The administration of safe general anaesthesia usually requires effective methods to establish an adequate airway and to ventilate the lungs. Recently, various supraglottic devices were introduced, which are particularly readily used for short ambulatory anaesthesias [1, 2].

The laryngeal mask airway (LMA) has become most popular; it enables effective maintenance of an adequate airway without the risk of laryngoscopy- and intubation-related complications [3]. The LMA is the method of choice in difficult intubations [4, 5].  

The Cobra perilaryngeal airway (PLA) provides similar possibilities. Its structural design differs from that of LMA, of a sealing cuff, in particular. Despite identical applications, Cobra PLA is not so popular as the laryngeal mask, which proved to be safe and effective in clinical anaesthesiology [6, 7].

The aim of the present study was to compare the Cobra PLA and LMA with respect to their safety, efficacy of ventilation and clinical usefulness during short general anaesthesias.

METHODS

With the approval of the Bioethical Committee of the Medical University of Lublin, the study encompassed patients scheduled for elective urological procedures (TURP, TURT, cystoscopy with biopsy, etc). The exclusion criteria were: age<18 years, reflux disease, diseases limiting the cervical spine mobility, goitre and the Mallampati score III or more.

Patients were premedicated with oral diazepam, 0.1 mg kg-1, 60 min before the onset of the procedure. In the operating room, they received 0.1 mg of fentanyl and 0.5 mg of atropine. After oxygenation, anaesthesia was induced with propofol in the dose of 2 mg kg-1. In the Jackson II position, the upper airway was instrumented with the randomly chosen LMA or Cobra devices. In patients with body weight 50-70 kg, the size-3 LMA or Cobra device was used, in those >70 kg – size 4. The LMA sealing cuff was inflated with air to the volume of 20 ml; the Cobra one – to 40 ml for size 3 and 50 ml for size 4. If needed, the cuffs were supplemented with air to provide the airway seal at the peak inspiratory pressure of 15 cm H2O, which was verified by auscultation of the neck region and analysis of capnographs.

The anaesthesia was carried out with the spontaneous respiration preserved using the mixture of 33% O2 and 66% N2O. Anaesthesia was deepened with fractionated doses of propofol, if need be. The ETCO2 values were maintained within the range of 35-45 mm Hg, supporting periodically the spontaneous respiration with IPPV.

The insertion difficulties were determined according to the arbitral 3-degree scale where A denoted easy insertion, B – difficult one requiring wider mouth opening or head bending and C – unfeasible. Moreover, comparative assessment involved the time required to insert the device and to achieve proper mode of spontaneous breathing (capnography), pressures in the sealing cuff and necessary additional sealing of the cuff as well as induced (forced) leak pressures – at manual IPPV.
In all patients, the values of SpO2, ETCO2, HR and MAP were recorded before induction of anaesthesia, after placement of the device, at 10 min of the surgical procedure, and after removal of the device.

Two hours after the completion of anaesthesia, patients were interviewed about sore throat, hoarseness, difficult swallowing, nausea or vomiting and other procedure-related adverse events.

The obtained data were analysed using the structure coefficient expressed as percentage, an arithmetic mean and standard deviation. Correlations between two variables were tested using the c2 test and the tests for two means in independent samples: Student’s t, the Cochran-Cox test - when variance equilibrium was not met or Mann-Whitney U test when the variable distribution was not normal. In all tests, p<0.05 was considered as significant.

RESULTS

The study involved 50 ASA physical status I and II patients aged 23-78 years.  In the LMA group, the laryngeal masks were used (n=25), in the Cobra group the Cobra PLA was applied (n=25). Both groups of patients were comparable with respect to age, body weight, height, duration of operation and anaesthesia (Table 1).

The time of LMA placement was 16.8±5 sec and was significantly shorter compared to that for the Cobra PLA – 33.0±19.6 sec (p<0.0001); leak pressures were markedly lower in LMA patients ( p<0.001) (Fig. 1).

In the majority of cases, devices were inserted without difficulties, additional interventions (B procedure) were necessary in 8% of LMA and 28% of Cobra cases. In one patient an attempt to use LMA failed and anaesthesia was continued with the Cobra device; in another one a significant leak of respiratory gases around the Cobra device was observed therefore it was removed and anaesthesia continued using the facial mask (Table 2). In both groups, in 24 patients the devices were fitted to the anatomical structure of the upper airway (no leak during induced inspiratory pressure of 15 cm H2O).

