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© 1999 International Anesthesia Research Society
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Abstract |
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In this randomized, controlled cadaver study, we tested the hypothesis that the standard laryngeal mask airway (LMA) and flexible laryngeal mask airway (FLMA) attenuate liquid flow between the esophagus and pharynx. Fifty fresh cadavers were studied in four LMA groups. Ten female cadavers had a size 4 LMA and 10 had a size 4 FLMA; 10 male cadavers had a size 5 LMA and 10 had a size 5 FLMA; 5 male and 5 female cadavers functioned as controls. The chest was opened, and the infusion set of a pressure-controlled, continuous flow pump was inserted into the esophagus and ligated into place. Esophageal pressure was increased in 2-cm H2O increments. Regurgitation pressure was the esophageal pressure at which fluid was first seen with a fiberoptic scope in the hypopharynx (control group) and above the cuff or within the bowl (LMA groups). This was performed in the LMA groups at 0–40 mL cuff volume in 10-mL increments. Mean (95% confidence interval) regurgitation pressure for the control group was 7 (6–8) cm H2O and for the LMA groups combined was 19 (17–20) cm H2O at 0 mL cuff volume, 47 (41–52) cm H2O at 10 mL, 51 (44–55) cm H2O at 20 mL, 52 (45–56) cm H2O at 30 mL, and 52 (45–55) cm H2O at 40 mL. The increase in regurgitation pressure with increasing cuff volume from 0 to 10 mL was statistically significant (P < 0.0001). Regurgitation pressure was higher for the LMA groups at all cuff volumes compared with the control group (P < 0.0001). There were no differences in regurgitation pressure among the LMA groups. We conclude that the correctly placed LMA and FLMA attenuate liquid flow between the esophagus and pharynx.
Implications: We have shown, in cadavers, that the correctly placed standard and flexible laryngeal mask airways attenuate liquid flow between the pharynx and esophagus.
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Introduction |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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One of the most limiting features of the laryngeal mask airway (LMA) is a lack of airway protection from regurgitated gastric contents (1). Controversy surrounds the theory that the presence of the LMA in the pharynx could promote regurgitation by reducing lower esophageal sphincter tone (2). Owens et al. (3) reported that lower esophageal regurgitation occurred in 54% of anesthetized patients with the LMA, and Valentine et al. (4) reported that lower esophageal regurgitation occurred in 80% of anesthetized patients with the LMA, but Joshi et al. (5) and Bapat and Verghese (6) were unable to detect any evidence of pharyngeal regurgitation. Perioperative regurgitation and aspiration of gastric contents with the LMA are rare clinical events with an approximate incidence of 0.1% and 0.02%, respectively (7). Two factors preventing pharyngeal regurgitation may be the persistent function of the upper esophageal sphincter (8) or mechanical blockade of the hypopharynx by the cuff (1). In cadavers, the LMA does not prevent regurgitation at an esophageal pressure of 78 cm H2O (9), but such high pressures occur only during vomiting (10), and there has been a report of regurgitation occurring when the cuff was deflated (11).
In this randomized, controlled cadaver study, we tested the hypothesis that the LMA and flexible laryngeal mask airway (FLMA) prevent liquid flow between the esophagus and pharynx.
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Methods |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Research and ethical committee approval was obtained, and all patients, or their relatives, consented to postmortem research. Fifty fresh cadavers (6–24 h postmortem) were randomly allocated into four LMA groups and a control group. Ten female cadavers had a size 4 LMA and 10 had a size 4 FLMA; 10 male cadavers had a size 5 LMA and 10 had a size 5 FLMA; 5 males and 5 female cadavers did not have an LMA inserted and functioned as controls. Cadavers with known upper esophageal or laryngopharyngeal pathology were excluded from the trial. A single experienced LMA user (>1500 uses) inserted/fixed the randomized device according to the manufacturer’s instructions (12). The insertion technique included full deflation of the cuff, careful placement of the cuff flat against the hard palate, and pushing the device into and along the posterior palato-pharyngeal curve using the index finger. Airway sealing pressure and fiberoptic positions were documented at zero volume with the cuff fully evacuated and after each additional 10 mL up to 40 mL. Air was used to fill the cuff. The fiberoptic position of the LMA was determined from the mask aperture bars using the following scoring system: 4 = only vocal cords visible, 3 = vocal cords plus posterior epiglottis visible, 2 = vocal cords plus anterior epiglottis visible, 1 = vocal cords not seen (13). The visibility of the hypopharynx was also determined from within the bowl of the LMA. Measurements were made in the supine position with the head/neck in the neutral position and the occiput on a firm pillow 5 cm in height. The airway sealing pressure 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 (14).
