The Cobra perilaryngeal airway (CPLA) is a relatively new supraglottic airway device. It seems that many in anaesthetic practice believe that use of CPLA has a limitation in that the risk of pulmonary aspiration is not prevented during controlled ventilation [1,2]. However, as shown with the case reports using the CPLA as a conduit to the larynx [3,4], the possibility of intubation through the CPLA is important in practice. CPLA has theoretical advantages in view of the small head size, ease of insertion, and short and wide-diameter shaft compared with the laryngeal mask airway (LMA) classic [1,5-7]. This study aimed to evaluate the success rates of blind and fibre-optic (FO)-guided intubation through the CPLA as a conduit, and identify the factors associated with its failure.
The institutional review board of our hospital approved this study and all patients gave written informed consent before taking part. In all, 49 patients, ASA class I or II, undergoing elective surgery were enrolled. We excluded subjects with any pathology of the neck, upper respiratory or upper alimentary tract, those at risk of aspiration of gastric contents and those who weighed less than 50 kg or greater than 80 kg. Upon arrival in the operating room, standard anaesthetic monitoring was used before induction of anaesthesia. Patients' heads were supported on a firm pillow. After preoxygenation with 100% oxygen, anaesthesia was induced with thiopental (5 mg kg−1), rocuronium (0.6 mg kg−1) and sevoflurane. Neuromuscular block was confirmed using a peripheral nerve stimulator (train of four count = 0/4) before airway manipulation. Anaesthesia was maintained with nitrous oxide, oxygen, fentanyl and sevoflurane during the procedure. The investigators were staff anaesthesiologists who had more than three encounters with CPLA insertion and had reviewed the literature. The CPLA size was selected according to the patient's weight (size 3 for weight <60 kg, size 4 for 60-80 kg) . The CPLA was inserted with the patient's head in a sniffing position. The deflated lubricated cuff was swept back along the shaft and inserted until resistance was felt, then pulled back slightly. The cuff was inflated. After every manoeuvre, adequacy of ventilation was assessed. Three criteria for success of ventilation were applied: adequate chest movement, expired tidal volume of more than 7 mL kg−1 at a peak airway pressure <15 cmH2O and square-wave capnography. Two attempts to insert CPLA were allowed and the insertion time (from insertion of the device into the patient's mouth to obtaining the adequate airway) was recorded. Further study of the device was stopped if the placement failed after two attempts. After securing the device, the FO view through the CPLA was evaluated and a blind intubation through the CPLA was attempted (a cuffed internal diameter (I.D.) 6.5 mm conventional polyvinyl chloride tube for no. 3 CPLA, I.D. 7.0 mm conventional tube for no. 4 CPLA). The tracheal tube (TT) was advanced with its inherent curve facing forward. The success of intubation was evaluated with a stethoscope, capnograph and chest movement. Then the TT was removed and, after some ventilation via the CPLA, an FO-guided intubation was tried. The TT was advanced over the fibrescope and the adequacy of FO-guided intubation was recorded. Only one attempt was allowed for tracheal intubation at each step. The CPLA was then deflated and removed leaving the TT in place by pushing the TT using the silicone rod of the LMA Fastrach (Vitaid Ltd, Toronto, Ont., Canada). After proper positioning of the TT had been confirmed by the CO2 curve, the TT was removed. The Cormack & Lehane (C&L) grading  and visible injury of the upper airway were evaluated under a conventional direct laryngoscope by one investigator, then the trachea was intubated. The patients were assessed for desaturation (<95% of peripheral oxygen saturation), stridor during the procedure as well as by the presence of a sore throat and dysphonia postoperatively.
Assuming the overall occurrence of successful ventilation in a standard LMA of 95% from previous reports, it was decided that a 20% difference in the overall occurrence of successful ventilation through the CPLA would be clinically important. In all, 49 patients with α = 0.05 and β = 0.1 would be necessary. χ2 test or unpaired t-test was used to compare the C&L grading, success rate and complication in the two groups according to the FO view. Results are presented as mean ± SD or actual numbers (%). A P value <0.05 was considered significant.
