Anesth Analg 1999;88:1418
© 1999 International Anesthesia Research Society
BRIEF COMMUNICATION
Mucosal Pressure, Mechanism of Seal, Airway Sealing Pressure, and Anatomic Position for the Disposable Versus Reusable Laryngeal Mask Airways
C. Keller, MD*, and
J. Brimacombe, MB, ChB, FRCA, MD
*Department of Anaesthesia and Intensive Care Medicine, Leopold-Franzens University, Innsbruck, Austria; and
Department of Anaesthesia and Intensive Care, Cairns Base Hospital, Cairns, Australia
Address correspondence and reprint requests to Dr. J. Brimacombe, Department of Anaesthesia and Intensive Care, Cairns Base Hospital, The Esplanade, Cairns 4870, Australia. Address e-mail to 100236,2343{at}compuserve.com
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Introduction
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Adisposable laryngeal mask airway (DLMA) constructed from medical-grade polyvinyl chloride has recently been introduced. Published data suggest that the DLMA performs similarly to the classic, silicone-based, reusable laryngeal mask airway (LMA) (1,2). Although the dimensions of the two LMA devices are identical, the DLMA tube is more rigid and the cuff less compliant (1). We considered that these structural differences would influence the way the cuff interacted with the pharyngeal tissues. We therefore tested the hypothesis that mucosal pressures, the mechanism of the seal, and the changes in airway sealing pressure (ASP) and anatomic position (judged fiberoptically) with increasing cuff volume differ between the two devices.
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Methods
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Twenty ASA physical status I or II adult patients were randomly allocated to receive either the LMA or DLMA for airway management. Patients were excluded if they were at risk of aspiration. Mucosal pressures were measured using seven 1.2-mm diameter strain gauge silicone microchip sensors (Codman® MicroSensorTM; Johnson and Johnson Medical Ltd, Bracknell, UK) attached to the external surface of the DLMA/LMA with clear adhesive dressing 0.45 mm thick (TegadermTM; 3M Health Care, St. Paul, MN) at the following locations (corresponding mucosal areas): anterior middle part of the cuff side (pyriform fossa); the posterior tip of cuff (hypopharynx); anterior base of cuff (base of tongue); posterior middle part of the cuff side (lateral pharynx); backplate (posterior pharynx); Posterior Tube 1 (distal oropharynx); and Posterior Tube 2 (proximal oropharynx). The sensor was orientated toward the mucosal surface and was accurate to 2% (3). The position, orientation, and accuracy of all the sensors was checked in vitro before and after use in each patient.
Anesthesia was induced with propofol 2.5 mg/kg and was maintained with 100% O2 and sevoflurane 1%–2%. Muscle relaxation was achieved with atracurium 0.5 mg/kg. An experienced DLMA/LMA user inserted and fixed the DLMA/LMA according to the manufacturer's instructions (4). A size 4 DLMA/LMA was used for all patients. The pilot balloon was attached via a three-way tap to a 10-mL syringe and a calibrated pressure transducer. The intracuff pressure was reduced to –55 cm H2O in vitro. Mucosal pressure, ASP, and fiberoptic position were documented at zero volume and after each additional 10 mL up to 30 mL. The fiberoptic position was determined by using an established scoring system (5). Measurements were made with the patient in the supine position with the occiput on a firm pillow 7 cm in height. ASP was measured by closing the expiratory valve of the circle system at a fixed gas flow of 3 L/min and noting the airway pressure at which the dial on the aneroid manometer reached equilibrium (6). The mechanism of seal was determined by assessing the relationship between ASP and mucosal pressure. A significant positive correlation suggests that the seal is formed by pressure against the mucosa, and a lack of correlation suggests that the seal is formed by the matching shape of the two surfaces (3).
Statistical comparisons of ASP, fiberoptic score, and mucosal pressure were made from data taken over the inflation range (0–30 mL). Statistical analysis was performed by using a paired t-test, Friedman's two-way analysis of variance, regression analysis,  2 test, and Pearson product-moment correlation coefficient. Unless otherwise stated, data are presented as mean (95% confidence intervals). Significance was taken as P < 0.05.
