1 Royal United Hospital, Combe Park, Bath, UK. 2 Assistant Professor, Kansai Medical University, 1015 Fumizono-cho, Moriguchi City, Osaka 5708507, Japan
Corresponding author LMA® is the property of Intavent Limited.
Declaration of interest. Dr Cook has received an honorarium from Intavent Limited.
Accepted for publication: April 3, 2003
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Abstract |
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Methods. We randomly allocated 72 patients to receive either the laryngeal tube or an LMA, and compared adequacy of controlled ventilation during anaesthesia (good: clear airway without complications; fair; clear airway with complications or suboptimal airway; or failed), leak pressure and the incidence of postoperative complications.
Results. Insertion was successful within 2 attempts in all 36 patients for the classic LMA and in 35 patients for the laryngeal tube. The mean leak pressure for the laryngeal tube (28 cm H2O) was significantly greater than that for the classic LMA (21 cm H2O) (P<0.001; 95% CI 3.610.0 cm H2O). Ventilation was good in 25 cases, fair in 11, and failed in no patients with the classic laryngeal mask airway; and good in 23, fair in 11 and failed in two for the laryngeal tube. There was no significant difference in adequacy of ventilation between the groups. The median peak airway pressure for the laryngeal tube (17.5 cm H2O) was greater than that for the classic LMA (16 cm H2O) (difference: 2 cm H2O; 95% CI 05 cm H2O). There was no significant difference in the incidence and severity of the postoperative complications between the two groups.
Conclusion. The laryngeal tube was as effective as the classic LMA during anaesthesia with controlled ventilation. There were similar operative and postoperative complications.
Br J Anaesth 2003; 91: 3738
Keywords: equipment, masks laryngeal; equipment, tubes laryngeal
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Introduction |
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The laryngeal tube can be used as an alternative to the classic LMA during maintenance of anaesthesia. There have been three studies comparing the efficacy of the laryngeal tube with the classic LMA.13 One study showed that the success rates of insertion was similar in the two groups, and the laryngeal tube provided a better seal than the classic LMA.1 In a second study,2 arterial blood gas analysis obtained 10 min after insertion of the test device showed that the laryngeal tube was as effective as the classic LMA. However, in the third report,3 the study was stopped because of a high failure rate with the laryngeal tube. In all studies, prototype laryngeal tubes were used. There has been no study comparing the current type of the laryngeal tube with other airway devices, such as the classic LMA, and no studies comparing these two devices during the entire course of anaesthesia.
Our aim was to compare the efficacy of the laryngeal tube with that of the classic LMA in paralysed patients during the entire course of anaesthesia, and to compare the sealing effects of the two devices.
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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. After preoxygenation, anaesthesia was induced with a target controlled infusion of i.v. propofol set to give a predicted plasma concentration of 47 µg ml1, and fentanyl 1 µg kg1. Neuromuscular block was with rocuronium 0.61.0 mg kg1, confirmed using a peripheral nerve stimulator (train-of-four count=0) before airway manipulation. Anaesthesia was maintained with a target-controlled infusion of propofol. The patients were then allocated randomly to one of two groups, in which the laryngeal tube or the classic LMA was specified for airway management. Randomization was by 72 sequentially numbered, sealed opaque envelopes containing equal numbers of the names of either of the two devices.
Insertion of the device
The laryngeal tube was inserted according to the manufacturers instructions. Briefly, before insertion, cuffs were deflated and a water-soluble lubricant (KY jelly) was applied to the cuffs. The patients head was extended on the neck (sniffing position). The tip of the laryngeal tube was placed against the hard palate behind the upper incisors and the device was slid down into the centre of the mouth until resistance was felt or the second bold black line on the tube had just passed between upper and lower teeth. The cuffs were inflated using a cuff inflator (VBM, Germany) until the intracuff pressure reached 8090 cm H2O. The pressure was then reduced to 6070 cm H2O.6 A size 3 was used for patients less than 155 cm, a size 4 for those between 155 cm and 180 cm, and a size 5 for those >180 cm in height.6
The classic LMA was inserted according to the manufacturers instruction manual.7 A size 4 was used in females and a size 5 mask for males.8 The back of the cuff was lubricated with KY jelly. The cuff was inflated using the same cuff inflator until the intracuff pressure reached 6070 cm H2O.7
In both groups, the breathing system was connected to the device. We assessed adequacy of ventilation 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. In the case of the laryngeal tube, the position was also allowed to be adjusted by gently pushing or pulling the device (whereas no such manoeuvre was allowed for the classic LMA). After any manoeuvre, adequacy of ventilation was re-assessed. If it was not possible to insert the device or ventilate through it, one more attempt at insertion was allowed. If placement had failed after two attempts, the study was abandoned and the airway maintained either through a classic LMA or such other airway device as suitable.
