Royal United Hospital, Combe Park, Bath, UK
Corresponding author
Accepted for publication: June 25, 2003
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Abstract |
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Methods. Thirty-two patients were studied using a randomized cross-over design. The primary outcome measure was airway seal pressure. Secondary outcome measures included peak and plateau airway pressures, time to achieve an airway, ease of insertion, airway manipulations required to achieve a patent airway and grade of fibre-optic laryngoscopy. The proportion of patients in whom good, fair or failed ventilation was achieved was also calculated.
Results. No significant difference was found in regard to seal pressure (PLMA, median 26.5 cm H2O, range 1040; LT, median 24, range 640; P=0.7, 95% confidence interval of the difference 3.5 to 4.0). There were two failures of insertion or ventilation in the LT group and none in the PLMA group. The peak airway pressure with the PLMA was lower than with the LT but the difference was clinically unimportant (PLMA, mean 16.2 cm H2O, SD 3.52; LT, mean 17.9, SD 5.21; P=0.02, 95% confidence interval of the difference 3.1 to 0.28). The PLMA took significantly less time to insert than the LT (PLMA, median 18.5 s, interquartile range 1426; LT, median 22, interquartile range 1536.5; P<0.02, 95% confidence interval of the difference 21.5 to 1.0). The PLMA gave a significantly better view on fibre-optic laryngoscopy than the LT (P<0.001, 95% confidence interval of the difference in grade 2.0 to 1.0). In the 16 patients in whom the PLMA was used during maintenance of anaesthesia ventilation was good in 15, fair in none and failed in one. The equivalent figures for the LT were good in nine, fair in six and failed in one (P=0.009). There was no significant difference in the plateau airway pressure, ease of insertion of the devices, number of manipulations required to achieve or maintain an airway, or in overall complications.
Conclusion. The two devices performed equally well in terms of seal pressure. The PLMA was quicker to insert. Efficacy of ventilation was significantly better with the PLMA than the LT. The PLMA allowed a significantly better view of the larynx with a fibre-optic laryngoscope, and may therefore be of more use in cases where visualization of the larynx is required.
Br J Anaesth 2003; 91: 67883
Keywords: equipment, masks laryngeal; equipment, tubes laryngeal
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Introduction |
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While there have been several studies comparing the classic laryngeal mask airway and LT, until recently all have used versions of the LT that are now obsolete.35 We have recently compared the classic laryngeal mask airway with the current LT during controlled ventilation.6 Performance of the two devices was similar; however, the airway seal pressure with the LT was higher than with the classic laryngeal mask airway and comparable to those in published studies with the PLMA.1 2
The aim of this study was to compare the seal pressure of the LT with that of the PLMA in paralysed patients during the entire course of anaesthesia. Secondary outcome measures were designed to examine the relative clinical utility of each device.
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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 at 47 µg ml1 and fentanyl 1 µg kg1. Neuromuscular block was produced by rocuronium 0.60.9 mg kg1 and was confirmed using a peripheral nerve stimulator (train of four count=0) before airway manipulation. 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 LT or the PLMA was used first for airway management. Randomization was by the use of 32 sequentially numbered, sealed opaque envelopes; 16 contained a slip of paper on which was written the letters PLMA and 16 the letters LT. These envelopes were opened after induction of anaesthesia and the indicated device was then used first.
Insertion of the device
The LT was inserted following the manufacturers instructions. 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 LT was placed against the hard palate behind the upper incisors and the device was inserted in the centre of the mouth until resistance was felt. If no resistance was felt the LT was positioned with the second bold positioning line on the tube between upper and lower incisors. The cuffs were inflated using a cuff inflator (VBM, Medizintechnik, Sulz, Germany) until the intracuff pressure reached 8090 cm H2O and was then reduced to 60 70 cm H2O.9 A size 4 was used for those between 155 and 180 cm and a size 5 for those above 180 cm in height.9
The PLMA was inserted according to the manufacturers instruction manual.10 An introducer was used in all cases. The back of the cuff was lubricated with KY jelly. A size 4 mask was used in females and a size 5 in males.11 The cuff was inflated using the same cuff inflator until the intracuff pressure reached 6070 cm H2O.10
Maintenance of the airway
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. For the LT, the position was also allowed to be adjusted, by gently pushing or pulling the device as recommended by the manufacturer.9 After any manoeuvre, adequacy of ventilation was reassessed. If it was not possible to insert the device or ventilate through it, one more attempt at insertion was allowed. If placement failed after two attempts, study of that device was abandoned and the second device was studied.