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Results
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All DLMAs/LMAs were inserted on the first attempt. The position, orientation, and accuracy of the sensors were identical before and after use. Demographic and overall data are presented in Table 1. Mucosal pressures over the inflation range were higher for the LMA at the base of the tongue (8 vs 5 cm H2O; P = 0.04) and were higher for the DLMA in the proximal oropharynx (1 vs 15 cm H2O; P < 0.0001). The highest mucosal pressures were with the DLMA in the proximal oropharynx at maximal cuff volume (Table 2). ASP for both devices increased with increasing intracuff volume from 0 to 10 mL (LMA P = 0.002; DLMA P < 0.0001) and 10 to 20 mL (LMA P = 0.004; DLMA P = 0.001), and it decreased from 20 to 30 mL (LMA P = 0.002; DLMA P = 0.005). There was no correlation between mucosal pressure and ASP at any location for either device.
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Table 2. Airway Sealing Pressures (ASP), Fiberoptic Score (FOS), and Mucosal Pressures with Increasing Cuff Volume
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Discussion
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Our data show that mucosal pressures are generally similar for the LMA and DLMA. However, mucosal pressures were higher for the LMA at the base of the tongue and higher in the proximal oropharynx for the DLMA. The former finding may be related to the softer tube of the LMA tube being curved more anteriorly, thus compressing the proximal cuff more firmly against the base of the tongue. The latter finding is probably related to the stiffer tube of the DLMA pressing more firmly into the anterior body of the cervical vertebrae. It has been recommended that tracheal mucosal pressures be <41 cm H2O (30 mm Hg) for safe prolonged intubation (7). This value was not exceeded for either device at any location. This suggests that the incidence of pharyngolaryngeal trauma will be low, provided that no mucosal damage is caused during placement or removal of the devices. Pressures exerted against the soft palate were not measured, but it is likely that these would be higher with the DLMA, which might have implications for uvula trauma.
In our previous study (3), we used a size 5 LMA and showed that the conformity of the cuff with the pharynx, rather than the pressure the cuff exerts on the pharyngeal mucosa, determines the efficacy of the seal. In the present study, we confirmed this for the size 4 LMA and showed that the mechanism of seal is similar for the DLMA. It is known that the ASP with the LMA is optimal and submaximal cuff volumes (8). Our data confirm these findings for the DLMA and show that the relationships between cuff volume and ASP, and cuff volume and anatomic position, are similar between devices.
We conclude that mucosal pressures for the LMA and DLMA are generally similar. The mechanism of seal for both devices depends on the degree of conformity with the pharyngeal tissues, rather than pressure against the mucosa. The changes in ASP and anatomic position with increasing cuff volume are similar between devices.
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References
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Brimacombe J, Keller C, Morris R, Mecklem D. A comparison of the disposable versus the reusable laryngeal mask airway in paralyzed adult patients. Anesth Analg 1998;87:921–4.[Abstract/Free Full Text]
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Verghese C, Berlet J, Kapila A, Pollard R. A clinical assessment of the single use laryngeal mask airway the LMA-UNIQUETM. Anaesth 1998;80:677–9.
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Brimacombe J, Keller C. A comparison of pharyngeal mucosal pressure and airway sealing pressure with the laryngeal mask airway in anesthetized adult patients. Anesth Analg 1998;87:1379–82.[Abstract/Free Full Text]
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Brain AIJ, Denman WT, Goudsouzian NG. LMA instruction manual. San Diego, CA:Gensia, 1995.
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Brimacombe J, Berry A. A proposed fiber-optic scoring system to standardize the assessment of laryngeal mask airway position [letter]. Anesth Analg 1993;76:457.
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Keller C, Brimacombe J, Keller K, Morris R. A comparison of four methods for assessing airway sealing pressure with the laryngeal mask airway in adult patients. Br J Anaesth 1999;82:286–7.[Abstract/Free Full Text]
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Lewis FR, Schlobohm RM, Thomas AN. Prevention of complications from prolonged tracheal intubation. Am J Surg 1978;135:452–7.[Web of Science][Medline]
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Keller C, Puehringer F, Brimacombe J. The influence of cuff volume on oropharyngeal leak pressure and fibreoptic position with the laryngeal mask airway. Br J Anaesth 1998;81:186–7.[Abstract/Free Full Text]
Accepted for publication March 5, 1999.
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Introduction
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