The laryngeal mask airway (LMA) is an established device during general anaesthesia with spontaneous as well as controlled ventilation, for management of the difficult airway and for airway management during resuscitation. However, the process of washing and preparation for repeat use is time consuming. More important, it has been shown that even repeat autoclaving does not remove protein deposits from the reusable LMA, thus potentially allowing disease transmission through residual biological debris .
Recently, several disposable LMAs have been introduced into clinical practice by different manufacturers, that differ slightly in the materials used and some constructive details, e.g. the shape of the mask's shoe and the presence or absence of mask aperture bars, each claiming an edge over the competitors in terms of handling, costs, or both .
The purpose of the present prospective, randomized, controlled trial was to compare the LMA Unique™ (LMA-U), the Ambu laryngeal mask (Ambu LM) and the Soft Seal laryngeal mask (Soft Seal LM) during positive pressure ventilation in non-paralysed patients undergoing routine surgical procedures.
After approval of the institutional review board and written informed consent was obtained, 120 ASA I–III patients undergoing general anaesthesia for routine minor obstetric surgical procedures were enroled in our study and randomly allocated (by opening of a sealed envelope) to the LMA-U (n = 40), Ambu LM (n = 40) or Soft Seal LM (n = 40) group, respectively. All patients undergoing surgical procedures with controlled ventilation not requiring endotracheal intubation were eligible. Patients with morbid obesity (BMI >35 kg m−2) or otherwise considered to have an increased risk of aspiration were excluded as well as patients with a limited interincisor distance (<2.5 cm) or a known difficult airway.
All patients were premedicated with midazolam 0.1 mg kg−1 orally 30 min before induction of anaesthesia. Devices were tested for leaks before insertion, lubricated on the tip and posterior surface with water-soluble lubricant, and the cuff was subsequently inflated with 5 mL air.
Routine monitoring was established including non-invasive blood pressure (NIBP), peripheral oxygen saturation (SPO2) and heart rate (HR; S/5, Datex- Ohmeda, Helsinki, Finland). Preoxygenation with 100% oxygen via face mask for 3 min was followed by standardized induction of anaesthesia with 2 mg kg−1 propofol and 0.1 mg kg−1 remifentanil, and anaesthesia was subsequently maintained with propofol (4–8 mg kg−1 h−1) and remifentanil (0.2–0.5 μg kg−1 h−1). Anaesthesia was monitored using bispectral index (BIS XP™; Aspect Medical Systems, Wallingford, USA).
After loss of eyelash reflex, the patient's head was positioned in a neutral position and the airway device inserted according to the manufacturer's instructions. Time to successful insertion was measured from ‘touching’ the device after loss of eyelash reflex until the first expiratory tidal volume >200 mL.
All devices were inserted by a single anaesthesiologist (H.F.), with experience using each of the study devices (>50 uses with each of the study devices and >1500 insertions of other LMAs). A size 4 LMA-U, a size 4 Ambu LM and size 4 Soft Seal LM were used for all patients. The cuffs (LMA-U, Ambu LM and Soft Seal LM) were inflated initially with 20 mL. Resulting cuff pressures were recorded in all patients (high fidelity pressure gauge; Smiths Medical, Kirchseeon, Germany; pressure range ±500 mmHg, accurate to ±0.5%). The airway was considered adequate if a tidal volume of 7 mL kg−1 was achieved with a typical square CO2 curve on the capnograph; inspiratory concentration of oxygen was set to 30%.
Airway leak pressure was determined by closing the expiratory valve of the breathing system at a fixed gas flow of 3 L min−1 and noting the resulting airway pressure (maximum allowed was 40 cmH2O) at which equilibrium was reached. Gastric inflation was assessed with a stethoscope placed on the epigastrium. Air entrainment into the stomach was detected while measuring oropharyngeal leak pressure. After taking baseline values, cuff volume in all three devices was increased by another 10 and 20 mL, respectively (total cuff volume 30 and 40 mL in all devices). Measurement of resulting cuff pressure and airway leak pressure was repeated and then volume was immediately returned to baseline values.
