For the management of the difficult airway, several devices are available. The laryngeal mask airway (LMA; Laryngeal Mask Company, Henley-on-Thames, UK), laryngeal tube (LT; VBM Medizintechnik, Sulz, Germany) and oesophageal-tracheal combitube (OTC; Tyco Healthcare, Mansfield, MA, USA) all have been advocated as emergency airway devices [1,2]. To date, only the LMA and the OTC have been implemented in the American Society of Anesthesiologists (ASA) difficult airway algorithm . Recently, a modification of the ‘classic’ LMA, the ProSeal laryngeal mask airway (PLMA™; Laryngeal Mask Company, Henley-on-Thames, UK) and a modification of the laryngeal tube airway; the laryngeal tube S (LTS®; VBM Medizintechnik, Sulz, Germany) have been introduced into clinical practice [4,5]. Both the PLMA and the LTS allow for gastric drainage via a separate lumen incorporated in the device. This may be advantageous especially in non-fasted emergency patients at risk for regurgitation and aspiration.
The success rate of an airway device in an emergency may be mainly related to the level of training and experience of the user . Therefore, to assure an adequate level of training, the airway device should be suitable for routine use in clinical practice. While there are several reports on the feasibility of the PLMA in this respect , the use of the OTC in daily routine is highly controversial [7-9] and apart from a preliminary study, the LTS has not yet been investigated in a systematic fashion. In the present study, we investigated whether usage of the PLMA, LTS and OTC would yield comparable results during routine surgical procedures.
After approval of the institutional review board and written informed consent was obtained, 90 ASA I-III patients undergoing general anaesthesia for routine minor obstetric surgical procedures were enrolled in our study and randomly allocated (by opening of a sealed envelope) to the PLMA (n = 30), LTS (n = 30) or OTC (n = 30) group, respectively. Patients with morbid obesity (body mass index > 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. Routine monitoring was established including non-invasive blood pressure, peripheral oxygen saturation (SpO2) and heart rate (HR; S/5, Datex-Ohmeda, Helsinki, Finland). Preoxygenation with 100% oxygen (O2) 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 min−1). Depth of anaesthesia was assessed using bispectral index (BIS XP™; Aspect Medical Systems, Wallingford, USA). Successful ventilation was assured with capnography and bilateral chest auscultation. Subsequently, patients were ventilated with a tidal volume of 8 mL kg−1 at a respiratory rate of 10 breaths min−1. 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. Adverse events during anaesthesia were recorded and defined as aspiration/regurgitation, hypoxia (SpO2 < 90%), bronchospasm/airway obstruction and dental trauma.
After loss of eyelash reflex, the patient's head was placed 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 of >200 mL.
The cuffs were inflated as follows: initially, the PLMA cuff was inflated with 20 mL, the LTS cuff with 70 mL and the OTC cuffs, pharyngeal and oesophageal, with 75 and 12 mL, respectively. Resulting cuff pressures were recorded in all patients (high fidelity pressure gauge, Smiths Medical, Kirchseeon, Germany; pressure range ± 500 mmHg, accurate to ±0.5%). Airway leak pressure was determined by adjusting the expiratory valve of the breathing circle to 40 cmH2O (fixed fresh gas flow 3 L min−1) and recording the airway pressure at which equilibrium was reached . By listening with a stethoscope over 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 mL (total cuff volume: PLMA 30 mL; LTS 80 mL; OTC pharyngeal cuff 85 mL); the oesophageal cuff of the OTC being kept at a volume of 12 mL during the study period. Measurement of resulting cuff pressures and airway leak pressures was repeated and then the volume was immediately returned to baseline values. If ventilation was impossible after insertion of the device, the position of the device was adjusted by gently pushing or pulling it and adequacy of ventilation was reassessed. If more than three attempts were necessary for placement of the device or more than 3 min were needed, the attempt was terminated, recorded as failed and the airway managed as clinically indicated. After successful placement of the device, a 14-F gastric catheter was advanced in the oesophagus via the separate drain tube in the PLMA and LTS group, while the distal tube was used in the OTC group, followed by aspiration of gastric fluid.
Variables studied were overall success rate for ventilation, times for airway insertion, cuff pressures and resulting leak pressure, expiratory tidal volumes, haemodynamic responses, subjective assessment of the handling of either device and the frequency of postoperative sore throat, dysphagia and hoarseness.
The study was performed by two anaesthesiologists experienced with the devices under investigation (>25 uses each), who inserted 15 PLMA, 15 LTS and 15 OTC each, respectively. A size 4 PLMA, a size 4 LTS or a ‘small adult’ size OTC were used for all patients.
