Mechanical closure of the vocal cords with the laryngeal mask airway ProSealTM
J. Brimacombe*,1,
C. Richardson1,
C. Keller2 and
S. Donald1
1University of Queensland, Department of Anaesthesia and Intensive Care, Cairns Base Hospital, The Esplanade, Cairns 4870, Australia. 2Department of Anaesthesia and Intensive Care Medicine, Leopold-Franzens University, A-6020 Innsbruck, Austria*Corresponding author
LMA® is the property of Intavant Limited.
Accepted for publication: October 2, 2001
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Abstract
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We report a case of mechanical closure of the vocal cords with the laryngeal mask airway ProSealTM (PLMA) in an anaesthetized, paralysed 59-yr-old, 88 kg female undergoing lower-limb vascular surgery. Near-complete airway obstruction developed after a size-5 PLMA was inserted and the cuff inflated with 20 ml of air. Fibreoptic inspection revealed that the PLMA was correctly positioned, but the vocal cords were closed. Withdrawal of air from the cuff and/or moving the head and neck into the sniffing position resolved this problem.
Br J Anaesth 2002; 88: 2967
Keywords: airway, technique; complications, obstruction; equipment, ProSeal laryngeal mask airway
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Introduction
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The laryngeal mask airway (LMA
) ProSealTM is a new airway device with a large double cuff that forms a better seal than the LMA ClassicTM, and a drainage tube designed to: (i) protect against regurgitation; (ii) facilitate passage of a gastric tube; (iii) reduce the risk of gastric insufflation; and (iv) facilitate the detection of malposition as air leaking up the drainage tube indicates incorrect tip placement.1 In a preliminary study of 120 patients, our group reported a first-time successful insertion rate of 90% and an overall successful insertion rate of 100%, all failures being associated with either difficulty manoeuvring the cuff into the pharynx or malposition.2 We describe a case of mechanical closure of the vocal cords with the PLMA that occurred despite easy placement and correct positioning.
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Case report
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A 59-yr-old, 88 kg, ASA III female presented for lower-limb vascular surgery expected to take 2 h. Her medical history included non-insulin-dependent diabetes mellitus and peripheral vascular disease. She did not wish to have a regional anaesthetic. Preoperative airway assessment was unremarkable. Anaesthesia was induced with midazolam 1.5 mg, fentanyl 75 µg and propofol 90 mg, and maintained with sevoflurane 13% in 33% oxygen and nitrous oxide. Muscle relaxation was with rocuronium 40 mg. Face mask ventilation was easy without the need for a Guedel oropharyngeal airway. A size-5 PLMA was inserted easily without the introducer and the cuff inflated with 20 ml of air. The PLMA was connected to the circle anaesthesia breathing system and manually assisted breathing commenced; however, there was near-complete airway obstruction, with tidal volumes of approximately 100 ml and peak airway pressures of >30 cm H2O without oropharyngeal air leak. The built-in bite block was between the teeth and there was no air leak from the drainage tube, suggesting correct placement. Fibreoptic inspection down the airway tube further suggested that the cuff was correctly placed because: (i) the epiglottis was within the bowl, but it was not downfolded; (ii) a full view of the vocal cords could be obtained; and (iii) the oesophageal inlet could not be seen. However, the vocal cords were almost completely closed. Before removal of the PLMA, the head and neck were moved from the neutral to the sniffing position by applying chin lift, which resulted in a sudden and complete relief of the obstruction, with tidal volumes of 700 ml and peak airway pressures of <20 cm H2O. To investigate this phenomenon, we observed the vocal cords and the epiglottis in the sniffing position (no obstruction) and the neutral position (obstruction) on several occasions. In the neutral position, the vocal cords were closed and the anteroposterior diameter of the glottic inlet reduced. In the sniffing position, the vocal cords were open and the anteroposterior diameter of the glottic inlet increased. We also found that adding air to the cuff caused the vocal cords to close and the anteroposterior diameter to decrease; removing air from the cuff caused the vocal cords to open and the anteroposterior diameter to increase. Fibreoptic inspection down the drainage tube revealed mucosal tissue in both the neutral and sniffing position (suggestive of correct tip placement); this was subsequently confirmed by easy passage of a gastric tube into the stomach. Surgery was completed using the PLMA with the head and neck in the sniffing position. Oxygen saturation remained >90% during the above sequence of events. There were no other perioperative problems.
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Discussion
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Three malpositions have been described for the PLMA. The first is when the PLMA is not inserted deeply enough, resulting in the tip sitting mid-pharynx. This results in air leaking up the drainage tube during positive-pressure ventilation and a poor seal. The second is when the PLMA tip impacts against the glottis. This results in airway obstruction and air leaking up the drainage tube.1 A third malposition is when the tip of the PLMA cuff is folded posteriorly.3 This results in failure of the drainage tube to perform its intended functions, but it may have no impact on seal or ventilatory function. In our case, the PLMA was correctly positioned and had a good seal, but it failed to function as a ventilatory device because of closure of the vocal cords.
The pathogenesis of vocal-cord closure was probably mechanical rather than physiological as the patient was paralysed and the vocal cords reacted to changes in cuff volume and head and neck position. We found that vocal-cord closure was also associated with a reduction in the anteroposterior diameter of the glottic inlet. We postulate that the mechanism of vocal-cord closure is that the PLMA cuff compresses the glottic inlet along the anteroposterior axis and reduces the tension of the vocal cords. This allows the arytenoid cartilages to rotate inwards and the vocal cords to close. Withdrawing air from the cuff and moving the head and neck into the sniffing position reduces the compressive force against the glottis and allows the arytenoids to rotate outwards and the vocal cords to open. Adopting the sniffing position probably reduces the compressive force by increasing the anteroposterior diameter of the pharynx.4 Interestingly, mechanical closure of the vocal cords has been reported with the classic LMA in association with cricoid pressure, and a similar mechanism proposed.5 Moving the head and neck into the sniffing position to correct mechanical vocal-cord closure should not be attempted if the patient has an unstable cervical spine.
One of the authors (JB) has noted mechanical vocal-cord closure occurring in 4 of 915 (0.4%) paralysed patients managed with the PLMA. We speculate that the larger distal cuff of the PLMA may cause more compression of the glottic inlet than the LMA for a given cuff volume. However, the performance of the PLMA as a ventilatory device appears to be similar to the LMA in healthy female patients.6 A well-known cause of mechanical airway obstruction with the LMA is downfolding of the epiglottis.7 Although a degree of epiglottic downfolding has been seen in 17% of patients with the PLMA, it rarely causes airway obstruction.2 This is probably because a downfolded epiglottis rests on the drainage tube and the accessory vent provides a clear passage for gas underneath the drainage tube. However, if the epiglottis is completely folded across the glottic inlet, airway obstruction can occur. Interestingly, epiglottic downfolding will also be improved by moving the head and neck into the sniffing position.8
We used the size-5 PLMA in this female patient and inflated the cuff with 20 ml of air. It is possible that selection of a small size and/or use of a smaller cuff volume would have prevented this problem. However, the size-5 PLMA is recommended for adults of 70100 kg9 and 20 ml of air is only half the maximum recommended cuff volume. The size-5 LMA has been shown to be suitable for females.10 11
In summary, we report a case of mechanical closure of the vocal cords with the PLMA that occurred despite easy placement and correct positioning. We recommend withdrawing air from the cuff or moving the head and neck into the sniffing position to resolve this problem.
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References
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