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GENERAL ARTICLES

A Comparison Between the PLA Cobra^TM and the Laryngeal Mask Airway Unique^TM During Spontaneous Ventilation: A Randomized Prospective Study

Luis Gaitini, MD^*, Boris Yanovski, MD^*, Mustafa Somri, MD^*, Sonia Vaida, MD^*, Tome Riad, MD^*, and David Alfery, MD

*Department of Anesthesiology, Bnai Zion Medical Center Faculty of Medicine, Technion Israel Institute of Technology, Haifa, Israel; and Vanderbilt University Medical Center and Anesthesia Medical Group, Nashville, Tennessee

Address correspondence and reprint requests to Luis Gaitini, MD, Director of the Anesthesiology Department, Senior Lecture of the Faculty of Medicine, Techion, Israel Institute of Technology, Haifa, Israel. Address e-mail to luis.gaitini{at}b-zion.org.il.

	Abstract

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The Laryngeal Mask (LMA) Unique and the Cobra Perilaryngeal Airway (PLA) are single-use supraglottic devices. There are no published studies comparing these devices during spontaneous ventilation. We compared the LMA Unique and the Cobra PLA with respect to 1) ventilatory variables during spontaneous ventilation, 2) time to achieve an effective airway, 3) airway intervention requirements, 4) cuff seal pressures, 5) fiberoptic score, and 6) perioperative adverse events. Eighty adult ASA physical status I–II patients undergoing general anesthesia for minor routine surgery were randomly allocated to LMA Unique or PLA Cobra for airway management. No statistically significant differences were found between the devices with respect to inspiratory tidal volume, expiratory tidal volume, end-tidal CO₂ concentration, respiratory rate, number and type of airway interventions required with placement, the fiberoptic score, and the incidence of perioperative adverse events. The oropharyngeal leak (seal) pressure was higher for the CobraPLA (27 ± 7 versus 21 ± 4 cm H₂O; P < 0.001). The oxygen saturation was higher (98.1% ± 1% versus 97.3% ± 2%; P = 0.02) in the LMA group. Time of insertion was shorter for LMA (23.7 ± 2 s versus 26.6 ± 7 s; P = 0.02) and insertion difficulty was less for LMA (P = 0.03). As these differences were not judged to be clinically important, both devices appear to be effective in establishing an adequate airway in patients who are spontaneously breathing under general anesthesia.

	Introduction

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The Laryngeal Mask Airway (LMA) Unique and the Cobra Perilaryngeal Airway (CobraPLA) are single-use supraglottic airway devices intended for use in spontaneously and mechanically ventilated patients undergoing general anesthesia. The LMA Unique is made of medical grade polyvinyl chloride (PVC) (Fig. 1); it is a disposable replica of the LMA Classic. Several studies have proven the safety and effectiveness of the LMA in clinical anesthesia (1,2).

View larger version (113K): [in this window] [in a new window]   Figure 1. The Laryngeal Mask Airway (LMA) Unique.

The CobraPLA is a new supraglottic airway device also comprised of PVC (Fig. 2). Studies of the CobraPLA are limited to this point, although there is one completed evaluation which compares it to the LMA Classic in mechanically ventilated patients (3). There are no published studies comparing the CobraPLA to the LMA Unique during spontaneous ventilation. We therefore embarked on a study of the two devices with the hypothesis that the two devices could perform similarly during spontaneous ventilation despite differences in their structural design. The chief aim of this prospective randomized study was to compare the CobraPLA and the LMA Unique with respect to 1) time to achieve an effective airway, 2) need for interventions to achieve an effective airway, 3) cuff seal (leak) pressure at an intracuff pressure of 60 cm H₂O, 4) fiberoptic score at the distal end of the devices, 5) ventilatory variables during spontaneous ventilation, and 6) adverse perioperative events.

View larger version (114K): [in this window] [in a new window]   Figure 2. The CobraPLATM Airway.

	Methods

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Eighty consecutive patients of ASA physical status I and II weighing between 50 to 100 kg, undergoing general anesthesia for minor routine surgical procedures in the supine position were randomly assigned to have either a CobraPLA or a LMA Unique used for airway management. The surgery types were from a wide range of routine general surgery, orthopedic, urologic, gynecologic, and plastic surgery of moderate duration. Exclusion criteria were age <18 yr, weight <50 kg, body mass index >35 kg/m², cervical spine disease limiting neck movement, a known difficult airway, emergency operations with a "full stomach," and patients with active gastroesophageal reflux. Ethics Committee approval was obtained for the study and written informed consent was obtained from each of the study subjects. The following data were collected for each patient: sex, age, height, weight, Mallampati score, body mass index, type of operation, and total time of anesthesia.

