Department of Anaesthesia, Royal United Hospital, Combe Park, Bath BA1 3NG, UK
Corresponding author. Declaration of interest. TMC has received an honorarium from Intavent Orthofix, the distributors of the classic Laryngeal Mask AirwayTM.
LMA® is the property of Intavent Limited.
Accepted for publication: June 19, 2003
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Abstract |
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Methods. Primary outcome was success of airway placement. Secondary outcomes included time to achieve an airway, airway manipulations required, complications during use and fibre-optic assessment of airway positioning.
Results. We planned to study 300 patients but interim analysis demonstrated the study should be stopped after 100 patients. The AMD was inserted on the first attempt less frequently than the cLMA (P=0.04). Eight AMDs and one cLMA could not be placed within three attempts (P=0.03). The AMD required more attempts (P=0.03) and more manipulations (P=0.02) and caused more complications (P=0.01) during insertion. During maintenance of anaesthesia, three AMDs and no cLMAs had to be removed as a result of complications. Seal pressure was better with the AMD than with the cLMA (AMD 25 cm H2O, cLMA 20 cm H2O, P=0.001). Efficacy of ventilation was better with the cLMA than with the AMD (P=0.005). On fibre-optic examination, positioning over the larynx was better with the cLMA than with the AMD (P=0.005). Two of 32 attempts to pass an orogastric tube via the AMD were successful. During recovery, two AMDs and no cLMAs required premature removal. Tolerance during emergence, the incidence of blood on the devices and the incidence of postoperative complications were equivalent between devices. Overall failure rate of the AMD was greater than that of the cLMA (P=0.001).
Conclusions. Successful insertion of the cLMA is more likely than that of the AMD. Insertion of the AMD required more attempts and caused a greater number of complications. Fibre-optic position was poorer than with the cLMA. When an airway is established, the AMD caused a greater number of complications during anaesthesia and failed more frequently than the cLMA. During recovery from anaesthesia, more complications occurred with the AMD. Overall performance of the AMD was poorer than with the classic LMA.
Br J Anaesth 2003; 91: 6727
Keywords: airway, ventilation; anaesthesia; equipment, airway management device; equipment, laryngeal mask airway
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Introduction |
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Methods |
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Standard anaesthesia monitors were attached before induction of anaesthesia. The patients head was supported on a firm pillow with the neck flexed and the head extended. After pre-oxygenation, anaesthesia was induced with fentanyl 1 µg kg1 and a target-controlled infusion of i.v. propofol at 47 µg ml1. Anaesthesia was maintained with a continuous target-controlled infusion of propofol. The patients were then allocated randomly to one of two groups, in which the AMD or cLMA was used for airway management. Randomization was by the use of sequentially numbered, sealed opaque envelopes containing the name of either of the two devices.
Insertion and establishing an airway
Size selection of the AMD was according to the manufacturers guidelines,6 using size 4 for those weighing 4575 kg and size 5 for those weighing more than 75 kg. As instructed by the manufacturer, when the investigator felt other factors, such as height or sex, suggested use of a different size, this was allowed. The AMD was inserted according to the manufacturers instructions,6 with the proximal cuff deflated and the distal cuff inflated with just enough air to close the orifice at the distal tip of the device. The distal cuff was lubricated and the AMD was inserted in a single movement until the distal cuff passed behind the base of the tongue and resistance was noted as it was located in the hypopharynx. After insertion of the device, the proximal cuff was inflated with 10 ml increments, until a gas-tight seal was obtained. The volume of air introduced into the proximal cuff and the resulting pressure were recorded.
In the cLMA group, a size 4 was used in females and a size 5 mask for males.7 The cLMA was inserted according to the manufacturers instruction manual.8 The back of the cuff was lubricated with KY jelly and the cLMA directed into place with an index finger placed at the anterior junction of the stem and cuff. The cuff was inflated using the cuff inflator until the intracuff pressure reached 60 cm H2O. For both devices, resistance to insertion was treated with jaw thrust initially and repositioning of the head and neck if this did not help.
In both groups, the breathing system was connected to the device. An initial assessment of airway patency and the ability to ventilate the lungs was made by gently squeezing the reservoir bag, observing the presence of end-tidal carbon dioxide waveforms and chest movement. If it was not possible to ventilate the lungs, the following airway manoeuvres were allowed: chin lift, jaw thrust, head extension, or flexion on the neck. After any manoeuvre, adequacy of ventilation was re-assessed. If it was not possible to insert the device or ventilate through it, two more attempts at insertion were allowed. If placement failed after three attempts, the patient was withdrawn from the study, insertion was recorded as a failure, and the second device was inserted.
