Anesth Analg 2003;96:1821-1822
© 2003 International Anesthesia Research Society
GENERAL ARTICLES
Storage Capacities of the Laryngeal Mask and Laryngeal Tube Compared and Their Relevance to Aspiration Risk During Positive Pressure Ventilation
Donald M. Miller, MB ChB, FFA(SA), PhD, and
Derek Light, HND(MechEng)
Department of Anaesthetics, Guy’s, King’s and St. Thomas’ School of Medicine, King’s College, London
Address correspondence and reprint requests to D. M. Miller, GKT Department of Anesthetics, 2nd Floor, NGH, Guy’s Hospital, London SE1 9RT, UK. Address e-mail to donald.miller{at}kcl.ac.uk
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Abstract
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IMPLICATIONS: Supraglottic airways used for positive pressure ventilation run the occasional risk of regurgitated liquid entering the lungs (aspiration). A dynamic model described here shows that the laryngeal tube has a larger liquid storage capacity between the two cuffs than the bowl of the laryngeal mask, with a consequent smaller aspiration risk.
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Introduction
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Doubts still remain concerning the use of the laryngeal mask airway (LM) with positive pressure ventilation (1), because of the risk of gastroesophageal insufflation leading to possible regurgitation and aspiration during anesthesia (2–4). A pilot study showed that the LM is more susceptible to gastroesophageal insufflation than the laryngeal tube (LT). The study was on 22 patients, in whom 3 (14%) had gastroesophageal insufflation with the LM and 0 with the LT (5). In the event of regurgitation, the upward ascent of liquid may be encouraged to enter the bowl of the LM rather than to pass posteriorly behind the LM, which leads us to consider aspiration risks. It is common sense that if regurgitation does occur, the risks of aspiration relate to the storage capacities available before the volume of liquid overflows into the lungs. The useful storage capacity of the two devices needs to be compared. The space in the pharynx between the two cuffs of the LT is possibly larger than the bowl of the LM. Therefore, larger quantities of regurgitated liquid may be contained before overflow into the bronchial tree, and aspiration is likely to occur.
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Methods
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This hypothesis was tested by using a model silicone rubber pharynx designed to accommodate the insertion of supraglottic airways that will seal and allow for positive pressure ventilation. In addition to the model pharynx allowing supraglottic airways to fit better than with commercially available models with appropriate sealing pressures, there is a similarity to the actual reality of the anatomical plumbing arrangement of the model and a facility for injecting volumes of regurgitation liquid via an esophageal tube (Fig. 1). The model pharynx was positioned to mimic a 10° head-down tilt as would occur, for example, during laparoscopic procedures.
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Figure 1. Aspiration lung model comprising a silicone rubber pharynx, shown containing a laryngeal tube airway (AW) with a liquid level of "regurgitated" liquid (shaded portion) that was injected via the esophageal tube (E) and syringe (S), with the tracheal tube (T) attached to a lung compliance (LC) model via a water trap (WT). A ventilator (V) was used to ventilate the LC model via attachment to the AW being tested.
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The lung model was ventilated via the airway at 15 cm H2O pressure, with a tidal volume of 450 mL at 10 breaths/min and an inspiratory/expiratory ratio of 1:4. The seal between the airway and the silicone pharynx was 26–27 cm H2O and was checked before and after each experiment. The seal pressures chosen were near the upper limit usually obtained with an LM, and the mean sealing pressure was recorded with the LT (5). After priming the dead space in the esophageal tubing (3 mL), 5–60 mL of water was manually injected into the model pharyngeal cavity via the "esophagus" by using a 60-mL syringe, at approximately 15 and 30 mL/s. The injection was timed to begin at the onset of expiration and was completed before the next inspiration. For this to be achieved, a long expiratory pause phase was provided. Water "aspirated" into the "trachea" was collected in a water trap between the "trachea" and lung model, and its volume was measured with an appropriately sized disposable syringe. Each experiment was performed three times, and the mean value was taken; measurements were made and recorded by an investigator blinded to the device being used. Standard deviations obtained from 7 measurements with 15 and 50 mL of "regurgitation" were 0.4 and 1.3 mL, respectively, indicating very small variability in the values presented.