Following completion of the surgical procedure, the airway devices were removed when the patients could open their mouths to respond to commands; the cuff of the device was immediately deflated before removal and the presence or absence of blood was noted.
Respiratory parameters were recorded directly after device insertion and 5 and 10 min thereafter, respectively; haemodynamic and BIS values were recorded before and after device insertion followed by measurement every 5 min throughout the study period. Capillary blood gas samples were taken before induction of anaesthesia, and 10 min following the first successful ventilation.
Adverse events during anaesthesia were recorded and defined as aspiration/regurgitation, hypoxia (SPO2 <90%), bronchospasm/airway obstruction and dental trauma.
Variables studied were overall success rate for ventilation, time for airway insertion, cuff pressure and resulting leak pressure, expiratory tidal volume, blood gas, haemodynamic response, subjective assessment of the handling of either device, and the frequency of postoperative sore throat, dysphagia and hoarseness.
Upon completion of the study protocol, the single experienced anaesthesiologist who inserted all devices gave a subjective assessment of both the insertion procedure that was rated as very easy, easy and difficult or failed (after three failed attempts or if more than 180 s were needed) and the handling of either device, that was rated as excellent, good, fair and poor. Furthermore, 6 and 24 h after surgery, all patients underwent a structured interview by an anaesthesiologist blinded to the airway device used. The interview comprised the degree of sore throat, hoarseness and dysphagia. Symptoms were graded as nil, moderate and severe. Patients were also unaware of the airway device used in their individual case.
Sample size calculation was based on two previous studies comparing the LMA-U with the ‘Classic’ LMA and the Soft Seal LM, respectively [3,4]; accordingly, we calculated the sample size to detect at least a 20% difference between devices for the primary end-point (time to successful ventilation) with an α-error of 0.05 and a power of 0.9. Non-parametric data between groups were analysed with the U-test, while parametric data were compared with unpaired t-test. Proportions and anaesthesiologists' experiences were compared with Fisher's exact test or χ2-test, as appropriate. Changes of parameters within each group over time were analysed with one way repeated analysis of variance with Bonferroni's test correction for multiple comparisons and Friedman's test with Dunn's post-test correction, as appropriate. Parametric data are given as mean ± standard deviation (SD), and non-parametric data are presented as median, interquartile range and range. P < 0.05 was considered significant.
Groups were comparable with regard to patient characteristics data, body weight and height, ASA classification, duration of surgery and number of predictors of a difficult airway (Table 1).
There were no significant differences in cardiorespiratory variables, expiratory tidal volumes and BIS values between the three groups at any time (Table 2). HR and arterial pressure decreased after induction of anaesthesia and remained stable thereafter (Table 2).
In the LMA-U group, a successful primary airway was established in 35 patients (87%) on the first attempt and in the remaining five patients (100%) on the second attempt. In the Ambu LM group, device insertion was successful in 35 patients (87%) on the first attempt, in three patients (95%) on the second attempt and in one patient (97%) after three attempts. In the Soft Seal LM group, insertion was successful in 35 patients (87%) on the first attempt, in 1 patient (90%) on the second attempt and in 2 patients (95%) after three attempts.
In one patient (3%) in the Ambu LM group and in two patients (5%) in the Soft Seal LM group device insertion failed even after three attempts and patients were managed by tracheal intubation. No gastric inflation occurred with either device.
Time of insertion was comparable with the LMA-U (median 19 s; range 8–57 s) and the Soft Seal LM (median 20 s; range 12–46 s) and significantly (P < 0.005 and P < 0.0001, respectively) shorter in the Ambu LM group (median 14 s; range 8–35 s). Insertion of the airway device was generally easier in the LMA-U and Ambu LM groups compared to the Soft Seal LM group. Insertion was judged ‘excellent’ in 75% of patients in the LMA-U group, in 70% of patients in Ambu LM group and in 65% of patients in the Soft Seal LM group (Table 3).