Time for insertion of the airway (from picking up the device to attaching it to the breathing system after inflation of the cuff) was measured in patients in whom it was possible to ventilate the lungs. Fresh gas was insufflated at 5 litres min1, the spill valve was occluded and the minimum airway pressure at which gas leaked around the airway device was determined. Peak airway pressure was not allowed to exceed 40 cm H2O.
Evaluation of ventilation
After securing the device, controlled ventilation in oxygen and air was started with a tidal volume of 7 ml kg1 and an inspiratory/expiratory ratio of 1:2. If expired tidal volume was below 7 ml kg1, tidal volume was increased up to 10 ml kg1 in an attempt to achieve this. The respiratory rate was adjusted to maintain the end-tidal carbon dioxide concentration in the normal range. Ventilation was judged to be optimal if the following four tests were passed: (i) adequate chest movement; (ii) an expired tidal volume of 7 ml kg1; (iii) stable oxygenation; and (iv) square-wave capnography. Ventilation was judged to be suboptimal if even one test was not passed.
The position of the device over the larynx was inspected with a flexible fibrescope. View was assessed with the tip of the fibrescope just exiting the bowl of the cLMA and just exiting the orifice of the AMD. View was graded 14: 1=vocal cords fully visible; 2=vocal cords partially visible or arytenoid cartilages visible; 3=epiglottis visible; 4=no laryngeal structures visible.9
In the case of the AMD, an attempt was made to pass an orogastric tube through the oesophageal cuff orifice. Two to eight millilitres of air was removed from the oesophageal cuff and a well-lubricated 1214 French gauge nasogastric tube was inserted through the access port, as recommended by the manufacturer.6 If the orogastric tube could not be advanced, the airway was inspected with the fibrescope and air was removed or added, until the distal cuff orifice was seen to open. A single further attempt at orogastric tube passage was then made. After attempting to pass the orogastric tube, air removed from the oesophageal cuff was replaced.
If airway obstruction occurred during maintenance of anaesthesia, or when manoeuvring the position of the device or the patients head and neck, removal and reinsertion of the device were allowed. The numbers of manipulations during insertion and maintenance of anaesthesia were recorded for each device. The allocated device was used for the duration of anaesthesia. The success rate for each device in establishing and maintaining a patent airway allowing ventilation without complications (efficacy) was recorded. Ventilation efficacy was recorded as good, fair or failed, with the following definitions: good=optimal ventilation without complications during anaesthesia; fair=optimal ventilation with complications or suboptimal ventilation; failure=failed insertion or abandonment of use. After completing these tests, spontaneous ventilation was allowed to resume if this was suitable for the surgical procedure.
Removal of the device
At the end of the operation, anaesthetic agents were discontinued while the airway device was left in place. The airway device was removed after the patient had regained consciousness and had responded to verbal command to open the mouth. However, if necessary (e.g. if airway obstruction or retching occurred), it could be removed before this point. Before removal, the proximal cuff of the AMD was opened to atmospheric pressure by attaching a syringe barrel to the pilot tube. The cuff of the cLMA was left inflated until the cLMA was removed from the patients mouth in accordance with the manufacturers recommendations. At removal, the presence or absence of secretions interfering with airway management and the presence or absence of blood on the device were recorded. Complications, defined earlier, that occurred during induction of, maintenance of and emergence from anaesthesia from the use of the device were recorded. Complications recorded were soft tissue damage, failure to establish an airway, partial or complete loss of airway patency, hypoxia, coughing, hiccupping, movement, laryngospasm, protrusion of the tongue, regurgitation and aspiration. Postoperatively, each patient was questioned to determine whether airway complications were present.
Statistical analysis
The primary aim of the study was to compare the clinical efficacy of the devices. Power analysis was based on detecting a difference of 10% from the incidence of successful insertion of the cLMA (reported as 9095%).10 Using a power of 80% and a significance level of 0.05, we predicted the need to study 300 patients.11 Interim analysis was planned after 100 and 200 patients.
Secondary outcome measures included time taken to achieve an airway, number of airway manipulations required to achieve an airway, ventilation efficacy, complications during use, and grade of view on fibre-optic laryngoscopy. This broad collection of data was designed to give a full evaluation of the comparative performances of the two devices.
Insertion time and seal pressure data were not normally distributed and the MannWhitney U-test was used to compare them. The 2 test or Fishers exact test was used as appropriate to compare the grade of laryngoscopy, numbers of manipulations, and complications and efficacy of ventilation between groups. Device intracuff pressure data were normally distributed and an unpaired Students t-test was used for analysis. Statistical analysis was performed using the statistical package Analyse-it (Analyse-it Software, Leeds, UK) and Microsoft Excel v6.0.