Two anesthesiologists with no financial interests in either device participated in the trial. Each had performed more than 100 insertions of the LMA Unique, more than 1000 insertions of the LMA Classic, and more than 100 insertions of the CobraPLA. The study protocol was as follows: patients were initially given midazolam 0.05 mg/kg and fentanyl 2 µg/kg IV during a 3-min period of administration of oxygen. General anesthesia was induced with propofol 2 mg/kg given IV. After induction, isoflurane end-tidal concentration up to 1.5% in 33% oxygen and 66% nitrous oxide was started and ventilation was controlled by facemask for 3 min, and then a supraglottic airway was inserted. The choice of airway was randomized by opening a sealed envelope immediately before induction. A size 3, 4, or 5 CobraPLA or size 3, 4, or 5 LMA Unique was used according to manufacturer's recommendations. In accordance with these recommendations the cuffs were inflated to 60 cm H₂O immediately after insertion. Cuff pressures were individually checked by manometer. The airway was judged to be effective and adequate if the operator achieved at least 6 mL/kg expiratory volume during gentle manual ventilation, at a peak airway pressure of 15 cm of H₂O, no oral leak as judged by stethoscope over the neck, and a normal square-wave capnograph trace. The expiratory volume data and the square-wave capnograph trace were obtained using the integrate spirometer and capnograph of the S/5^TM Anesthesia Delivery Unit (EDU) (Datex-Ohmeda, Helsinki, Finland).

The total time to achieve an effective airway was measured as the time after the anesthesiologist removed the facemask until the square-wave capnograph tracing was observed after insertion. Two attempts at insertion were permitted. If unsuccessful (as judged by an inability to insert the device through the mouth or by a total lack of ventilation), the patient's airway was managed with a conventional endotracheal tube. If insertion of the device allowed some ventilation but was judged inadequate by the above-described criteria, further maneuvers were undertaken to properly position the device. These adjustments included minor interventions (adjusting head or neck position or changing depth of insertion) or major interventions (applying jaw lift, reinsertion of the device, or changing its size).

Once an effective airway was achieved, oropharyngeal cuff leak pressures were obtained by closing the expiratory valve of the anesthesia circuit with a fixed gas flow rate of 3 L/min and noting the airway pressure at which equilibrium was reached. The maximum allowed airway pressure during this period of evaluation was 40 cm H₂O (4). Patients were apneic during this time either as the result of the depth of anesthesia or by performing a brief period of hyperventilation and bringing their carbon dioxide level less than their apneic breathing threshold. After this measurement, assessment of the distal airway position was made by advancing a fiberoptic bronchoscope to the distal end of each device and scoring the view according to the following fiberoptic score: 4 = only vocal cords visible; 3 = vocal cords plus posterior epiglottis visible; 2 = vocal cords plus anterior epiglottis visible; 1 = vocal cords not visible but functions adequately; 0 = vocals cords not visible and functions inadequately (5). Respiratory variables were measured by a Datex AS-5 monitor.

The following data were recorded every 5 min commencing from the establishment of spontaneous ventilation until the device was removed: oxygen saturation, inspired and expired tidal volumes (TV), respiratory rate, fraction of inspired oxygen, end-tidal anesthetic gas concentrations, end-tidal carbon dioxide concentration, and the integrity of flow volume loops to monitor for leak. At the end of the operation isoflurane and nitrous oxide were discontinued and the patients were allowed to breathe 100% O₂. The airway device was removed after the patients had awakened to the point when they could open their mouths to command. Each device was examined for the presence of blood and the mouth was inspected for dental or mucosal trauma. Perioperative adverse events related to the investigational devices, if they occurred, were documented and graded as follows: mild = coughing or gagging on insertion, hiccups, gastric insufflation; moderate = bronchospasm, airway, obstruction, blood staining of the device, oral or tongue pain, sore throat, hoarseness, difficulty in swallowing, dysphagia and dysphonia; severe = hypoxia, regurgitation, aspiration, dental trauma, soft tissue trauma, gross blood-staining of the device. A blinded research assistant interviewed patients after release from the postanesthetic care unit (PACU) and then again 24 h postoperatively. Patients were interviewed to determine the incidence and severity of postoperative adverse events.

