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*Department of Anesthesiology, University of Texas Medical School at Houston, Houston, Texas; Department of Anaesthesiology, Gentofte University Hospital, Hellerup, Denmark; University Hospital Mannheim, Institute of Anesthesiology and Intensive Care Medicine, Mannheim, Germany; Hôpital Jeanne de Flandre, Clinique d’Anesthésie Réanimation, Lille Cedex, France
Address correspondence and reprint requests to Carin A. Hagberg, MD, Department of Anesthesiology, The University of Texas Medical School at Houston, 6431 Fannin Street, MSB 5.020 Houston, TX 77030. Address e-mail to carin.a.hagberg{at}uth.tmc.edu.
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Abstract |
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Introduction |
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The Ambu® LMA is a new disposable SGA manufactured from polyvinyl chloride and is currently available in 3 adult sizes (Fig. 1). The device consists of three key elements: an airway tube, a mount member, and a cuff. All three elements are molded together in one piece to minimize the risk of separation of the device. The mount member with the cuff and the airway tube are bent at a 90° angle, following the anatomy of the hypopharynx, pharynx, and mouth, to facilitate proper insertion. The cuff is thin and contoured to fit the hypopharynx so that, when properly positioned, the distal tip of the cuff sits in the upper esophageal sphincter and the proximal end rests at the base of the tongue. The bowl of the mask is open with no aperture bars and faces the glottis (Fig. 2).
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In this international multicenter study, we evaluated the clinical performance of the Ambu LMA in nonparalyzed patients receiving general anesthesia for elective surgery during positive pressure ventilation. The primary objectives were to determine insertion success rate, time of insertion, sealing efficacy, fiberoptic position, stability of the device in different head positions, and intraoperative and postoperative complications. A first attempt success rate of 90% has been reported for the cLMA (2–4); this was also expected to be achieved with the Ambu LMA.
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Methods |
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2.5 cm, disease of the oral cavity, recent upper respiratory tract infection, or increased risk of aspiration were excluded from the study. Other exclusion criteria included surgery of the head or neck, surgery of the thoracoabdominal cavities, or procedures not performed in the supine position. After administration of 100% oxygen for 3 min, anesthesia was induced with alfentanil 20 µg/kg or fentanyl 3.5 µg/kg followed by propofol 2.5 mg/kg. Anesthesia was maintained with propofol (bolus injections or continuous infusion) and additional administration of opioids (i.e., fentanyl, alfentanil, or remifentanil). After anesthetic induction, the ability to perform facemask ventilation was graded as follows: easy (only chin lift required), adequate (jaw thrust required), difficult (requiring jaw thrust and oral airway), or failed (no CO2 trace, no chest wall movement). Once adequate facemask ventilation was established, an appropriately sized Ambu LMA was inserted by an experienced user (>10 Ambu LMA insertions) according to the manufacturer’s recommendations: #3 for patients weighing 30–50 kg, #4 for patients 50–70 kg, and #5 for patients >70 kg. Patients’ lungs were ventilated with positive pressure ventilation using a mixture of air and oxygen. No inhaled anesthetics or muscle relaxants were used.
The cuff of the Ambu LMA was inflated with the recommended volume of air (20, 30, and 40 mL for sizes 3, 4, and 5 LMAs, respectively). The time from picking up the Ambu LMA until the first tidal volume registered on the airway monitor was documented as insertion time, and the number of placement attempts was recorded. A failed attempt was defined as removal of the device from the mouth. Three attempts were allowed before device use was considered a failure. The airway was considered adequate if the minimal expired tidal volume was 6 mL/kg, peripheral oxygen saturation 
95%, and CO2 45 mm Hg with a respiratory rate of 12–14 min, an inspiratory/expiratory ratio of 1:2, and a fresh gas flow of 3 L/min without an oropharyngeal leak or gastric insufflation. Any patient exhibiting an oropharyngeal leak or gastric insufflation was discontinued from the study and excluded from any data analysis. Inflation volumes and intracuff pressures were recorded with both the recommended inflated volume and the inflation volume to achieve an intracuff pressure of 60 cm H2O using a digital manometer (Pressure gauge, Welch Allyn, Skaneateles Falls, NY).
