that no blood samples were analysed to assess propofol blood concentration during insertion. The Diprifusor device does not take into account the patient's age, but is currently used in clinical anaesthesia practice. Pharmacodynamic and pharmacokinetic of propofol had been reported to be different depending on the patient's age and consequently influence the Cet [26-29]. Nevertheless, the mean age of patients in our study, and the number of patients in each age range if we classify, as other studies [28], the patients as young (18-49 yr), middle-aged (50-69 yr), and elderly (=70 yr) was similar (Table 1). No opioids or muscle relaxant or local anaesthetics were used in this study to evaluate the Cet of propofol alone, unless the use of propofol alone is not of clinical routine practice. Studies associating propofol with opioids will have to be developed in the future.

Finally, we can conclude that the insertion of LT needs lower propofol doses than the LMA in a TCI of propofol alone. Consequently, the LT seems to induce less haemodynamic side-effects than the LMA with regard to anaesthetic requirement. However both groups had a high Cet of propofol and many hypotensive events (30% in LT, 55% in LMA group) during induction and before insertion. Further studies will be necessary to evaluate the merit of an association of propofol with premedication, opioids, or local anaesthetics to diminish the doses of propofol required for correct insertion with less haemodynamic effects, and to be as close as possible to current clinical practice.

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Keywords:

LARYNGEAL MASK; PROPOFOL; ANAESTHESIA INTRAVENOUS

© 2005 European Society of Anaesthesiology