Blood gas analyses revealed sufficient oxygenation with either device (Table 4). In vivo cuff pressure was lowest with the Soft Seal LM, while airway leak pressure was higher in Ambu LM and Soft Seal LM groups (Table 5). Generally, both cuff pressure and airway leak pressure increased with increasing cuff volume (Table 5).
No major adverse event occurred during the intra- and immediate postoperative period in any patient in the LMA-U group. However, one patient in both the Ambu LM and Soft Seal LM group experienced bronchospasm/airway obstruction 20 min after device insertion.
Blood staining after removal of the devices (2%) was comparable in all groups. There were no differences regarding postoperative airway morbidity (Table 6).
The reusable ‘Classic’ LMA is an ingenious supraglottic airway device with well established properties for airway management in both routine and emergency cases, and has evolved as a standard airway adjunct for ventilation of patients not requiring endotracheal intubation. Several alternative LMAs have recently been introduced into clinical practice, most of them intended for single use. Apart from cost-effectiveness, disposable devices may be advantageous since there is no risk of transmitting diseases .
The first single-use device, the LMA Unique™ was released in 1997 (LMA Company, Henley on Thames, UK). The main difference to the ‘Classic’ LMA is the material used (polyvinyl chloride instead of medical-grade silicon rubber), and the device performed similarly to the reusable LMA . More recently, two other disposable LM have been introduced, the Ambu LM (Ambu A/S, Ballerup, Denmark) and the Soft Seal LM (Portex Ltd, Hythe, UK). Both are devoid of mask aperture bars, which are patent protected until 2008. The Ambu LM features a special curve that claims to emulate natural human anatomy . This is intended to ensure that the patient's head remains in a natural, supine position when the mask is in use. The absence of mask aperture bars of the Soft Seal is claimed to facilitate access for flexible fibreoptic scopes. Furthermore, a one-piece design with no step between tube and shoe is supposed to assist in easy placement . So far, there are only limited data available as to the safety and efficacy of these devices, and it is conceivable that either the different materials used in the reusable and disposable devices or the slightly differing shape of the disposable LMAs may influence the accept–reject profile . To our knowledge, the aforementioned LMAs have never been compared with each other in a controlled trial.
Our study was performed in non-paralysed patients for elective general anaesthesia with intravenous (i.v.) agents during controlled ventilation. In this scenario, the Soft Seal LM provided the highest ventilation seal at all cuff volumes applied with the lowest intracuff pressures. This was not associated with higher airway morbidity. While first time insertion rate was comparable between devices, the Soft Seal LM had the highest failure rate after three attempts (5%) and was more likely to be rated inferior regarding handling. This was also reported in a recent audit of the Soft Seal LM, where failure rate (3.8%) and first time success rate (83.8%) were very similar to our study . In contrast, insertion of the LMA-U was rated ‘excellent’ in 75% of patients.
Several investigations have compared the LMA-U and the Soft Seal LM with the ‘Classic’ LMA. Generally, both devices have been found to be comparable to the ‘Classic’ LMA [3,6,9–12]. The main difference was the lack of nitrous oxide diffusion into the cuffs of the single-use LMAs, which was attributed to the different materials used for production. In the study by Paech and colleagues, however, the Soft Seal LM was more difficult to insert and associated with increased postoperative sore throat .
More recently, several trials have compared the LMA-U with the Soft Seal LM both in spontaneously breathing and paralysed patients [4,14–16]. Tan and colleagues reported a significantly higher oropharyngeal leak pressure in the Soft Seal LM (21 ± 6 cmH2O vs. 16 ± 6 cmH2O), while there was no difference with respect to first time success rates and insertion times . A higher proportion of patients in the Soft Seal LM group experienced sore throat, and blood was found in 32% of patients in the Soft Seal LM group (9% LMA-U group). Similar results were reported in a large trial performed in spontaneously breathing adults . First time success rates were comparable (91% Soft Seal LM vs. 96% LMA-U), while first insertion times slightly differed (41.5 vs. 38.1 s). The Soft Seal was more often rated as difficult to insert and was more likely to result in mucosal trauma after the first insertion (10% vs. 4%). Again, efficacy of seal (tested at 20 cmH2O) was better with the Soft Seal LM (59% vs. 39%) . Cook and colleagues also reported on a higher seal pressure, a more difficult insertion, more manipulations and an inferior overall usefulness comparing the Soft Seal LM with the LMA-U . Taking these studies and our own results together, it is suggested that the Soft Seal LM provides a better airway seal but is inferior regarding ease of insertion and handling compared with the LMA-U.