Upon completion of the study protocol, the anaesthesiologist gave a subjective assessment of the handling of either device, which was rated as excellent, good, fair and poor. Between 18 and 24 h after surgery, all patients underwent a structured interview by a medical student blinded to the airway device used. The interview asked the degree of sore throat, hoarseness and dysphagia. Symptoms were graded as nil, moderate or severe. Patients were unaware of the airway device used in their individual case.
Sample size calculation was based on two previous studies with the PLMA and LTS. We calculated the sample size to detect at least the difference between the devices which had been described previously [4,5] for the primary end-point (time to successful ventilation) with an α-error of 0.05 and a power of 0.9. However, after enrolling half of the total number of patients planned, an interim analysis was performed and the Institutional Review Board considered it unethical to complete the study due to the high incidence of severe sore throat and dysphagia in the OTC group.
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 (ANOVA) using Bonferroni 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.
There were no significant differences between groups with regard to patient characteristics data and number of predictors of a difficult airway (Table 1).
Cardiorespiratory variables, expiratory tidal volumes and BIS values remained stable throughout the procedure and did not differ significantly between groups except for a higher peak airway pressure in the OTC group (Table 2).
In the PLMA group, a successful primary airway was established in 26 patients (87%) on the first attempt and in the remaining four patients (100%) on the second attempt. In the LTS group, device insertion was successful in 28 patients (93%) on the first attempt and in the remaining two patients (100%) on the second attempt. In the OTC group, 26 patients (84%) and one patient (90%) were ventilated successful on the first and second attempt, respectively; in three patients (10%), device insertion failed even after three attempts and patients were managed by tracheal intubation. Gastric catheter placement was successful in all patients at the first attempt.
Placement of the distal lumen of the OTC in the oesophagus was proved in all patients by aspiration of gastric fluid. Time required for the first adequate ventilation was significantly (P < 0.0001) shorter in the PLMA (median 29 s; IQR 25-48 s; range 10-161 s; success rate 100%) and in the LTS group (38 s; 30-44 s; 13-187 s; 100%) compared to the OTC group (75 s; 48-98 s; 35-183 s; 90%). In vivo cuff pressures and airway leak pressures increased with the inflating cuff volume in all devices and were the highest in the OTC group at both oropharyngeal cuff volumes, 75 and 85 mL (Table 3).
Patients in the PLMA and the LTS group complained significantly less about sore throat (P < 0.001 and 0.05) and dysphagia (P < 0.001 and 0.02) postoperatively compared to the OTC group, while there was no difference regarding the incidence of hoarseness (Table 4). Subjective assessment of handling was comparable with the PLMA and the LTS, but inferior with the OTC (Table 5).
In the OTC group, one patient started coughing and retching after insertion of the device, and gastric fluid regurgitated through the distal tube; laryngoscopic examination showed no fluid in the hypopharynx and the case was completed uneventfully.
The main findings of our prospective, randomized study show the following:
- Both the PLMA and the LTS proved to be suitable for airway management during routine surgical procedures.
- In the PLMA group cuff volumes often used in daily clinical practice result in in vivo cuff pressures largely above recommended values.
- The OTC was inferior in technical aspects of airway management (i.e. time to successful ventilation, failure rate) and showed the highest in vivo cuff pressures.
- Airway morbidity following anaesthesia was lowest with the PLMA and highest with the OTC.
According to the use of a device in daily clinical practice, a different proportion of anaesthesiologists will become familiar with alternative airway devices . The PLMA and the LTS both showed acceptable times for device insertion, airway leak pressures and post-anaesthesia airway morbidity. There seem to be some advantages of the PLMA in terms of patient comfort as reflected by a significant lower incidence of sore throat and dysphagia in the PLMA group. On the other hand, the LTS may offer an advantage in patients at an increased risk of aspiration (non-fasted, gastro-oesophageal reflux disease, emergency cases) due to the safe airway seal together with the possibility of free gastric drainage. In a former study comparing the LMA and LT using a model rubber pharynx, the LT showed significantly increased storage capacities for regurgitated fluids . However, the PLMA probably by forming a better seal than the classic LMA, attenuates liquid flow between the oesophagus and the pharynx more effective than the classic LMA, as recently demonstrated in a cadaver study . On the other hand, improper PLMA placement  as well as upper airway obstruction in spontaneously breathing patients [14,15] may not provide sufficient protection against aspiration. The difference between the PLMA and the LTS in this respect, therefore, remains speculative.