The study population consisted of 15 men and 34 women with a mean age of 46.4 ± 14.7 yr, and a body weight of 61.6 ± 10.5 kg. In all, 95.9% (47/49) of patients had adequate ventilation through the CPLA. In two cases ventilation was not possible after two attempts to insert the CPLA; hence the following protocol did not proceed. The average insertion time of the CPLA was 7.6 ± 4.1 s. No events of desaturation or stridor were encountered while inserting the CPLA. Blind intubation was successful in 17 (36.2%) cases. The success rate of an FO-guided intubation through the CPLA was 83.0%. Table 1 shows the FO view of the larynx through the CPLA. The patients were divided into two groups in accordance with the FO view of the larynx. Group 1 (16/47, 34.0%) included patients with an obstructed view by an anterior grill of the CPLA head and a view of the anterior epiglottis (downfolded epiglottis) with or without a visible larynx. Group 2 (31/47, 66.0%) included the remaining patients. There was a significant difference in the success rates of intubation through the CPLA between the two groups (Table 2). The rates of the FO view and successful intubation through the CPLA were not different according to the C&L grading (Table 3). Totally, 42.6% of patients showed blood staining after removing the device and complications such as trauma, oedema confirmed under direct laryngoscope. Totally, 68.1% of patients had complications including dysphonia and sore throat at discharge from the recovery area. In all cases, the complications were transient and minor.
The ASA Difficult Airway Algorithm recommends the use of a laryngeal mask as a conduit for tracheal intubation in patients with an unexpected difficult airway . However, there are some disadvantages with the technique of an intubation through the standard LMA. For example, a standard TT, when fully inserted through an appropriately sized standard LMA, enters the trachea by only 1-2 cm . Therefore, there is a risk of accidental extubation when the LMA is removed [11,12]. We also experienced unexpected extubation after removing the standard LMA in a difficult airway case. In addition, the LMA also restricts the size and type of the TT that can be placed through it . LMA Fastrach is designed to overcome many of the problems of the LMA classic, it has a specially designed ramp that guides the tube into the trachea with much greater certainty . In addition, various techniques to overcome problems associated with tracheal intubation through the LMA classic have been tried [10-12,14-16]. The CPLA may be more useful than LMA classic as a conduit for blind or FO-guided intubation because it has structural differences such as a shorter (22 vs. 27 cm for each no. 3 size) and wider diameter stem. Therefore, a CPLA carries a lower risk of accidental extubation when the CPLA is removed. In this study, the CPLA could be removed without accidental extubation in all cases. In addition, the CPLA might be easier to insert than the LMA with no need for any airway manipulation even in cases with limited mouth opening . However, the CPLA has some disadvantages. One is the low success rate of blind intubation. Another is the increased risk of pulmonary aspiration related to an absence of a distal cuff and a positioning of the head at a more proximal site (hypopharynx) [6,7].
In this study, for blind and FO-guided intubations, the success rates were not different according to the grading of C&L, indicating a degree of difficult intubation under direct laryngoscopy. However, the success rate was different in association with the FO view through the CPLA. There was a low success rate for blind intubation when the anterior epiglottis was downfolded or the openings of the anterior grill of the CPLA were inadequate. In such cases, the excessive force applied to the TT to overcome the resistance for the tube passage might displace the TT, resulting in some dislodgement into the perilaryngeal space or oesophagus. The proper placement of the CPLA is essential for the success of blind intubation. For the LMA classic, when the mask is in a perfect central position (45-60%) [17,18], any of the blind insertions has some chance of success. The success rates of blind and FO-guided intubations via the CPLA in this study (36.2% and 83.0%, respectively) did not appear different from the success rates achieved blindly (51%) or using the FO scope (82%) via an LMA classic . Therefore, the CPLA may have a place in rescue airway management.
It was reported that 50% of the CPLA had positive blood traces when the airways were removed, while only 17% of the LMA had positive blood traces (P < 0.01) . In addition, 50% of the patients suffered from a sore throat in the CPLA group, which was significantly higher than that in the LMA group (P < 0.05) . In this study, 42.6% of study participants had signs of blood staining on the device and 68.1% of patients had complications such as dysphonia, dysphagia or sore throat. In our study it is likely that repeated manipulations of the airway during the procedure have contributed to the high complication rate of the CPLA. However, recently, one study also reported higher blood staining in the use of CPLA (35.2%) . A higher chance of further oedema or blood would exacerbate an already bad airway situation during an emergency airway.
This study has some limitations in that cases of predicted difficult airway were not selected and the performance of the Cobra was evaluated under fully paralysed conditions Further studies would be needed to address these issues.
From this study, although the CPLA showed a comparable success rate of FO-guided intubation to the LMA classic, the overall blind intubation failure rate through CPLA was relatively high at 64%. The CPLA could provide adequate ventilation but not reliably assist blind tracheal intubation through it. Caution will be needed against airway trauma.
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