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Original papers

The laryngeal mask and VBM laryngeal tube compared during spontaneous ventilation. A pilot study

Miller, D. M.; Youkhana, I.; Pearce, A. C.

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European Journal of Anaesthesiology: September 2001 - Volume 18 - Issue 9 - p 593-598

Abstract

Introduction

The design of the laryngeal tube (VBM Medizintechnik GmbH, Sulz, Germany) (LT) is based on the oesophageal obturator airway. The oesophageal-tracheal Combitube® was presented as an improvement of the obturator airway, providing an open oesophageal tube to relieve the stomach. It was designed to provide positive-pressure ventilation of the lungs by means of its blind insertion into the mouth, pharynx and oesophagus or trachea of unconscious patients in need of ventilatory support [1,2]. Likewise, the laryngeal tube airway device (Figure 1) is designed to be inserted blindly into the oesophagus. It comprises a curved tube, slightly ‘S’ shaped, with two cuffs, one for sealing gas leakage ‘downwards’ via the oesophagus and the other for sealing gas leakage ‘upwards’ via the mouth. The upward sealing cuff should be located at the base of the tongue and pharynx. The transmission of gases between the airway tube and the larynx takes place via an anterior opening in the tube between the two cuffs.

Figure 1.
Figure 1.:
Diagram (a) and photograph (b) of the laryngeal tube showing small oesophageal cuff and larger pharyngeal cuff and slightly ‘S’-shaped tube with one anterior opening.

Trials of the device in controlled ventilation of the lungs demonstrate that the device is effective [3] when used for this purpose. Nevertheless, the manufacturers' claim that it is also an effective device during spontaneous ventilation as well. This is important to confirm if it is to be considered a viable alternative airway to the laryngeal mask.

Methods

Regional Ethics Committee approval and written consent from participants was obtained to conduct this study using patients undergoing minor surgery during general anaesthesia. Up to 60 fasted healthy adult patients, of ASA status II or I and above, were recruited above the age of 17 years. Patients were excluded if they were at risk for pulmonary aspiration or suffered from diabetes mellitus, morbid obesity, gastroesophageal reflux or were pregnant. The patients chosen for the study were suitable for using a spontaneous ventilation mode of breathing during general anaesthesia.

Patients were enrolled in the study into one of two groups in a randomized, single-blinded protocol. One group was to receive a laryngeal mask for airway management and the other group the laryngeal tube. Two studies were conducted, one by each of two anaesthetists in order to assess the existence of user bias. To minimize the effect of a possible learning curve bias, the anaesthetists were allowed five practice comparisons of each laryngeal mask airway and laryngeal tube to gain experience with the device before the trial began. The results of the comparisons were then entered into a sequential analysis chart. Both anaesthetists had greater than 10 years' experience. Airway size was determined by the physical characteristics of the patient. Mallampati scoring in all patients was recorded [4].

General anaesthesia was induced identically in both groups with fentanyl 1 µg kg−1 followed a minute later by propofol 2–3 mg kg−1 or more if required. One minute later either a laryngeal mask (LM) or laryngeal tube was placed for airway management. Size 3.0 or 4.0 laryngeal masks were used for females, and size 4.0 or 5.0 laryngeal masks for males, and insertion was performed as outlined in the manufacturers' instruction booklet. Twenty millilitres of air was injected into the size 3 LM, 30 mL into size 4 and 40 mL into size 5 LM. The patient's weight and size, as outlined in the manufacturers' instruction booklet, determined the sizes of laryngeal tubes (3–5). The laryngeal tube was inserted to a depth indicated by the recommended marks on the tube, which correspond to the level of the teeth. The laryngeal tube has two cuffs that must be inflated utilizing a cuff pressure inflation device with gauge, which was supplied by the manufacturer. The pharyngeal cuff was inflated to at least 80 mmHg then deflated to the desired pressure between 60 and 70 mmHg. Next, the oesophageal cuff was over pressurized then deflated to the desired pressure of 60–70 mmHg. When insertion of the laryngeal mask or laryngeal tube was unsuccessful, the patient received a supplemental dose of propofol up to 1 mg kg−1, and the head repositioned and a further attempt made. If the laryngeal mask or laryngeal tube could not be placed on the third attempt, it was recorded as a failure and the study was terminated. The airway was then secured in the most suitable manner (usually by means of a laryngeal mask) determined by the anaesthetist. Anaesthesia was maintained with nitrous oxide and isoflurane in oxygen in the usual way. Ventilation of the lungs was manually assisted until respiratory efforts were regular. The patients were then allowed to breathe spontaneously throughout the procedure and no muscle relaxants were administered. Supplemental fentanyl, 50–100 µg intravenously, was given if analgesia was inadequate and patient movement occurred at the time of surgical stimuli. Regional blocks were used as appropriate for pain relief.