The pressures in sealing cuffs after inflating them with the set air volume were significantly higher in LMA compared to the Cobra PLA (Fig. 2). Supplementation of cuff volumes was necessary in 28 % of LMA and in 64% of Cobra patients (p=0.01) (Table 2).

The recorded values of haemodynamic parameters and blood saturation were comparable in both groups of patients throughout the procedure. There were no significant differences found at the successive stages of the study and in intergroup comparisons (Table 3).

The adverse reactions on removal (cough, glottic spasms) as well as throat mucosa injuries occurred in 2 (8%) patients following Cobra PLA. In the postoperative period, none of the patients complained of sore throat, difficult swallowing, nausea or vomiting.

DISCUSSION

The key element of general anaesthesia is to establish an adequate airway. Supraglottic masks of various designs used for this purpose require proper knowledge of methods of their application and clinical experience. This is confirmed by our observations as the time to insert the Cobra devices was substantially longer due to their bigger sizes compared to smaller LMA devices. Opinions of other authors vary [8, 9]. Moreover, there are some reports demonstrating comparable times and difficulties in airway instrumentation using both devices [10, 11], with muscle relaxants cases included [12]. Longer times of Cobra insertions did not decrease blood saturation in our patients.  
Pressures in low-volume cuffs of the LMA were substantially higher compared to those in high-volume cuffs of Cobra devices. Moreover, the latter more often required additional sealing.

Cuff pressures leading to leakage of air  were significantly higher for the Cobra PLA compared to LMA. Similar results were reported by other authors [8, 11], also during laparoscopic gynaecological procedures with mechanical lung ventilation [10]. Indeed, during mechanical ventilation the value of pressure at which the leak occurs seems relevant and therefore the producers of devices determined the acceptable peak pressure at 20 cm H2O.

In our study, in one case, trace amounts of gastric contents were visible on the Cobra device after its removal, although the course of anaesthesia was uneventful. According to Cook and colleagues [13], regurgitation and aspiration of gastric contents to the airway was observed in two Cobra cases. One of these cases concerned anaesthesia for the laparoscopic gynaecological procedure with mechanical lung ventilation and the peak  inspiratory pressure of 12 cm H2O. In that case, the Cobra device twisted along its axis resulting in increased pressure in the respiratory system.  The mask displacement led to ineffective ventilation, filling of the stomach with the mixture of respiratory gases and regurgitation. During insertion of the Cobra PLA, as opposed to LMA, the definite tissue resistance is not encountered, which may lead to its improper placement [10].
In two Cobra cases, after removal of the device, adverse events developed - glottic spasms and persistent cough, which quickly subsided after the treatment provided. In another two cases, blood traces were found on the device surface after the procedure. It is claimed that the incidence of tissue injuries in Cobra cases is significantly higher [10, 14, 15].

Both types of masks appear equally useful for airway maintenance during short-term general anaesthesias. However, in one case, proper placement of the LMA was unfeasible. An attempt was made to insert the Cobra device, which ended with success. Both the placement and further lung ventilation were easy. In this particular case, rigidity and more solid structure of Cobra appeared to be advantageous.

The literature reports widely document similar cases, e.g. effective use of this mask in difficult intubation of a patient anaesthetized for renal transplantation [16] or in a patient with stridor following thyroidectomy [17]. Moreover, the Cobra PLA was found extremely useful in a child with the Desbuquois syndrome, with multiple defects involving, among other things, the facial skeleton and thoracic cavity, in whom difficult intubation was anticipated [18].  However, our experiences suggest that in some cases the provision of a fully patent airway might be unfeasible and other devices or alternative methods of management are required.

CONCLUSIONS

1. LMA and Cobra devices are equally useful for establishing an adequate airway during short general anaesthesias.

2. The insertion time for LMA is shorter than that for Cobra PLA.

3. LMA cuff pressures are higher compared to Cobra devices.

4. Induced leak pressures around the cuff are higher in Cobra than in LMA devices.

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REFERENCES

1.   Bein B, Scholz J: Supraglottic airway devices. Best Practice & Research Clinical Anaesthesiology 2005; 19: 581-593.

2.   Nestorowicz A, Wosko J, Olszanski W, Morshed K: Dozylna indukcja anestezji zzyciem maski krtaniowej. Anaesthesiol Intensive Ther 2000; 32: 169-172.