The time for insertion of the airway (from the 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.
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. The peak airway pressure was not allowed to exceed 40 cm H2O.
Ventilation of the lungs
After securing the device, controlled ventilation in oxygen and air was started with a delivered 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. If the expired tidal volume was below 7 ml kg1, the delivered tidal volume was increased up to 10 ml kg1 in an attempt to achieve this level. Four tests of ventilation were applied: (i) adequate chest movement; (ii) an expired tidal volume of 7 ml kg1; (iii) stable oxygenation; and (iv) square-wave capnography. If all tests were passed, ventilation was recorded as optimal. Ventilation was recorded as suboptimal if any one test was not passed.
Fibre-optic inspection of airway position
A fibrescope was inserted to assess airway positioning. The fibre-optic view was assessed with the tip of the fibrescope in the bowl of the PLMA and at the orifice of the LT. In the case of the LT, the view from both ventilation orifices was inspected and the best view recorded. The view was graded from 1 to 4: 1=vocal cords fully visible; 2=vocal cords partially visible or arytenoid cartilages visible; 3=epiglottis visible; 4=no laryngeal structures visible.12
Crossover
After the above tests had been completed for the first device, neuromuscular paralysis was reconfirmed (train of 4 count=0). The first device was removed and the second device was studied as described above. If airway obstruction occurred during maintenance of anaesthesia, manoeuvring the position of the device or the patients head and neck or removal and reinsertion of the device was allowed. The number of manipulations of the device during insertion and maintenance of anaesthesia was recorded for each device.
Ventilation efficacy
The second device was used for the duration of anaesthesia and we investigated the success rate for each device in establishing and maintaining a patent airway allowing ventilation without complications (efficacy). 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 without complications; and failed=failed insertion or abandonment of use.
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 had regained consciousness and had responded to a 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 LT were deflated, whereas the cuff of the PLMA was not (in accordance with the manufacturers recommendations).10 At removal, the presence or absence of secretions interfering with airway management and of blood on the device was recorded. Complications, defined a priori, that occurred during induction of, maintenance of and emergence from anaesthesia from the use of the device were recorded.
Postoperative period
After operation, each patient was questioned to determine whether airway complications were present. Because of the crossover design of the study, these data were not used for analysis.
Statistical analysis
The primary aim of the study was to compare the leak pressures between two groups. On the basis of the results of our previous studies, power analysis showed that the study had 80% power to detect a difference in seal pressure of 5 cm H2O. Secondary outcome measures included peak airway pressure, time taken to achieve an airway, ease of insertion, number of airway manipulations required to achieve an airway, ventilation efficacy and grade of view on fibre-optic inspection of the larynx.
Insertion time and seal pressure data were not normally distributed and the Wilcoxon signed rank test was used. The 2 test or Fishers exact test was used as appropriate to compare the grade of fibre-optic view of the larynx, numbers of manipulations, complications and efficacy of ventilation between groups. The data for peak airway pressure, plateau pressure and device intracuff pressure were normally distributed and the paired 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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Discussion |
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The crossover design of this study increased its power, allowing us to treat each patient as their own control and answer our clinical question while studying the minimum number of patients. The success rate of obtaining a patent airway through both the PLMA and the LT was high and in keeping with recent reports on both devices.6 Ventilation was also adequate in most cases. Both devices required few manipulations during anaesthesia and were tolerated well during emergence, but the numbers studied were too small to make robust conclusions about stability, tolerability or safety.