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Results |
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The AMD was inserted on the first attempt less frequently than the cLMA (AMD 35/50, cLMA 45/50, P=0.04) (Table 2). The cLMA was inserted successfully within three attempts more frequently than the AMD (AMD 42/50, cLMA 49/50, P=0.03). The AMD required more attempts at insertion (AMD 72 vs cLMA 56, P=0.03), more manipulations (AMD 1.2, cLMA 0.4 per patient, P=0.02) in more patients (AMD 30/50, cLMA 17/50, P=0.02). The AMD caused more complications during insertion (AMD 0.62, cLMA 0.1 per patient, P=0.01) in more patients (AMD 13/50, cLMA 4/50, P=0.03), and took longer to place (AMD median 35 s, cLMA 15 s, difference between medians 20 s, 95% confidence interval 1223 s). Optimal ventilation was achieved in 40 cases with the AMD and 45 with the cLMA (P=0.26) (Table 2).
During maintenance of anaesthesia, three AMDs had to be removed because of complications (airway obstruction). No cLMAs were removed. The use of the AMD was associated with more complications (AMD 23, cLMA 2, P=0.0017) in a greater number of patients (AMD 12 of 42, cLMA 2 of 49, P=0.003) (Table 3). Efficacy of ventilation was better with the cLMA than with the AMD (P=0.005) (Table 3). Seal pressure was better with the AMD than the cLMA (AMD 25 cm H2O, cLMA 20 cm H2O, P=0.001) (Table 2). Positioning over the larynx was better with the cLMA than with the AMD (P=0.005) (Table 2).
Thirty-two attempts were made to pass an orogastric tube through the AMD. None was successful without fibre-optic assistance, and with it only two (6%) were successful. In 18 cases where passage of an orogastric tube was not attempted, this was generally because of inadequate time or because airway manipulations or removal prevented it.
During recovery, two AMDs were removed because of airway obstruction and oxygen desaturation. No cLMAs needed to be removed. The number of complications occurring in recovery was greater with the AMD than with the cLMA (AMD 15, cLMA 1, P=0.02). The airways were similarly tolerated during emergence, the incidence of blood on the devices was similar, and the incidence of postoperative complications was equivalent (Table 3). Overall failure rate of the AMD was greater than that of the cLMA (cLMA 1 of 50, AMD 13 of 50, P=0.001).
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Discussion |
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The two groups were similar in most characteristics. However, there were minor differences: the cLMA group contained more males and their operations were of shorter duration, and the AMD group had a larger number of Mallampati class 3 patients (Table 1). We do not consider that the results are accounted for by these minor differences in group composition.
Analysis of our secondary outcome measures allows detailed comparison of the performance of the two devices. This demonstrates that the AMD may fail at any point during the anaesthetic, and that overall its performance is poorer than that of the cLMA, with an increased number of airway manipulations required and complications noted at all phases of anaesthesia. The modifications made to the device appear to have had little effect on its clinical efficacy, as the results of this study are in broad agreement with our findings with the previous version of the AMD in 2001.3
Airway seal pressure was better with the AMD than the cLMA. However, this did not lead to an improvement in the ability to provide controlled ventilation through the device. The cLMA was more likely to provide an airway in which uncomplicated controlled ventilation could be achieved, which we refer to as ventilation efficacy. Fibre-optic examination of the airway position revealed the orifice of the AMD to be positioned over the larynx (grade 12) in 64% of cases compared with 94% with the cLMA. This is partly explained by the size and shape of the devices orifices.
The manufacturers literature states that the AMD may be used to allow access for a suction catheter into the oesophagus via the lower cuff channel.6 We were not successful on any of 32 attempts, and even when a fibrescope was used to confirm opening of the lower cuff channel we were only successful in two cases. Under fibre-optic inspection, we observed that withdrawal of only a fraction of a millilitre of air from the distal cuff of the AMD led to the closed channel opening then reclosing. This makes the successful passage of a suction catheter or gastric tube unlikely. If a device is needed that allows passage of a gastric or suction tube, the ProSeal LMATM or Laryngeal Tube SondaTM are better alternatives than the AMD, with success rates of orogastric tube passage of 90100%.1214
In conclusion, we have demonstrated that successful insertion is more likely with the cLMA than with the AMD. Insertion of the AMD requires more attempts and causes a greater number of complications. Even when an airway is established, the AMD requires a greater number of manipulations during anaesthesia to maintain a patent airway and fails more frequently than the cLMA.
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References |
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11 Altman DG. Practical statistics for medical research. London: Chapman and Hall, 1991; 45560
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