The primary variables studied were oxygenation and adequacy of ventilation. Secondary variables were time to achieve an effective airway, airway intervention requirement, cuff seal leak pressure, fiberoptic score, and perioperative adverse events. Sample size was determined for the primary variables (O₂ saturation and end-tidal CO₂ concentration) using the following information from previous studies: standard deviations are 2% and 6 mm Hg for the 2 variables, respectively, and a minimal change of 5% saturation or 5 mm Hg is statistically significant. If a statistically significant difference in insertion time resulted in a decrease in oxygen saturation to less than 90% for one of the devices, it was considered to be clinically significant. Sample size calculation was performed assuming a two-sided test with = 0.05 and power of 0.9. We used the two-sided independent Student's t-tests to analyze continuous data, the Mann-Whitney U-test for ordinal data, and Fisher's exact test for categorical data.

	Results

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There were no significant differences between the two groups with respect to demographic and surgical data (Table 1). No statistically significant differences were found between the devices with respect to inspiratory TV (TV_ins) (P = 0.82), expiratory TV (TV_exp) (P = 0.86), end-tidal CO₂ concentration (P = 0.74), and respiratory rate (P = 0.54). O₂ saturation for the CobraPLA was 97.3% ± 1.67%, and for the LMA Unique 98.1% ± 1.05%. This difference of 0.76% (95% confidence interval, 0.15%-1.4%) was statistically significant (P = 0.02) (Table 2).

The time to achieve an effective airway for the CobraPLA was 26.6 ± 7.1 s and for the LMA Unique 23.7 ± 2.47 s, a difference which reached statistical significance (P = 0.02). Insertion difficulty was significantly greater with the CobraPLA (P = 0.03), but there were no failures of placement with either device. No patients recorded an arterial saturation to less than 90% during insertion. The two devices did not differ with respect to fiberoptic scores (P = 0.69) or the incidence of postoperative adverse events such as blood staining (P = 1.0) or sore throat in PACU (P = 0.62). No other adverse events in PACU and 24 h after the surgery were detected (Table 3). The oropharyngeal leak pressure was significantly higher for the CobraPLA (27.0 ± 7.16 cm H₂O) than it was for the LMA Unique (21.0 ± 4.21 cm H₂O) (P < 0.001) (Table 3). The number and type of airway interventions required for each device were similar (P = 0.28) (Table 4).

	Discussion

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Cobra PLA and the LMA Unique function equally well in most aspects of airway management, using spontaneous ventilation of patients undergoing operations of short to intermediate duration. With respect to our primary study end-points, there was a slight, yet statistically significant, difference in achieved oxygen saturation, whereas end-tidal CO₂ concentrations recorded were not different. However, the mean difference for oxygen saturation of <1% (97.3% for CobraPLA and 98.1% for LMA) clearly has no clinical significance, as both values reside in an acceptable range for patients undergoing general anesthesia and represent only minimal differences in oxygen-carrying capacity. Further, no cases of oxygen saturation <95% occurred with either device.

The degree of hypercapnia that occurred with the two devices was as expected and acceptable; during spontaneous ventilation during general anesthesia there is a gradual increase in the level of end-tidal carbon dioxide. This increase depends on the level of anesthesia obtained and the end-tidal concentration or plasma levels of the respiratory depressant drugs that are used (6).

The relatively similar insertion performances that were obtained may be related to the same relative anatomic location of each device, despite their different structural designs. The cuff of the LMA Unique surrounds the laryngeal inlet and forms a seal with the hypopharyngeal tissues. The distal ventilation hole has two epiglottic bars, which prevent the epiglottis from entering and obstructing the airway gas channel, especially during expiration. The distal end of the CobraPLA (the "Cobra head") has a geometric triangular shape that serves to hold soft tissues away from the distal grill where respiratory gases enter and exit. The grill, in turn, helps to further prevent hypopharyngeal tissues from obstructing the distal ventilatory hole. These obstruction protective mechanisms, in our opinion, are important design features for each device that allow ventilation to occur without impairment. Still, there are controversies in the medical literature concerning the importance of obstructive protective mechanisms such as the LMA's epiglottic bars (7).