There have been several other studies of LT performance and indeed two in which the LT was compared with the PLMA.13 14 However, modifications to the design of the LT were carried out in September 2001 and only one study (currently in press) has used the LT that is now commercially available.6 Whether the results from previous studies using older devices and prototypes can be extrapolated to the current device is unknown.
Two recent studies have compared the performance of the LT with that of the PLMA. Brimacombe and colleagues13 compared the devices in similar conditions to this study, with controlled ventilation in paralysed patients. They found that the airway seal pressures were similar between the two devices, whereas expired tidal volumes were larger, end-tidal carbon dioxide concentration lower and the incidence of airway obstruction during anaesthesia lower for the PLMA than for the LT. Although these authors used the previous version of the LT, their results are in broad agreement with ours.
Figueredo and colleagues14 compared the two devices during anaesthesia in 70 patients breathing spontaneously. First-attempt insertion rates were poorer for the LT than for the PLMA (but low in both groups compared with our results) and the LT group showed a lower tidal volume and less frequent hands-free ventilation. More airway manoeuvres were required in the LT group to maintain a patent airway. There are several criticisms of this study. First, the PLMA is a device primarily designed for use during controlled ventilation15 and it has been commented that in order for its performance to be optimized it should be used in this manner.16 Previous versions of the LT have performed particularly poorly during spontaneous ventilation,5 and while the new version probably performs better its role in patients breathing spontaneously has yet to be established.6 Compared with the large airway orifice of the PLMA, the small gas exchange orifices of the LT and the fact that these orifices are frequently not positioned directly over the laryngeal inlet are likely to increase the work of ventilation during spontaneous breathing.
In a recent study6 we noted that the peak airway pressure with the LT was higher than with the classic laryngeal mask airway. We speculated that this might be due to the higher resistance of the small airway orifices of the LT, or the possibility that the LT was not lying over the laryngeal inlet. A novel feature of this study is that the position of the airway devices was examined fibre-optically. The vocal cords were visible in 91% of cases with the PLMA and in only 42% of cases with the LT. The positioning of the PLMA is in keeping with or better than that reported in other studies.1 2 In a recent report, Genzwuerker and colleagues17 reported high levels of success with visualization of the larynx and airway instrumentation in 10 patients after practice on a mannekin. They suggested that the LT would be a useful device for catheter exchange tube insertion. Genzwuerker used a previous version of the LT and minor manipulations were needed in four of 10 patients to optimize the view, whereas we used the currently available device and examined the position of the device once in a functional position. Our findings suggest that the PLMA will perform better than the LT when access to the trachea is required. The relatively narrow diameter of the PLMA (similar to that of the reinforced laryngeal mask airway) is against its use for directly instrumenting the trachea, but the lack of grille bars and of an epiglottic elevator (compared with the classic laryngeal mask airway and intubating laryngeal mask airway respectively) may offer advantages for catheter exchange techniques.
While it was not a primary aim of our study to determine the quality of ventilation, we found that our ability to achieve a patent airway and optimal ventilation without subsequent complications (which we call ventilation efficacy) was higher using the PLMA than with the LT. While this finding is potentially important and the difference did reach statistical significance, we believe the finding should be interpreted with caution as it was not the primary aim of this study to examine this aspect of performance. Further studies, appropriately powered, may confirm or refute this.
Two of 18 LTs broke during the study, as has happened in our previous work. No PLMA failed. The durability of the LT would be usefully examined in a longitudinal study of its use.
The recently introduced Laryngeal Tube Sonda (LTS), like the PLMA, incorporates a drain tube to allow separation of the gastrointestinal and respiratory tracts. In future it would be useful to compare the performance of the PLMA and LTS.
We conclude that the LT and PLMA performed equally well in terms of laryngeal seal pressure. The PLMA was slightly quicker to insert. Ventilation was significantly more reliable with the PLMA. The PLMA allowed a significantly better view of the larynx with a fibre-optic laryngoscope, and may therefore be of more use in cases where visualization or access to the larynx is required.
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Potential conflict of interest |
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References |
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