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Department of Anaesthesia, Royal United Hospital, Combe Park, Bath, England
To the Editor:
I wish to comment on the recent paper by Figueredo et al. (1) in which the authors compared the ProSealTM Laryngeal Mask Airway (PLMA) with the Laryngeal Tube® airway (LT) during spontaneous breathing in 70 anesthetized patients. I am concerned that there are inaccuracies in the report but also that neither device was used to its best effect and that this is reflected in their results.
The authors state that the PLMA and the LT are designed for use "during spontaneous-breathing general anaesthesia." I believe this is misleading. First, the PLMA is designed for use with controlled ventilation (2) and this has been stated by the inventor, Dr. Brain (3). Second, the LT has been reported to perform particularly poorly during spontaneous ventilation (4). Newer versions of the LT (last modified in 2002) perhaps perform better (5). It is unclear from the paper which version of the device Figueredo et al. used.
The authors report lower success at insertion for both the PLMA and LT as noted in the paper. The paper does not state whether the PLMA introducer tool was used for some or all of the cases. There may indeed be a learning curve for placement of the PLMA and it is unlikely that after 10 insertions the authors had reached the top of this (6). This may have contributed to the poor first time insertion with the PLMA. The authors state that with both devices "only one forward pushing movement, without backward movement was allowed." The insertion instructions for the LT state "if ventilation is not sufficient reposition the tube by pushing it either distal or pull proximal (7)." Therefore, for both devices the insertion technique appears to have been suboptimal and this is reflected in the performance of each device.
In the case in which gastric inflation and regurgitation appear to have occurred it is likely that the PLMA was misplaced, as not only was ventilation "insufficient" but passage of a gastric tube via the PLMA drain tube was not possible (8). When these factors occur it is likely that the tip of the PLMA is folded over in the hypopharynx and this will lead to problems that were witnessed here (9). Use of an algorithm to check position may improve performance of the PLMA (9,10).
The authors further state that recommended intracuff pressure with the LT higher than that with the PLMA. The manufacturers of the LT actually recommend an intracuff pressure of 60-70 cm H2O, not 60–80 as reported (7). The manufacturers of the PLMA advise inflation until a "just seal" is achieved with maxima of 30 and 40 mL for a size 4 and 5 PLMA (11). Where these volumes are used it is likely to result in higher pressures in the cuff of the PLMA than those recommended for the LT (12).
The authors claim the LT is designed to prevent aspiration. This is not stated in the product literature. There is no evidence that the LT does protect against aspiration and the recent development of the Laryngeal Tube Sonda (LTS) by the same manufacturers, which incorporates a drain tube similar to the PLMA, makes this clear.
In summary, the authors have not compared the two devices in the modes for which they were designed, insertion technique is likely to have been suboptimal. The results appear to reflect these weaknesses and perhaps do a disservice to both devices.
References
Department of Anesthesiology, Torrecardenas Hospital, Almería, Spain Department of Anesthesiology, Hospital Costa del Sol, Málaga, Spain
In Response:
We thank Dr. Cook for his comments. We welcome the opportunity to provide the following itemized responses to his numerous questions.
1. Neither the ProSeal Laryngeal Mask Airway (PLMA) nor the Laryngeal Tube (LT) contraindicated with spontaneous ventilation; the article by Dr. Brain merely proposed that positive pressure was preferable (1). Nevertheless, the single study in which data had been collected for spontaneous and positive pressure ventilation suggests that both modes of ventilation are effective and, probably, safe (2).
2. That LT has been reported to perform poorly during spontaneous ventilation in one study (3) does not mean that its use should be proscribed in clinical practice or that other clinical trials need not be conducted in this mode of ventilation.
3. The new version of the laryngeal tube (LTS) (4) has been available in Europe since June 2002 (5). Our study commenced at the beginning of 2002, when the second version of the LT (6) was in use. Clearly, had we used the new modified version (LTS) we would have stated it as such in the manuscript.
4. The paper does not state whether the PLMA introducer tool was used. The simple answer is because it was not used. So as to compare the ease of insertion, this needed to be done in an equivalent manner in both devices and, in all the cases, the index finger method was used.
5. We do not presume to have reached the top of the learning curve after 10 insertions before starting our trial but, surely, neither have Cook et al. who, according to their own publication (7) had inserted the PLMA a minimum of five times before starting the trial. In our trial we considered it more important to have had an equivalent experience-of-use of both devices. The learning curve was not one of the objectives of the study.
6. To evaluate the ease of insertion of the devices we employed a strict methodology so that we would be able to detect significant differences (if any) between groups using a smaller number of patients. It is curious that our first-time-right insertion rate of 77% with the PLMA was considered "poor" by Dr. Cook while, in his own study (7), the first-attempt success rate with the same device was only marginally better at 81%. A quick statistical comparison indicates that the difference between the values in the two studies (77% vs 81%) is not significant (
2 test; P = 0.21) nor are there differences in the numbers of failures after the three allowed attempts (i.e., 0% vs 1%, in the two studies, respectively).
7. In one of the patients in whom we had observed gastric insufflation together with insufficient ventilation, we were unable to propose a mechanism to explain the insufflation (the patient was in spontaneous ventilation). Using the drainage tube test we detected the malposition of the PLMA and we had stated in the manuscript that "the problem was solved by mobilizing the device."
8. At the time of preparing our study protocol, we were guided by the instructions manual provided in 2000, as we had stated in the article. Not being prescient, we can be forgiven for not intuiting the present recommendations. So that both groups were approached in an equivalent manner, we inflated the cuffs of both devices up to a predetermined pressure independently of the volumes necessary. The equivalence between the devices in the pressure exerted against the pharyngeal mucosa is an essential condition to evaluate, impartially, the incidence of postoperative pharyngolaryngeal morbidity (8,9).
9. Our starting point, that the LT was designed to prevent aspiration, was based on the article which described the device to the scientific community (10). More recent studies clearly establish that "the LT was designed to prevent the risk of regurgitation by blocking the esophagus" (11). The improvements introduced into the new LTS do not absolve the intents and purposes with which the previous version was manufactured.
10. In summary, we believe that we reported a correctly performed study, with the added merit of being the first to compare these devices in one of the accepted modes of ventilation. The insertion technique was correct, according to the manufacturer’s instruction, but with a more scrupulous method of evaluation of the outcomes. The awareness of the deficiencies in the performance of a device is what should stimulate a manufacturer to improve its products while our function as clinical investigators is to try to detect, with impartiality, "these types of weaknesses."
References
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