After removal of the anterior chest wall to expose the lungs and mediastinum, the esophagus was incised 10 cm below the level of the cricoid cartilage. The infusion set of a pressure-controlled, continuous-flow pump (Arthrex, Innsbruck, Austria) was inserted through the esophageal stump and ligated into position 5 cm below the cricoid cartilage. The pump was accurate to ±2 cm H2O at flow rates of 0–1600 mL/min over a pressure range of 0–300 cm H2O and was calibrated against a water manometer before use. Two fiberoptic scopes were inserted: one was positioned in the laryngopharynx to provide a view of the posterior laryngopharynx and proximal rim of the cuff (supracuff); another was passed through the LMA/FLMA tube and positioned with a view of the bowl of the LMA/FLMA and the glottic inlet (bowl). In the control group, a single fiberoptic scope was positioned in the laryngopharynx to provide a view of the hypopharynx.
The pressure in the esophagus was increased from 0 cm H2O in 2-cm H2O increments every 15 s. In the LMA groups, the esophageal pressure was noted when water first became visible in the supracuff space and/or the bowl of the LMA/FLMA. This measurement was performed at zero cuff volume and after each additional 10 mL up to 40 mL. Between measurements, the water was removed from the pharynx and lungs using the suction port of the fiberoptic scope, the infusion set was opened, and all fluid was drained from the esophagus. Care was taken to avoid displacement of the LMA/FLMA. Any fiberoptic displacement of the LMA/FLMA between measurements was noted. In the control group, the esophageal pressure was noted when water first became visible in the hypopharynx. The accuracy of the fiberoptic detection of fluid was confirmed by noting fluid dripping from the bag of the infusion set. Regurgitation pressure was defined as the esophageal pressure at which fluid was first seen in the hypopharynx (control group) or above the cuff or within the bowl of the LMA/FLMA (LMA groups).
Sample size was based on data from a pilot study of 10 cadavers in which the regurgitation pressure for the LMA and FLMA was measured for a type I error of 0.01 and a power of 0.9. The distribution of data was determined using Kolmogorov-Smirnov analysis. Statistical analysis was with paired t-test (normally distributed data) and Friedmann two-way analysis of variance (nonnormally distributed data). 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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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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There were no demographic differences among groups (Table 1). All LMAs were successfully inserted at the first attempt. Airway sealing pressure and fiberoptic position were generally similar (Table 2). There was a significant increase in airway sealing pressure for all LMA devices with increasing cuff volume from 0 to 10 mL (P < 0.0001) and 10 to 20 mL (P = 0.0001), but there were no significant increases thereafter. There was no fiberoptic displacement of the LMA or FLMA between measurements, and the hypopharynx was not visible in any cadaver in the LMA groups. Regurgitation pressure for the control group was 7 (6–8) cm H2O. Regurgitation pressure was significantly higher for all LMA groups at all cuff volumes compared with the control group (P < 0.0001) (Table 3). The increase in regurgitation pressure with increasing cuff volume from 0 to 10 mL was statistically significant (P < 0.0001) but did not change significantly with further increases in cuff volume. There were no differences in regurgitation pressure among the LMA groups. The regurgitation pressure at which fluid leaked into the bowl and above the cuff was similar for all LMA groups.
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Discussion |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Our data show that the LMA and FLMA provide some protection against pharyngeal regurgitation in the cadaver model. When correctly positioned, the tip of the cuff sits in the cone-shaped hypopharynx immediately above the upper esophageal sphincter and posterior to the cricoid cartilage. Once the cuff is inflated with 10 mL of air, the hypopharynx is effectively blocked until a critical esophageal pressure is reached. In effect, the tip of the cuff and the upper esophageal sphincter function as a ball valve. Adding more air did not increase the efficacy of the blockade. Recently, the efficacy of the seal with the respiratory tract was shown to be primarily related to the matching shape of the cuff and pharynx, rather than to the pressure the cuff exerts against the pharynx (15). This concept may also apply to the seal with the gastrointestinal tract. Regurgitation pressure was similar for both LMA devices, which suggests that the force transmitted along the semirigid shaft of the LMA does not confer any benefit. No hypopharyngeal malpositions were detected in our series, but it is likely that LMA devices only provide protection if the cuff tip is correctly positioned in the hypopharynx.
Strang (9) studied 10 cadavers with the LMA in situ and showed that cricoid pressure prevented regurgitation when the esophagus was filled with a barium solution at a pressure of 78 cm H2O, but when cricoid pressure was released, regurgitation occurred in all cadavers. Our data are consistent with this latter finding, as we showed that 95% of cadavers regurgitate at pressures >55 cm H2O. During spontaneous gastroesophageal reflux, intragastric pressure equals esophageal pressure with the creation of a common cavity (16). Gastric pressure in the starved human in the supine position is normally <10 cm H2O and rarely exceeds 30 cm H2O (17). We have shown that, with 10 mL of air in the cuff, the correctly positioned LMA/FLMA could potentially protect most patients from regurgitation up to esophageal pressures of 30–40 cm H2O. Vanner et al. (8) reported that mean upper esophageal sphincter pressure in nonparalyzed anesthetized patients with the LMA is 35 cm H2O.