3.   Zimmert M, Zwirnert P, Kruse, Braun U: Effects on vocal function and incidence of laryngeal disorder when using amask airway in comparison with an endotracheal tube. Eur J Anaesthesiol 1999; 16: 511-515.

4.   The American Society of Anesthesiologist Task Force on Management of the Difficult Airway: Practice guidelines for management of the difficult airway: aAnesthesiology 1993; 78: 597-602.

5.   Crosby ET, Cooper RM, Douglas MJ: The unanticipated difficult airway with recommendation for management. Can J Anaesth 1998; 45: 757-776.

6.   Brimacombe J: Analysis of 1500 laryngeal mask uses by one anaesthesist in adults undergoing routine anaesthesia. Anaesthesia 1996; 54: 76-80.

7.   Verghese C, Brimacombe J: Survey of laryngeal mask airway usage in 11, 910 patients: safety and efficacy for conventional and nonconventional usage. Anesth Analg 1996; 82: 129-133.

8.   Gaitini L, Yanovski B, Somri M, Vaida S, Riad T, Alfery D: Abetween the PLA Cobra and the Laryngeal Mask Airway Unique during spontaneus ventilation: aprospective study. Anesth Analg 2006; 102: 631-636.

9.   Agrò F, Barzoi G, Gallì B: The CobraPLA in 110 anaesthetized and paralysed patients: what size to choose? Br J Anaesth 2004; 92: 777-778.

10. Galvin E, van Doorn M, Blazquez J, Ubben J, Zijlstra F, Klein J, Verbrugge S: Arandomized prospective study comparing the Cobra Perilaryngeal Airway and Laryngeal Mask Airway-Classic during controlled ventilation for gynecological laparoscopy. Anesth Analg 2007; 104: 102-105.

11.  Akca O, Wadhwa A, Sengupta P, Durrani J, Hanni K, Wenke M, Yucel Y, Lenhardt R, Doufas A, Sessler D: The new perilaryngeal airway (CobraPLA) is as efficient as the laryngeal mask airway (LMA) but provides better airway sealing pressures. Anesth Analg 2004; 99: 272-278.

12.  Gaitini LA, Somri MJ, Kersh K, Yanovski B, Vaida S: Acomparison of the laryngeal mask airway Unique, pharyngeal airway X press and perilaryngeal airway Cobra in paralysed anesthetized adult patients. Anesthesiology 2003; 99: A149.

13.  Cook TM, Lowe JM: An evaluation of the Cobra Perilaryngeal Airway: study halted after two cases of pulmonary aspiration. Anaesthesia 2005; 60: 791-796.

14.  Van Zundert A, Al-Shaikh B, Brimacombe J, Koster J, Koning D, Mortier E: Comparison of three disposable extraglottic airway devices in spontaneously breathing adults. Anesthesiology 2006; 104: 1165-1169.

15.  Lee JJ, Kim JA, Gwak MS, Kim MH: Evaluation of the Cobra perilaryngeal airway (CPLA) as an airway conduit. Eur J Anaesthesiol 2007; 24: 852-855.

16.  Szmuk P, Ezri T, Akca O, Alfery D: Use of asupraglottic airway device – the CobraPLA – in a‘difficult to intubate/difficult to ventilate’ scenario. Acta Anaesthesiol Scand 2005; 49: 421-423.

17.  Agrò F, Carassiti M, Barzoi G, Millozzi F, Galli B: Areport on the diagnosis and treatment of acute postoperative airway obstruction with the CobraPLA. Can J Anaesth 2004; 51: 640-641.

18.  Szmuk P, Matuszczeak M, Carlson R, Warters R, Rabb M, Ezri T: Use of CobraPLA for airway management in aneonate with Desbuquois syndrome. Case report and anesthetic implications. Pediatric Anesthesia 2005; 15: 602-605.

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Address:

*Agnieszka Wrońska-Sewruk
I Klinika Anestezjologii i Intensywnej Terapii UM w Lublinie
ul.  Janczewskiego 8, 20-910 Lublin
tel.: 0-81 724 43 32, fax: 0-81 724 45 50
e-mail: anest@am.lublin.pl

Received: 02.03.2009
Accepted: 20.04.2009