A sequential analysis was chosen, as there were no previous studies to indicate whether the device was appropriate for spontaneous breathing. This technique is an application of the binomial distribution and involves the use of charts suitable for use only in an either/or situation. The assessment was based simply on a decision that airway A was better, the same or worse than airway B. Pairs of patients were entered into the trial, and one received A and the other B, according to a random selection process. The sequential analysis chart that was chosen had a P-value=0.01. The results obtained from each of the two anaesthetists were recorded separately.

Ease of establishment of the airway was measured using the following criteria and scoring system. These were used to determine satisfaction with the resultant airway during spontaneous ventilation only. For each variable there was a score of zero if there were no problems or one if there was a problem. If the use of the airway had to be abandoned on induction or if airway control was lost intraoperatively, after having secured a satisfactory airway initially, it scored a maximum of five automatically. The criteria may be summarized:

  • • Time to insert the airway device: > 10 s=1; < 10 s=0;
  • • Further manipulation of the device or patient after initial insertion in order to secure a clear airway: manipulation required=1; not required=0;
  • • Number of insertions to secure the airway: more than one attempt=1; first attempt=0;
  • • Lowest oxygen saturation: < 95%=1; never below 95%=0;
  • • Abandonment of technique on induction or loss of airway intraoperatively: Score=5.

A score of four or less was considered a success. The ceiling maximum score was five.

The inflation pressures at which gas began to leak around the cuffs of the two devices were measured. The comparison was subjected to analysis by Student's t-test after an F-test was used to ensure a sufficiently small variance ratio necessary to qualify similarity of population groups.

Results

Demographic variables were evenly distributed between the two groups with a two-thirds male preponderance, a mean age of 46 ± 19.5 years (19–81 years) and mean body weight of 75.6 ± 12.7 kg (62–103 kg). Mallampati scores were equivalent in both groups without any patient exceeding a score of three in either group.

The detailed results of the airway scores are recorded in Table 1. For the first five laryngeal tube ‘learning curve’ patients a collective 90% failure rate occurred. The subsequent findings revealed a preferable score in the case of the laryngeal mask in all of the next seven pairs in the case of anaesthetist A. The superiority of the laryngeal mask was demonstrated at the 99% confidence level. For anaesthetist B, 10 pairs were required to show this level of confidence. Abandonment in the use of the laryngeal tube occurred in five out of seven occasions in the case of anaesthetist A and five out of 10 occasions in the case of anaesthetist B. Pooled results with the 17 comparisons using Student's t-test for two samples assuming unequal variances gave P < 0.000001. In each case in which the laryngeal tube had to be abandoned, rescue using the laryngeal mask was achieved successfully. On two occasions, satisfactory positive-pressure ventilation of the lungs using the laryngeal tube was possible despite the unsatisfactory airway noted during spontaneous ventilation with evidence of partial airway obstruction. This was characterized by ‘see-saw’ movements of chest and abdomen, and CO2 retention. Removal of the laryngeal tube and replacement by a laryngeal mask resulted in a many fold increase in tidal volume and a falling PCO2. Anaesthetist B obtained a satisfactory airway in seven out of 10 patients using manual controlled ventilation of the lungs with the laryngeal tube.