Performance of the Ambu LM in our study showed major differences with respect to insertion times and airway leak pressure to the recently published multicentre evaluation (insertion time: 14 vs. 45 s; airway leak pressure 18 vs. 24 cmH2O) . The large variability of insertion times recorded in the latter investigation may reflect a different level of training of the attending anaesthesiologists or differences in the patient population enroled. Even at a cuff volume of 40 mL we were unable to generate a mean leak pressure above 21 cmH2O. A leak pressure of 24 cmH2O as reported by Hagberg and colleagues would be superior to both the ‘Classic’ and the competing disposable LMAs and comparable to the Pro Seal LMA, that has performed outstanding in this respect , and further investigations are necessary to verify this property.
Interestingly, intracuff pressures in individual patients were largely above recommended values (60 cmH2O) even at the lowest cuff volume (20 mL) inflated. Differences between devices, however, may not predict a differing postoperative airway morbidity. The in vivo cuff pressures measured in our study are in good agreement to values reported by Keller and Brimacombe in cadavers . Since in vivo cuff pressures in cadavers were only moderate predictors of the applied mucosal pressures, differences found may not reflect the pressure exerted on the mucosa. Nevertheless, in vivo cuff pressures may be of clinical significance. In cadaver studies investigating supraglottic airway devices, mucosal pressures became critical at very high in vivo cuff pressures [19,20]. More important, increasing the cuff volume in our study did not improve airway leak pressure in a clinically relevant fashion, and may even deteriorate efficacy of seal and increase postoperative airway morbidity, as demonstrated previously with the ‘Classic’ LMA [21,22]. Since cuff volumes often used in daily clinical practice with LMAs were found to result in cuff pressures largely above recommended values, a cuff pressure control should be performed after each device insertion. Efficacy of seal may vary dependent on the individual patient's laryngopharyngeal anatomy. Therefore, applying fixed cuff volumes may not be recommended, may contribute largely to airway morbidity  and should be replaced also in the instruction manuals by insufflating the lowest cuff volumes resulting in an acceptable airway leak pressure guided by meticulous cuff pressure monitoring.
Some limitations of our study should be noted. The present study was performed in patients with normal airways during controlled positive pressure ventilation by an investigator trained in the use of each device. Therefore, we cannot comment on results obtained in spontaneously breathing patients, during difficult airway management or with naïve users, though we speculate that the magnitude of the differences found is quite similar. It may be criticized, that we inflated the cuffs of the LMAs with a fixed volume rather than adjusting them to a predefined cuff pressure. However, in daily clinical practice pressure manometers are rarely used and not always available, and we sought to study a clinically relevant scenario.
In conclusion, all three disposable LMAs proved to be suitable for controlled ventilation during routine surgical procedures. In vivo cuff pressures were found largely above recommended values even at the lowest cuff volumes applied. Increasing cuff volume did not improve airway leak pressure in a clinically relevant fashion.
We are grateful to the LMA Company, Bonn, Germany, and to the Ambu Company, Ballerup, Denmark, for supplying the devices used in this investigation. We are indebted to Juergen Hedderich, PhD, for statistical advice and to Bernd Kuhr, RN, for enthusiastic support.
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Keywords:© 2007 European Society of Anaesthesiology
LARYNGEAL MASKS, comparison; RESPIRATION ARTIFICIAL; EQUIPMENT AND SUPPLIES; ADVERSE EFFECTS