Filling the cuff of the PLMA with a predefined cuff volume (20 and 30 mL, respectively) resulted in cuff pressures largely above recommended values. This may be of clinical significance, since in daily clinical practice, recommended intracuff pressure often is ignored and cuff overinflation unnoticed, especially during short procedures with spontaneously breathing patients . Of note in the LTS, predefined cuff volumes for emergency cases resulted in cuff pressures matching recommended values. Interestingly, a cuff volume of 70 mL in the LTS group was sufficient to provide an acceptable airway leak pressure, thus avoiding cuff overinflation in most patients. In contrast, the OTC proved to be inferior in most aspects of practical use, such as time to successful ventilation, number of failed attempts and handling as well as in terms of patient satisfaction and comfort. The high cuff pressures found in the present investigation are in good agreement to mucosal pressures in the cadaver study of Keller and colleagues which were obtained with sophisticated, high fidelity recording technology. More important, the type of complaints of the patients in the OTC group (dysphagia, sore throat) match the anatomical region where the high cuff pressure is applied (pharynx and hypopharynx), while there was no difference regarding hoarseness between groups, since the vocal cords normally are not passed with the device. A significantly increased airway morbidity with the OTC compared to the LMA was also reported in another study investigating both devices during routine surgery . The high cuff pressures in the OTC group may be harmful for the oropharyngeal mucosa if applied for a longer period of time.
Post-anaesthesia airway morbidity has gained widespread attention, especially in a health environment where cost containment is essential and patient satisfaction is of high priority . A high incidence of sore throat and dysphagia, as found in the present study with the OTC, may, therefore, discourage anaesthesiologists from using a device probably causing patient dissatisfaction and increased length of hospital stay.
Some limitations of our study should be noted. Due to the limited sample size, there may be some differences between groups which were missed. However, we found significant differences in most of the important primary and secondary study end-points. The present study was performed in patients with normal airways during controlled positive pressure ventilation by investigators 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 are similar in patients with anticipated difficult airways. It may be criticized, that we inflated the cuffs of the PLMA and the LTS with a fixed volume rather than adjusting them to a predefined cuff pressure, as recommended by the manufacturer. However, in daily clinical practice as well as in an emergency or out of hospital situation, pressure manometers are rarely used and not always available. It is well known, that in vivo cuff pressures are only moderate predictors of the applied mucosal pressures. The in vivo cuff pressures measured with the PLMA and the LTS in our study are in good agreement with values reported by Keller and Brimacombe in patients and awake volunteers [19,20]. However, in these studies the measured in vivo cuff pressures did not result in mucosal pressures thought to impede pharyngeal mucosal perfusion, except for the LTS inflated with 80 mL at the posterior laryngopharynx. In contrast, in vivo OTC cuff pressures in the present study closely match mucosal pressure in a study in fresh cadavers and awake volunteers . This discrepancy may be explained, at least in part, by the fact that the latex material used for the cuff in the OTC is very rigid and, therefore, less compliant than the low-pressure cuffs used in the PLMA and the LTS, respectively. In the latter devices, effective mucosal pressure is mainly influenced by cuff conformity with the pharynx. Nevertheless, in vivo cuff pressures may be of threefold significance. Firstly, cuff volumes often used in daily clinical practice with the PLMA were found to result in cuff pressures largely above recommended values and, therefore, a cuff pressure control should be performed after each PLMA insertion. Secondly, the inflating cuff volume in the LTS closely matches the resulting in vivo cuff pressure, thus indicating a suitable alternative especially for use in an emergency and out of hospital situation. And finally, with the OTC in vivo cuff pressure monitoring seems to be suitable to assess effective mucosal pressure.
Efficacy of seal may vary dependent on the individual patient's laryngopharyngeal anatomy. Thus, applying fixed cuff volumes may not be recommended, may contribute to airway morbidity  and should be replaced in the instruction manuals by using the lowest cuff volumes resulting in an acceptable airway leak pressure guided by meticulous cuff pressure monitoring. Intracuff pressure monitoring should become routine practice with any extraglottic airway device.
The results of our study suggest that the LTS and the PLMA are advantageous compared with the OTC with respect to the time required for insertion of the device, failure rate and patient comfort. In conclusion, the OTC is largely discounted for routine use. Since the success of emergency airway management is influenced by the degree of training and familiarity with the device used, the OTC may not be the first choice in emergency airway management in the future, although this device predominantly has influenced out of hospital airway management in the past decade.
We are grateful to LMA Company, Henley-on-Thames, UK; to VBM Medizintechnik, Sulz, Germany, and to Tyco Healthcare, Mansfield, MA, USA, for supplying the devices used in this investigation.
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