Anew laryngeal mask device, the ProSeal laryngeal mask airway (PLMA; Laryngeal Mask Company, Henley-on-Thames, UK), has been developed with a modified cuff intended to improve the seal and a drainage tube intended to prevent aspiration by providing a bypass channel for regurgitated fluid. The efficacy of seal is approximately 10 cm H2O higher for the PLMA than the classic laryngeal mask airway (LMA) in paralyzed, anesthetized patients (1); however, there are no published data assessing the efficacy of the drainage tube for airway protection. In this randomized, controlled cadaver study, we determined whether the PLMA prevents aspiration of regurgitated fluid.
The PLMA (Figures 1 and 2) is made from medical grade silicone and has the following new or modified features (intended purpose): A, a dorsal cuff (pushes the ventral cuff into the periglottic tissues to improve the seal); B, a drainage tube that travels from the tip, through the bowl and alongside the airway tube (for passage of an ≤18-gauge gastric tube, for venting regurgitated fluid and to provide information about device position); C, a built-in bite block; D, a locating strap on the anterior distal tube (prevents the finger or introducer slipping off the tube); E, the ventral cuff is larger proximally (to improve seal by plugging gaps) and contained posteriorly by a bucket-shaped section of the distal tube; F, an accessory vent under the drainage tube in the bowl (prevents pooling of secretions and acts as an accessory ventilation port; G, a double tube configuration (increases stability); H, a wire-reinforced airway tube (prevents the double tube configuration from being too stiff); I, a deeper bowl than the standard LMA (facilitates a better fit in the pharynx). The PLMA does not have a semirigid backplate and does not have mask aperture bars although the drainage tube functions as a mask aperture bar for the accessory vent.
We studied five male and five female supine cadavers (6–24 h postmortem). Research and ethical committee approval was obtained and patients, or their next of kin, consented to postmortem research. Cadavers with esophageal or laryngopharyngeal pathology were excluded.
After removal of the anterior chest wall, the esophagus was incised 10 cm below the level of the cricoid cartilage. The infusion set of a calibrated, pressure controlled, continuous flow pump (AR-6450; Arthrex, Innsbruck, Austria) accurate to ±2 cm H2O with flow rates 0–1600 mL/min) was inserted through the esophageal stump and ligated into position, 5 cm below the cricoid cartilage. An esophageal pouch was created to prevent water flowing distally. A fiberoptic scope was positioned in the laryngopharynx to provide a view of the hypopharynx. Esophageal pressure (EP) was increased from 0 cm H2O in 2-cm H2O increments every 15 s and the EP was noted from the pump when water first became visible (the control).
An experienced PLMA/LMA user inserted/fixed the PLMA/LMA into each cadaver in random order by using the technique recommended for the LMA. A size 4 PLMA/LMA was used for all cadavers. Two fiberoptic scopes were used; one was positioned in the oropharynx to provide a view of the proximal portion of the cuff and another was passed through the PLMA/LMA airway tube and positioned to provide a view of the laryngopharynx. EP was increased as in the control and the EP noted when water first appeared above and below the cuff. This was performed at zero cuff volume and repeated after each additional 10 mL up to 40 mL. For the PLMA, these measurements were made with the drainage tube clamped (PLMA clamped) or unclamped (PLMA unclamped). When the PLMA drainage tube was unclamped, the EP at which water was seen in the tube was noted. Measurements for the PLMA/LMA were made with the head/neck in the neutral position, and for the control with chin lift applied. Between each mea- surement, the water was removed from the pharynx and lungs and the infusion set opened and all fluid drained from the upper esophagus.
The EP at which fluid was first seen with a fiberoptic scope in the hypopharynx (the control), and above or below the cuff (PLMA clamped/unclamped and LMA), or seen directly in the drainage tube (PLMA-unclamped) was noted. To provide general information about position, the oropharyngeal leak pressure and the fiberoptic position (whether the vocal cord was visible from the distal end of airway tube) were determined at 20 mL cuff volume with the LMA and PLMA unclamped. Statistical analysis was with a paired t-test, Friedman’s two-way analysis of variance, and a χ2 test. Significance was taken as P < 0.05.