Perhaps regurgitated fluid is preferentially directed toward the trachea by the shape/location of the LMA cuff (18). We have shown that regurgitated fluid appears above the cuff and in the bowl at the same esophageal pressure. Strang (9) showed that 60% of cadavers who regurgitated with the LMA also aspirated. In clinical practice, the incidence of regurgitation is more frequent than that of aspiration (0.1% vs 0.02%) (7). Possible explanations for the differences in the ratio of regurgitation to aspiration between cadavers and anesthetized humans are that, in nonparalyzed anesthetized humans, vocal cord closure can still occur, and that respiration may alter the dynamics of the fluid movement.
Cadaver studies should always be interpreted cautiously because they do not mimic the precise conditions found in anesthetized patients. Compared with the anesthetized human, our cadaver model had a lower temperature and more rigid esophago-pharyngeal musculature, and there were no changes in intrathoracic pressure because the chest was open to atmosphere and ventilation did not occur. It is also possible that slight movement of the LMA tip may occur during the respiratory cycle, but this has not been proven. However, there has been a reasonable match between cadaver and noncadaver studies of cricoid pressure (19). Furthermore, we found airway sealing pressures, fiberoptic position, and first-time insertion success rates similar to those in paralyzed, anesthetized humans (15).
We conclude that the correctly placed LMA/FLMA attenuate liquid flow between the pharynx and esophagus.
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Acknowledgments |
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The authors Dr. G. Mikuz, Dr. Alison Berry, and Arthrex (Innsbruck, Austria) for their help.
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References |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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- Brimacombe J, Brain AIJ, Berry A. The laryngeal mask airway: review and practical guide. London:WB Saunders, 1997.
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Rabey PG, Murphy PJ, Langton JA, et al. Effect of the laryngeal mask airway on lower oesophageal sphincter pressure in patients during general anesthesia. Br J Anaesth 1992;69:346–8.
[Abstract/Free Full Text] - Owens TM, Robertson P, Twomey C, et al. The incidence of gastroesophageal reflux with the laryngeal mask: a comparison with the facemask using esophageal lumen pH electrodes. Anesth Analg 1995;80:980–4.[Abstract]
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Valentine J, Stakes AF, Bellamy MC. Reflux during positive pressure ventilation through the laryngeal mask. Br J Anaesth 1994;73:543–5.
[Abstract/Free Full Text] - Joshi GP, Morrison SG, Okonkwo NA, White PF. Continuous hypopharyngeal pH measurements in spontaneously breathing anesthetized outpatients: laryngeal mask airway versus tracheal intubation. Anesth Analg 1996;82:254–7.[Abstract]
- Bapat P, Verghese C. Laryngeal mask airway and the incidence of regurgitation during gynecological laparoscopies. Anesth Analg 1997;85:139–43.[Abstract]
- Brimacombe J, Berry A. The incidence of aspiration associated with the laryngeal mask airway: a meta-analysis of published literature. J Clin Anesth 1995;7:297–305.[Web of Science][Medline]
- Vanner RG, Pryle BJ, O’Dwyer JP, Reynolds F. Upper oesophageal sphincter pressure during inhalational anesthesia. Anaesthesia 1992;47:950–4.[Web of Science][Medline]
- Strang TI. Does the laryngeal mask airway compromise cricoid pressure? Anaesthesia 1992;47:829–31.[Web of Science][Medline]
- Marchand P. A study of the forces productive of gastro-oesophageal regurgitation and herniation through the diaphragmatic hiatus. Thorax 1957;12:189–95.
- Griffin RM, Hatcher IS. Aspiration pneumonia and the laryngeal mask airway. Anesthesia 1990;45:1039–40.[Web of Science][Medline]
- Brain AIJ, Denman WT, Goudsouzian NG. LMA instruction manual. San Diego, CA:Gensia, 1995.
- Joshi S, Sciacca RR, Solanki DR, et al. A prospective evaluation of clinical tests for placement of laryngeal mask airways. Anesthesiology 1998;89:1141–6.[Web of Science][Medline]
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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] -
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] - Dent J, Dodds WJ, Friedman RH, et al. Mechanism of gastroesophageal reflux in recumbent asymptomatic human subjects. Invest 1980;65:256–67.
- Holloway RH, Hongo M, Berger K, McCallum RW. Gastric distension: a mechanism for postprandial gastroesophageal reflux. Gastroenterology 1985;89:779–84.[Web of Science][Medline]
- Riddell PL, Philpott B, Brain AIJ, et al. Aspiration pneumonia and the laryngeal mask airway. Anaesthesia 1991;46:418–9.[Medline]
- Brimacombe J, Berry A. Cricoid pressure. Can J Anaesth 1997;44:414–25.[Web of Science][Medline]
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