The oropharyngeal leak pressure was determined with an intracuff pressure of 60 cm H2O by closing the expiratory valve of the breathing circle circuit at a fixed gas flow of 3 L/min and noting the pressure at which an oropharyngeal leak occurred. The maximum pressure allowed was 40 cm H2O. The presence of any gastric inflation was evaluated by auscultation of the stomach. The position of the Ambu LMA was evaluated with a fiberoptic bronchoscope by positioning the tip of the bronchoscope at the end of the airway tube of the Ambu LMA. The fiberoptic view of the glottis was scored according to an established scoring system as follows: 0 = failure to function with no cords seen, 1 = cords not seen but function adequate, 2 = cords plus anterior epiglottis seen, 3 = cords plus posterior epiglottis seen, 4 = only cords seen (5).
In Center III, the stability of the device was evaluated in 5 different head positions: 1) head on a standard pillow, 2) head rotated 90° to the left side, 3) head rotated 90° to the right side, 4) head with chin lift on a standard pillow, and 5) head flat on table without a pillow. Possible effects of positioning included the development of an oropharyngeal leak, gastric insufflation, changes in end-tidal CO2 concentration, or chest movements during and after every position change. Any changes in these variables were noted and recorded after 5 tidal volume breaths.
During maintenance of anesthesia heart rate, mean arterial blood pressure, Spo2, respiratory rate, end-tidal CO2 concentration, presence of a square end-tidal CO2 wave, Fio2, expiratory tidal volume, oropharyngeal leak pressure, and gastric insufflation were documented every 5 min. Any adverse events (i.e., aspiration/regurgitation, bronchospasm, airway obstruction, coughing, gagging, vomiting, or hiccupping) and corresponding interventions were recorded.
Before tracheal extubation, patients received 100% oxygen and the final intracuff pressure was recorded before deflation of the cuff. Once anesthesia was discontinued, the Ambu LMA was removed after the return of spontaneous ventilation, protective airway reflexes, and the patient’s ability to follow commands. Any coughing during extubation, blood staining on the device, or trauma to the tongue, lips, or teeth was recorded.
Postoperative interviews were performed at 1 h and 12–24 h after the procedure. Patients were queried regarding the presence of a sore throat, dysphonia, or dysphagia and requested to grade these symptoms as mild, moderate, or severe.
Sample size was based on previous studies of the cLMA and its insertion success rate as 90%. Using a 
2 test with a 5% two-sided significance level, a sample size of 101 would be need to have an 85% power to detect a 10% difference in the insertion success rate of the Ambu LMA from the insertion success rate of the cLMA. The sample size was increased to 120, as the expected dropout rate was 16%. For statistical analysis, the SPSS Version 11.5 for Windows (SPSS, Chicago, IL) was used.
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Results |
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The mean ± sd (range) values for age, height, and weight were 43 ± 14 yr (18–65 yr), 171 ± 8 cm (155–196 cm), and 72 ± 13 kg (50–107 kg), respectively, with BMI 24 ± 3 kg/m2 (18–30 kg/m2). The men:women ratio was 47:71, and all patients were ASA I/II. Data regarding the performance of the Ambu LMA are presented in Table 1.
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The majority of the patients underwent gynecologic 38 (32.2%) or orthopedic 29 (24.6%) procedures. The remaining 51 (43.2%) patients underwent urological, abdominal, or vascular procedures. The mean length of anesthesia was 67 ± 41 min (25–250 min). Facemask ventilation was graded as easy in 101 (85.6%), adequate in 15 (12.7%), and difficult in 2 (1.7%) patients. Failed facemask ventilation did not occur.
Two patients required a different size mask than recommended according to their weight because of inability to obtain an adequate seal. Both were size 4 Ambu LMAs. One was changed to a size 5 and the other to a size 3 (after first trying a size 5). A defective pilot balloon was responsible for the change to a different mask of the same size in 1 patient.
In 105 (89%) patients, the immediate recovery period was uneventful. Coughing during removal of the device occurred in 1 (0.8%) patient. In 10 (8.5%) patients, a small amount of blood was present on the cuff of the LMA after removal; in 1 (0.8%) patient, the lip was cut during insertion and a small cut under the tongue was observed. There was no incidence of aspiration/regurgitation, bronchospasm, airway obstruction, coughing, vomiting, or hiccupping.
Although all 118 patients were questioned 1 h after surgery, only 92 patients were available for the second interview. A mild or moderate sore throat was reported by 6 (5.1%) and 2 (1.7%) patients, respectively within 1 h postoperatively. After 12–24 h, only 2 (2.2%) patients complained of mild sore throat. One patient (0.8%) complained about dysphonia at 1 h and 24 h after surgery. Mild and moderate dysphagia was present in 3 (2.5%) and 1 (0.8%) patients, respectively, 1 h after surgery. There were no complaints of dysphagia at 12–24 h.