There was a small but statistically significant difference in the insertion times for the 2 devices, with the LMA providing an adequate airway faster (by 2.87 seconds) than the CobraPLA. However, no patient in either group experienced an arterial saturation less than 90% during the time of insertion. The times to achieve an effective airway with both devices are similar to those obtained with previous earlier evaluations of the LMA (8) and CobraPLA (3), although in the Akca et al. (3) study differences in insertion times did not reach statistical significance. Despite the fact that the insertion techniques are not identical, placement of each device is relatively simple and straightforward; our results reflect the short learning curve when using the CobraPLA, as our experience with LMA placement (both LMA Classic and Unique) far exceeded that of the CobraPLA.

As in previous studies (3,9), we found cuff seal pressures consistently more than 20 cm H₂O when using the CobraPLA. The higher cuff seal pressure with the CobraPLA, however, represents an advantage only when patients are being mechanically ventilated, and even in that situation ventilatory pressures must be limited to avoid gastric insufflation. During spontaneous ventilation high cuff seal pressures are unnecessary, as airway pressures remain quite low. Thus, with spontaneous ventilation the cuffs function primarily to help stabilize the devices, hold soft tissues away, and prevent leakage of gases during unimpeded respiratory exchange. If one floods the posterior pharyngeal cavity with fluid when using an LMA, the resultant pressure at the posterior pharyngeal wall is approximately 10 cm H₂O (10), so it is evident that very little pressure from the cuffs against the pharyngeal wall is required to ensure a successful seal. In addition, intracuff pressures may not translate exactly into mucosal pressures, as these cuffs are inflated in an environment where soft tissues gently expand away from the inflated cuffs. As a result, extremely low cuff pressures may be adequate in some situations during which spontaneous respiration is occurring when using supraglottic devices.

Fiberoptically determined position of the distal breathing hole was similar for both devices. The Fiberoptic Scores confirm that both the CobraPLA and the LMA Unique occupy an excellent anatomic location to ensure either unimpeded ventilation or passage of an endotracheal tube should the latter become necessary.

The perioperative adverse events rate was infrequent for both devices (Table 3). The incidence of pharyngeal trauma, as evidenced by blood on the surface of the LMA and the CobraPLA after removal, concords with reports from previous studies (3,8). The LMA Unique is relatively soft and the breathing tube is contoured to the patient's airway anatomy, whereas the CobraPLA has a soft flexible distal tip that helps to guide it into proper position. The occurrence of sore throat in PACU could have been related to the direct physical effects of insertion and removal of the devices; however, as seen in Table 3, they did not reach statistical significance. The causes of postoperative adverse events such as sore throat after general anesthesia using supraglottic devices are multifactorial, with the overall incidence influenced by the depth of anesthesia at the time of insertion, the method of insertion (11), the number of insertion attempts (12), the duration of the anesthesia (12), the mode of ventilation used (13), and the type of postoperative analgesia provided (14). In assessing postoperative adverse events, we attempted to control some of the above variables by limiting the insertion attempts to two, using the same intracuff pressures, and limiting surgery time to a moderate amount. Finally, there were no severe adverse events resulting from the use of either device.

Our study has a number of limitations. First, the intraoperative data were obtained by an unblinded observer and thus may have introduced a possible source of bias. Second, the experience of the anesthesiologists placing the devices was much greater with the LMA than it was with the CobraPLA. Third, our results may not be applicable to patients who are mechanically ventilated. Fourth, we do not know if these results would be duplicated with use during surgery of long duration.

In conclusion, both the CobraPLA and the LMA Unique appear to be effective in establishing a clinically patent airway in patients who are spontaneously breathing under general anesthesia during operations of short or intermediate duration. They share similar insertion and functional characteristics and have an acceptably infrequent rate of complications.

	Footnotes

 
Dr. Alfery is the inventor of the CobraPLA (Engineered Medical Systems, Indianapolis, Indiana) and receives royalties on sales.

"Cobra PLA" is a trademark of the Engineered Medical Systems, and all references to Cobra PLA are to this trademark. "LMA Unique" is a trademark of The Laryngeal Mask Company, and all references therein to LMA are to this trademark.

Accepted for publication August 24, 2005.

	References

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a colleague Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (17) Citing Articles via Google Scholar Google Scholar Articles by Gaitini, L. Articles by Alfery, D. Search for Related Content PubMed PubMed Citation Articles by Gaitini, L. Articles by Alfery, D.