Table 1
Table 1:
The scores obtained with each device based on the criteria stated in the text. The first five pairs of results are in parentheses as they were considered the learning component of the study and not included in the 17 pairs of the sequential analysis. They are shown here as a record of the learning curve using the LT device

We had difficulty in differentiating between laryngospasm and mechanical obstruction as a cause of airway obstruction. We estimated that laryngospasm occurred using the laryngeal tube, in two out of the seven occasions with the technique employed by anaesthetist A. With anaesthetist B, this occurred only once on insertion during the 10 cases. With the five practice uses of the laryngeal tube, the estimated occurrence of laryngospasm with anaesthetists A and B was in two and three occasions respectively.

Respective cuff leakage inflation pressures achieved using the laryngeal tube and laryngeal mask were 30.6 ± 7.1 cmH2O and 23.3 ± 8.1 cmH2O which were significantly different, P < 0.05.

Discussion

As an airway device for use in spontaneous ventilation accompanied by a rescue rate of 50% (anaesthetist B) or greater (anaesthetist A), the laryngeal tube is clearly not an acceptable alternative to the laryngeal mask. This double pilot study, with two sequential analyses having the same result provides good evidence to support this statement. The study with anaesthetist B utilized a slightly different protocol for inserting the laryngeal tube with only a slight improvement to obtain the same result, 10 as opposed to seven comparisons to show statistical significance at the 99% confidence level.

The laryngeal tube was used in a manner that expected it to be an equivalent device to the laryngeal mask, i.e. special airway manoeuvres to overcome the difficulty with securing a satisfactory airway were not performed. The adjustments recommended by the manufacturer included pushing the airway in or out to find the ideal position. This was partially effective. It may be argued that insufficient time to learn how to use the device was provided for in our study. This could be a valid criticism, as there appeared to be some evidence that the success rate using the laryngeal tube was increasing in the case of anaesthetist B throughout the study. However, such an extended learning curve is not present in the use of the laryngeal mask, which is more successful in less experienced hands than the anaesthetists involved in this trial [5,6]. Is it ethically justifiable to use a device such as the laryngeal tube that requires more extensive training than is shown here? The point of the sequential analysis is to limit the extent of investigations, which are obviously not beneficial to patients. Moreover, user variability is also present, with anaesthetist B being a little more successful during the trial. Anaesthetist B had already a 100% failure in the first five practice attempts using the device according to the suggested protocol and therefore varied the technique of insertion with immediate improvement. He partially inflated the lower cuff first so that its position was located above the cricopharyngeus muscle. This located the position of the laryngeal tube at a more reliable level. Nevertheless, there was still great difficulty in maintaining a good airway with a 50% rescue rate.

Another deviation from the protocol by anaesthetist B was that the manufacturer's method of insertion using the midline approach was found to be difficult. A much easier and successful method of inserting the device was to pass it alongside the tongue and to avoid the midline. This also decreased the chance of producing laryngospasm. Anaesthetist B found that the difference was most notable between the technique used in the first five and the next 10 uses of the laryngeal tube. Laryngospasm was more commonly encountered when inserting the laryngeal tube compared with the laryngeal mask.

What was most distressing when using the laryngeal tube was the loss of airway control during the procedure. This happened on two occasions. Anaesthetist A followed the protocol strictly, whereas anaesthetist B had developed a different approach after the first five practice comparisons and therefore used a different protocol for the trial. It is likely that had anaesthetist B followed the same protocol as used by anaesthetist A, he may have encountered the same degree of failure. After all, he encountered 100% failure with the first five practice attempts when using the recommended protocol.