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Discussion |
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The insertion time in our study was defined to be the time from picking up the device to the first recorded tidal volume on the airway monitor, as previously defined by Brimacombe et al. (2,9). Different authors define the beginning and end-point of the insertion time differently (6,10), and some do not specify the cornerstones of the interval at all. Thus, reported insertion times can not always be compared. Nonetheless, insertion time of the Ambu LMA appears to be longer than the LMA Unique TM (uLMA) but similar to the ProSeal TM LMA (pLMA), given similar points of reference (9).
In all 118 patients enrolled in this study, the criteria for an effective airway were met. The mean oropharyngeal leak pressure was slightly higher (25.66 cm H2O) for the size 5 than for the size 4 (22.8 cm H2O) Ambu LMA. The mean leak pressure for all 3 sizes was 24.09 ± 5.44 cm H2O, which is higher than those usually achieved with the cLMA (18–22 cm H2O) but lower than that with the pLMA (27–29 cm H2O) (2,11,12). Thus, our study demonstrates that the Ambu LMA provides an effective seal for positive pressure ventilation in this patient population.
Correct anatomical placement was documented in 99.2% of the patients. In 78.8% of patients, the Ambu LMA was found aligned in the midline of the airway with the vocal cords and the anterior or posterior portion of the epiglottis was seen, whereas only cords were visualized in 12% of the patients. Most studies show little or no correlation between fiberoptic position and function for LMA devices (13), which is demonstrated in this study as well. Although the absence of aperture bars has not been found to influence fiberoptic position, absence of these bars may allow easier passage of instruments through the device into the respiratory tract, possibly facilitating passage of a fiberoptic bronchoscope, in combination with the Aintree airway exchange catheter (Cook Critical Care, Bloomington, IN).
The stability of the device in different head positions was tested in 30 patients in one of the participating centers. No changes in the performance of the device or ventilatory criteria were documented after all position changes. Thus, the Ambu LMA may be a useful SGA for cases in which head movement may be necessary for surgery. LMA devices have been used in patients in the lateral or prone position; few investigators evaluated the stability of LMAs with different head positions (14–16). Interestingly, although these studies did not find major changes in the performance of the LMAs in different head positions, the oropharyngeal leak pressure was found to increase with flexion of the head, whereas it decreased with head extension. Okuda et al. (17) observed the same pattern in the stability of LMA in children age 1.5–8 years.
The incidence of postoperative complaints, such as sore throat, dysphonia, and dysphagia 1 and 24 hours after surgery, was less than the incidences reported with comparable devices despite the inclusion of more female patients (2,8,18). It is likely that the soft material of the Ambu LMA cuff together with a frequent first attempt insertion rate and a 100% success rate with the second attempt contributes to the decreased incidence. Whether this already very infrequent incidence can be further reduced by reducing the cuff pressure just above an appropriate seal pressure might be an interesting subject for further investigation.
The incidence of blood staining on the device (8.5%) after removal is also less than the incidence reported for most other SGAs (9,18,19). Verghese et al. (18) found an incidence of 18% for the cLMA and uLMA. Finally, the incidence of minor adverse effects, such as coughing on the device during removal or minor mucosal trauma, was very infrequent and comparable with the cLMA.
Limitations of the study included: 1) this was an initial evaluation of the overall performance of the Ambu LMA in nonparalyzed, anesthetized patients and was noncomparative by design, 2) the patient population was limited to elective patients with normal airway anatomy who were nonparalyzed and anesthetized with total IV anesthesia and controlled ventilation, 3) only one center tested the performance of the Ambu LMA in different head positions, 4) the use of 4 centers might have created center deviations, which is also seen in the study results.
We conclude that the Ambu LMA is a safe, reliable and well tolerated single-use SGA. It is easy to insert and provides an effective seal during positive pressure ventilation in nonparalyzed patients undergoing elective surgery. The Ambu LMA also affords stability with head positioning and has an infrequent incidence of postoperative pharyngolaryngeal morbidity with its use.
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Footnotes |
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Accepted for publication June 9, 2005.
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References |
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This article has been cited by other articles:
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  S. J. Vaida and U. A. Yodfat Angulation of the Airway Tube in the AMBU Laryngeal Mask Could Be Responsible for Improved Insertion Success. Anesth. Analg., July 1, 2006; 103(1): 264 - 264. [Full Text] [PDF]
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 Disposable Laryngeal Mask Performs Well Journal Watch Emergency Medicine, January 17, 2006; 2006(117): 7 - 7. [Full Text]
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