The mean age, height, and weight were 76 (range 56–85) yr, 167 (range 156–183) cm, and 68 (range 55–81) kg, respectively. Mean EP without any airway device was 9 (range 8–10) cm H2O. EP was always higher for the PLMA clamped and LMA compared with the control (P < 0.0001) (Table 1). The mean EP for the PLMA unclamped was similar to the control at 10 (range 8–13) cm H2O. For the PLMA unclamped, fluid appeared from the drainage tube in all cadavers at 10–40 mL cuff volume and in 8 of 10 cadavers at zero cuff volume. There was an increase in EP at which fluid was seen above and below the cuff with increasing cuff volume for the LMA from 0 to 10 mL (P < 0.0001), but there were no significant changes, thereafter. There was an increase in EP at which fluid was seen above and below the cuff for the PLMA clamped with each incremental increase in cuff volume (P < 0.01). The EP at which fluid was seen above the cuff was higher for the PLMA clamped than the LMA at 40 mL cuff volume (P = 0.006). The EP at which fluid was seen below the cuff was higher for the PLMA clamped than the LMA at 20–40 mL cuff volume (P < 0.04). For the PLMA clamped and LMA, fluid appeared simultaneously above and below the cuff at all cuff volumes. Oropharyngeal leak pressure was higher for the PLMA (29 [95% CI, 14–40] vs 21 (15–27), P < 0.001), however, the vocal cord visibility was similar (PLMA 6 of 10, LMA 9 of 10).
Our previous study (1) of paralyzed, anesthetized patients showed that the PLMA can provide a more effective seal than the LMA. The current cadaver study shows that the correctly placed PLMA allows fluid in the esophagus to bypass the pharynx and mouth when the drainage tube is open, and attenuates liquid flow between the esophagus and pharynx when the drainage tube is closed. The LMA does not prevent aspiration of regurgitated fluid, but attenuates liquid flow between the esophagus and pharynx, as previously demonstrated (2). Aspiration with the PLMA-unclamped only occurred when the cuff was fully deflated, suggesting that a small volume of air is required to isolate the glottis from the esophagus and to correctly align the drainage tube with the esophagus.
The EP at which liquid flow occurred between the esophagus and pharynx was higher for the PLMA clamped than the LMA at higher cuff volumes. This may be related to the improved airtight seal of the PLMA. Liquid flow between the esophagus and pharynx occurred at progressively higher EPs for the PLMA, but not the LMA at cuff volumes greater than 10 mL. This suggests that the seal around the esophageal inlet progressively increases with increasing cuff volume.
The EP at which liquid flow occurred was sometimes higher than considered safe for the esophagus for both the PLMA and LMA (3). Vanner and Pryle (3) reported esophageal rupture in 2 of 10 cadavers when a cricoid force of 40 newtons was applied and EP was 50 cm H2O. We found no esophageal rupture in this or our previous study (2); however, our esophageal pouch may have been shorter and our cadavers fresher than Vanner and Pryle’s. Esophageal rupture has never been reported with the LMA. Some clinicians might consider clamping the drainage tube as a solution to preventing air leakage. However, air leakage up the drainage tube during positive pressure ventilation usually implies malposition and the PLMA should be reinserted or a different size used. We do not recommend clamping the drainage tube.
Our study was conducted in cadavers and the applicability of our findings to anesthetized patient is uncertain. However, cadavers have been used in cricoid pressure (4) and cervical spine motion studies (5). Brimacombe et al. (6) provided some evidence that pharyngeal compliance is similar in fresh cadavers and paralyzed anesthetized patients. Interestingly, there have been two case reports in which an earlier prototype PLMA prevented aspiration in anesthetized patients (7,8).
We conclude that in the cadaver model, the correctly placed PLMA allows fluid in the esophagus to bypass the pharynx and mouth when the drainage tube is open. Both the LMA and PLMA with a closed drainage tube attenuate liquid flow between the esophagus and pharynx. This may have implications for airway protection in unconscious patients.
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