Time for insertion of the airway (from time of 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.
To measure the seal pressure, fresh gas was insufflated at 5 litre min1, the spill valve occluded and we recorded the minimum airway pressure at which gas could either be heard leaking around the airway device or the airway pressure plateaued. Peak airway pressure was not allowed to exceed 40 cm H2O.
Maintenance of anaesthesia
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. 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. If airway obstruction occurred during anaesthesia, manoeuvring the position of the device or the patients head and neck, or removal and reinsertion of the device were allowed. The number of manipulations of each device during insertion and maintenance of anaesthesia was recorded for each device.
Removal of the device
At the end of the operation, anaesthetic agents were discontinued while the device was left in place. The device was removed after the patient regained consciousness spontaneously and responded to verbal command to open the mouth. However, if necessary (e.g. airway obstruction or retching occurred), it could be removed before this point. Before removal, the cuffs of the laryngeal tube were deflated, whereas the cuff of the classic LMA was not (in accordance with the manufacturers recommendations7). At removal, the presence or absence of secretions interfering with airway management and of blood on the device was recorded. Complications, were defined, a priori, with the use of the devices during induction, maintenance, and emergence.
Postoperative period
Within 2 h and again at 24 h after surgery, each patient was questioned to determine the following complications: sore throat (constant pain, independent of swallowing), dysphagia (difficulty or pain with swallowing), sore jaw, dysphonia (difficulty or pain with speaking), numbness of the tongue or the oropharynx, blocked or painful ears, reduced hearing, or neck pain. Each complication was graded as: none, mild, moderate or severe. Those patients discharged from hospital within 24 h after surgery were followed-up by telephone.
Statistical analysis
The primary aim of the study was to compare the success rates of two airway devices in establishing a patent airway allowing ventilation without complications. The secondary aim was to compare the leak pressures between two groups. We calculated the number of patients required for the study based on these two factors. The rate of the classic LMA providing a patent airway is 95100%.9 Our hypothesis was that there would be no difference in the efficacy of the two devices in terms of ventilation during anaesthesia, with a clinically important difference being 10%. Seventy-two patients would be required to assess this hypothesis, with a power of 80% and a one-sided 95% confidence interval (CI), using an equivalence test.10 The mean airway pressure at which gas leaks around the classic laryngeal mask airway ranges from 18 to 20 cm H2O (SD 5 cm H2O).11 12 We considered that an increase in leak pressure of 30% would be clinically significant. Twenty-two patients would be required to detect this difference with a power of 80%. Therefore, we studied 72 patients.
The 2-test was used to compare the efficacies of the two devices: good (optimal ventilation without complications during anaesthesia), fair (optimal ventilation with complications or suboptimal ventilation), and failure (failed insertion or abandonment of the use). Plots of normal scores showed that the data for leak pressures were normally distributed. Therefore, the Students t-test was used to compare the leak pressures between two groups. For additional information, we also applied the third hypothesis test (
2-test) to compare the incidence of postoperative complications between groups. For this analysis, the number of patients was counted for no, mild, moderate or severe complication. The data for 02 h and 24 h after surgery were pooled for analysis and for each patient, with the severest score recorded. P<0.05 was considered significant.
We noticed after the start of the study that there might be a difference in the peak airway pressures during anaesthesia between the laryngeal tube and classic LMA. Therefore, the confidence interval was used to compare this factor. The data for insertion time and for the peak pressure were not normally distributed. Therefore, the 95% CIs were calculated for the median difference of insertion time, the median difference of peak airway pressure, and the mean difference of the leak pressures, between the two devices.
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Results |
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The median peak airway pressure for the laryngeal tube (17.5 cm H2O) was greater than that for the classic LMA (16 cm H2O) (95% CI 05 cm H2O).
Removal of the device
The airways were equally well tolerated during emergence from anaesthesia: there was no need to remove the airway before the patient regained consciousness with either device. Complications during emergence and recovery from anaesthesia were few and mostly minor (Table 3). One patient in the classic LMA group developed laryngospasm, stridor and mild desaturation. One patient in the laryngeal tube group developed airway obstruction and mild desaturation. On removal, blood was present on one device in each group (Table 3).
Postoperative period
Postoperative complications that could be attributed to the airway device were observed in one-third of patients in each group (Table 3), and there was no significant difference in incidence and severity between the groups.