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Results
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The respective storage capacities for the LM and LT were 3.5 and 15 mL. The storage capacity in Figure 2 was taken as the regurgitated volume before aspiration occurred, and this finding agreed with the actual quantity of water measured within the airway devices. Once regurgitation exceeded these quantities, aspiration took place in a direct linear relationship to the volume regurgitated.
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Discussion
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The findings of this aspiration model suggest that because the storage capacity of the LM is significantly less than the LT, the LM is more vulnerable than the LT to the risk of aspiration if regurgitation occurs during positive pressure ventilation. The volumes involved may appear trivial; however, volumes in the order of 10 to 15 mL in starved patients may be regurgitated during positive pressure ventilation, because the average volume in stomachs of fasted lean and obese patients is 26 mL (8). It would appear from this model that regurgitation of 
5 mL when using the LM is likely to be associated with aspiration. Although this volume aspirated may be small and usually clinically insignificant, it may actually be sufficient to affect the course of anesthesia, and it cannot be considered an accepted norm.
It may be questioned whether the model accurately reflects what is likely to happen in clinical practice. If the spatial arrangement of the plumbing in the model is appropriate to normal anatomy and the sealing pressures for supraglottic airways are what one would expect during positive pressure ventilation, it is difficult to imagine what other more important factors would overrule these essential features of the model.
The use of the LM in positive pressure ventilation is debatable (1); however, its use should be questioned more seriously in laparoscopies, with the patient in lithotomy and head-down positions, where regurgitation is more likely (6). In the study, the 60% incidence of reflux with the LM showed that when reflux occurred, the reflux material tended to be anterior rather than posterior, that is, into the bowl of the LM. If the quantity of regurgitated liquid before aspiration occurs is so small in the case of the LM, the risks are greater than formerly appreciated. It is always better to have zero aspiration than a small amount, no matter how trivial. This model was set up with the head-down position in mind. It is quite possible that in the horizontal position, the storage capacity for both airways is less than demonstrated here.
Other important mechanisms relevant to a consideration of aspiration protection include the likely risks of gastroesophageal insufflation and regurgitation and their prevention. The factors described in this brief report have not been measured before. They should be considered when choosing to ventilate patients with supraglottic airways.
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Footnotes
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No financial benefit from manufacturers was received. VBM Medizintechnik GmbH, Sulz, Germany, donated the laryngeal tube.
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References
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- Sidaras G, Hunter JM. Is it safe to artificially ventilate a paralysed patient through the laryngeal mask? The jury is still out. Br J Anaesth 2001; 86: 749–53.[Free Full Text]
- Akhtar TM, Street MK. Risk of aspiration with the laryngeal mask. Br J Anaesth 1994; 72: 447–50.[Abstract/Free Full Text]
- Devitt JH, Wentsone R, Noel AG, O’Donnell MP. The laryngeal mask airway and positive-pressure ventilation. Anesthesiology 1994; 80: 550–5.[ISI][Medline]
- Weiler N, Latorre F, Eberle B, et al. Respiratory mechanics, gastric insufflation pressure, and air leakage of the laryngeal mask airway. Anesth Analg 1997; 84: 1025–8.[Abstract]
- Asai T, Kawashima A, Hidaka I, Kawashi S. The laryngeal tube in comparison with the laryngeal mask: insertion, gas leak and gastric insufflation. Br J Anaesth 2002; 89: 729–32.[Abstract/Free Full Text]
- McCroy C, McShane AJ. Laryngeal mask airway is associated with reflux in the lithotomy position. Br J Anaesth 1996; 77: 693.
- Asai T, Murao K, Shingu K. Efficacy of the laryngeal tube during intermittent positive pressure ventilation. Anaesthesia 2000; 55: 1099–102.[ISI][Medline]
- Juvin P, Fèvre G, Merouche M, et al. Gastric residue is not more copious in obese patients. Anesth Analg 2001; 93: 1621–2.[Abstract/Free Full Text]
Accepted for publication February 24, 2003.
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Top
Abstract
Introduction
) and (LT) laryngeal tube (
), at a regurgitation rate of 30 and 15 mL/s.