We propose three mechanisms to explain the loss of airway control intraoperatively. The one relates to the fact that the lower cuff was not fully inflated when anaesthetist B inserted it initially. The cuff was passed to the level of the upper oesophageal sphincter. As nitrous oxide diffuses into the cuff, the pressure rises and the cuff inflates further. As it does so, if it is slightly below or above the middle of the cricopharyngeus sphincter, it may pull the device inwards or push it further out. If it pulls it inwards, the anterior aperture in the laryngeal tube may be obstructed in the upper oesophagus. If it moves in the outward direction, the lower cuff may become obstructive to the laryngeal opening.

Deflating the lower cuff appears to clear the airway obstruction; inflating the cuff again produces obstruction. Inflating the cuff causes the epiglottis to fold downwards and to close against the glottis. This is the natural response to swallowing a bolus of food and is likely to be due to the effect of the lower cuff forcing the larynx in an anterior direction. The oesophageal cuff in the case of the Combitube® is located lower in the oesophagus and is therefore less likely to interfere with the epiglottis closing mechanism.

A third possible mechanism is that the epiglottis may obstruct the single anterior opening. If this is the reason, then more than one opening or two lateral openings may result in an improvement. The Combitube® certainly benefits from a series of openings.

On occasions, the use of intermittent positive pressure ventilation of the lungs was successful despite the laryngeal tube needing to be abandoned as an airway during spontaneous ventilation. The burden of an increased work of breathing in the presence of partial airway obstruction is easily overcome by a lung ventilator but not so easily by a patient's respiratory efforts. This may constitute the essential difference between controlled and spontaneous ventilation. Partial airway obstruction increases the work of breathing with resultant respective increase in sub-atmospheric and positive alveolar pressures during inspiration and expiration in both controlled and spontaneous ventilation. In the former, the work is performed by the lung ventilator. During spontaneous breathing it is performed by the patient so resulting in an increase in CO2 production and less CO2 elimination. This may partially explain the more favourable results in the initial study using intermittent positive ventilation of the lungs in human beings. This is understandable, when one bears in mind that one of the early obturator airways, e.g. the Combitube®, was designed for intermittent positive ventilation of the lungs. However, positioning of the Combitube® is not straight forward, requiring considerable training [7]. This problem may even be greater in the case of the laryngeal tube. However, the Combitube® has been used successfully in spontaneous ventilation [8]. The most significant difference in the design is the lower position of the oesophageal cuff already mentioned. It is possible that variants of the obturator airway will make suitable alternatives to the laryngeal mask for use in spontaneous ventilation if the cuff was located lower in the oesophagus. Perhaps, modifying the laryngeal tube to achieve this may constitute an improvement in the laryngeal tube design.

Higher airway pressures could be generated using the laryngeal tube (mean blow off or leak pressure of over 30 cmH2O) than with the laryngeal mask (mean blow off or leak pressure of 23 cmH2O). This finding may be more related to size of laryngeal mask usage than an observation of consequence. The laryngeal tube airway is also possibly safer in the circumstances of regurgitation risk. The cuff pressure in the oesophagus is likely to obstruct regurgitation fluids more effectively and should any fluids get above the cuff, they are less likely to be funnelled into the trachea than in the case of the laryngeal mask.

Airway manoeuvres to improve success with use of the laryngeal tube will no doubt arise and, with a lot more experience, success will improve. If the lower cuff is not inflated, greater success in its use is likely. Used in this way, it would be similar to a cuffed oral pharyngeal airway (COPA airway) or a cuffed pharyngeal tube and the advantage of preventing regurgitation nullified. However, the manufacturers are now producing the laryngeal tube with only one pilot tube (Figure 1 a). As such, it is unlikely to become a commonly used device for use in spontaneous ventilation. We have to conclude, at this stage of its development and experience, that it is not a viable alternative to the laryngeal mask for use in spontaneous ventilation.

Acknowledgments

We are grateful to VBM Medizintechnik GmbH, Sulz, Germany, who loaned us the Laryngeal Tubes and cuff inflation devices for the purpose of this trial.

References

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Keywords:

DEVICE, laryngeal mask airway; RESPIRATION, SPONTANEOUS

© 2001 European Academy of Anaesthesiology