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Discussion |
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The success rate of obtaining a patent airway through the laryngeal tube was high (97%). This result supports previous reports, where the success rate was 94100% immediately after insertion of the device.1 2 6 13 14 The laryngeal tube provided a patent airway as frequently as the classic LMA, not only during induction of anaesthesia, but also during maintenance of anaesthesia.
The airway seal pressure achieved with the laryngeal tube was significantly greater than that with the classic LMA (Table 2), which also supports a previous study.1 2 In this regard, the laryngeal tube may be more useful than the classic LMA for controlled ventilation.
The increased seal pressure of the laryngeal tube is achieved with two minor costs: the laryngeal tube required more re-adjustment because of airway obstruction and required a greater airway peak pressure during anaesthesia. The greater peak airway pressure with the laryngeal tube could be because the laryngeal tube is narrower and has smaller distal apertures, which will increase the resistance. The laryngeal tube could also be placed in a suboptimal position and cause partial obstruction of the airway. The classic LMA is designed to occupy the hypopharynx so that, provided an appropriate size is chosen, the device is unlikely to be inserted too deeply. In contrast, the laryngeal tube can potentially be inserted too deeply so that its distal apertures face the hypopharyngeal wall, rather than the laryngeal inlet. This would lead to airway obstruction or ventilation via the side holes of the device. The incidence of malposition is not known, since the position has not been studied using a fibreoptic endoscope.
The clinical relevance of the greater airway pressure with the laryngeal tube is unclear, but in our patients, we found no apparent problems. Nevertheless, high resistance in the airway would increase the work of breathing during spontaneous ventilation so that the laryngeal tube would not be as suitable as the classic LMA during spontaneous breathing.
Miller, Youkhana and Pearce3 compared the classic LMA with a prototype laryngeal tube in anaesthetized patients who were breathing spontaneously. They found the laryngeal tube virtually impossible to use, with 25 of 27 uses being abandoned. The reason for the high failure rate in their report is not clear. The use of a prototype is a possible explanation, but it is possible that there were technical problems,15 since in patients in whom ventilation was controlled, placement of, and ventilation through, the laryngeal tube was often unsatisfactory in their study,3 but almost always successful in other reports.1 2 5 13 In the current study, we found the laryngeal tube was effective for controlled ventilation and encountered no problems when the patient progressed from controlled to spontaneous ventilation. Miller stated that he has found that the success rate of adequate ventilation through the new laryngeal tube is much higher than that for the prototype (Miller, personal communication, 2002).
Although the efficacy of the devices was statistically equivalent there were two failures in the laryngeal tube group which were salvaged by use of the classic LMA. No failures occurred in the classic LMA group. Sub-optimal ventilation with classic LMA occurred most frequently because of an imperfect seal, and with the laryngeal tube because of obstruction. This drawback of the classic LMA (frequent leak) may be less with the LMA ProSealTM. After completion of this study, Brimacombe and colleagues16 reported a study comparing the efficacy of the laryngeal tube with the ProSealTM. They found that the air leak pressures were similar between the two devices, whereas expired tidal volumes were larger, the end-tidal carbon dioxide concentration was less and the incidence of airway obstruction during anaesthesia was less with the ProSealTM.16 However, these authors used a previous version of the laryngeal tube, so it is not known how the new laryngeal tube performs compared with the ProSealTM.
One concern with the use of the laryngeal tube was that the device might not be tolerated by the patient during emergence from anaesthesia, because of its relatively large proximal cuff. However, as with the classic LMA, the laryngeal tube could be left in place until the patient had regained consciousness and opened the mouth to command, with no complications in most patients.
Another concern was pressure damage to the oropharynx.17 However, the incidence and extent of sore throat, dysphagia, dysphonia, or numb mouth were similar. For example, the incidence of sore throat with the use of the laryngeal tube is similar to that reported after the classic LMA.18 19 Nevertheless, we detected apparent ischaemic changes to the tongue in two of 36 patients in whom the laryngeal tube was used. In these patients, no ischaemic changes were observed after deflating the cuffs.
Two of 18 laryngeal tubes broke during the study. One proximal cuff tore on teeth and one returned from sterilization with the proximal cuff ruptured. It will require more widespread use of the device to determine if there is a problem with durability.
In summary, the laryngeal tube allowed a greater pressure for controlled ventilation compared with the classic LMA, but peak airway pressure was greater and more manipulations were required during maintenance of anaesthesia.
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Acknowledgement |
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References |
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