The ProSeal™ laryngeal mask airway (PLMA™) (Laryngeal Mask Company, San Diego, CA, USA) is a supraglottic airway device with a larger cuff than the Classic™ laryngeal mask airway to produce a better seal. The PLMA is also equipped with a drainage tube to permit insertion of a gastric tube and evacuation of gastric content.1 The presence of the drainage tube reduces the risk of aspiration, which is the major concern of the Classic laryngeal mask airway, especially when the device is used with positive pressure ventilation.2 Malpositioning of the PLMA is common in clinical practice because its soft cuff can fold over onto itself.3 While a dedicated introducer (commonly known as an “introducer tool”) is recommended by the manufacturer to facilitate insertion of the PLMA,1 difficulties can still be encountered during insertion. The PLMA insertion success rate at first attempt has been reported as 82-87%, which is lower than the insertion success rate of the Classic laryngeal mask airway.2,4,5

Many methods have been proposed to facilitate insertion of PLMAs, including insertion of a gastric tube,6 a suction catheter,7 or an Eschmann® tracheal tube introducer (commonly known as a gum elastic bougie)8 into the drainage tube. These techniques help to prevent the PLMA soft cuff from folding over and help to decrease the incidence of malpositioning. However, all of these techniques involve protrusion of the guiding instrument beyond the tip of the drainage tube, which potentially can cause oropharyngeal trauma. In contrast to these methods, the Flexi-Slip stylet (Willy Rüsch AG, Kernen, Germany) is kept within the drainage tube.9 The Flexi-Slip stylet has a soft tip mounted on a malleable coated wire. It is intended primarily to facilitate tracheal intubation, but it also fits into a PLMA drainage tube. We demonstrated previously that the Flexi-Slip stylet could help facilitate PLMA insertion, although the insertion was difficult using the introducer.10 However, the Flexi-Slip stylet technique has not been evaluated in a randomized controlled trial. The purpose of this study was to investigate the efficacy of the Flexi-Slip stylet technique with regard to facilitating PLMA insertion. The specific aims were to compare the Flexi-Slip stylet technique with the introducer technique in terms of insertion success rates, insertion times, and incidence of airway complications.

Methods

The study was approved by the hospital ethics committee (E-DA Hospital, Kaohsiung City, Taiwan) and informed consent was obtained from all participants. One hundred and sixty consecutive adult patients (American Society of Anesthesiologists physical status I-II; ages 18-80 years) scheduled for elective surgery under general anesthesia with the PLMA were enrolled in the study. Patients with an anticipated difficult airway, a full stomach, coagulopathy, or pre-existing sore throat were excluded. One of the authors (S-C.Y.) managed a computer-generated table of random numbers that were placed in sealed envelopes, and the patients were assigned randomly to one of two groups once medication to induce anesthesia was administered in the operating room. In one group, the PLMA was inserted with an introducer (Introducer group). In the other group, a Flexi-Slip stylet was used to facilitate PLMA insertion (Flexi-Slip stylet group). Patients were unaware of the method of insertion.

In the operating room, standard monitoring was applied with the patient in the supine position and head resting on a 5-cm high pillow. The size of the PLMA was selected according to the manufacturer’s weight-based formula (size 3 for weight < 50 kg, size 4 for weight 50-70 kg, and size 5 for weight > 70 kg).11 The PLMA cuff was fully deflated and lubricated on both surfaces with a water-based lubricant. After preoxygenation with a face mask for three minutes, anesthesia was induced with fentanyl 2 μg·kg−1 iv and propofol 1.5-2 mg·kg−1 iv. Random allocation was performed by opening the sealed envelope immediately after induction of anesthesia. When loss of corneal reflex and unresponsiveness to jaw thrust maneuver were observed,12 the PLMA was inserted using an introducer or a Flexi-Slip stylet, according to the test groups previously allocated. If adverse responses (e.g., gagging, coughing, body movement) occurred during the jaw thrust maneuver or PLMA insertion, additional boluses of propofol 20-30 mg iv were given to achieve adequate anesthesia. The total dose of propofol was documented. All PLMA insertions were performed by the same experienced anesthesiologist (> 500 uses of both techniques).

In the Introducer group, the PLMA with the introducer in place was inserted according to the manufacturer’s instructions.13 The introducer technique involved inserting the cuff into the patient’s mouth, maintaining pressure against the palate, and then advancing the PLMA into the hypopharynx until resistance was felt. The introducer was then removed. In the Flexi-Slip stylet group, a 5.6-mm Rüsch Flexi-Slip stylet (Willy Rüsch AG, Kernen, Germany) was lubricated slightly with a water-based lubricant and passed down the drainage tube to a position just proximal to the end of the drainage tube (Fig. 1A). After the proximal end of the stylet was bent backwards 180°, the PLMA was bent with a near 90° angle at the junction of the cuff and the airway tube (Fig. 1B). The Flexi-Slip stylet technique involved the following steps (Fig. 2): (1) The anesthesiologist opened the patient’s mouth with the non-dominant hand while holding the middle portion of the PLMA airway tube in the dominant hand; (2) The PLMA was inserted into the patient’s mouth; (3) The cuff was advanced into the pharynx past the oropharyngeal angle with a simple wrist motion similar to that required for insertion of a laryngoscope behind the base of the tongue; (4) The anterior surface of the cuff was positioned directly adjacent to the surface of the tongue, and a sliding motion was effected along the curve of the oropharynx without lifting the tongue; and (5) After the cuff passed the base of the tongue, the stylet was removed, and the PLMA was advanced until tactile resistance was felt. In all cases, the cuff was inflated with air to achieve an intracuff pressure of 60 cm H2O after placement of the PLMA into the pharynx and removal of the assisting tool (introducer or Flexi-Slip stylet).

Fig. 1
figure 1

Setting of the Flexi-Slip stylet technique. (A) Tip of stylet (arrow) is positioned 2.0 cm from the distal end of the drainage tube; (B) The ProSeal laryngeal mask airway is bent to form a near 90° angle at the junction of the cuff and the airway tube, with the rigid portion of the stylet extending 5-6 cm into the cuff (arrow: tip of stylet)

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Fig. 2
figure 2

Procedure of the Flexi-Slip stylet technique (informed consent was obtained from the pictured patient): (A) Opening the mouth; (B) Inserting the laryngeal mask into the mouth; (C) Rotating the laryngeal mask caudad toward the base of the tongue with a wrist motion; (D) Removing the stylet and advancing the airway tube until resistance is felt

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After connecting the airway tube to the breathing system, successful placement of the PLMA was evaluated according to the following criteria: (1) successful insertion of the PLMA into the pharynx; (2) absence of PLMA malposition, as evidenced by successful insertion of a suction catheter through the drainage tube with no air leak; and (3) effective ventilation. Air leakage was detected by using a stethoscope to auscultate the mouth, the proximal end of the drainage tube, and the epigastrium during manual ventilation. Effective ventilation was defined as a square wave capnograph tracing with an expired tidal volume > 8 mL·kg−1 and end-tidal carbon dioxide < 45 mmHg during gentle manual ventilation.14 If the criteria for successful placement were not fulfilled, the insertion attempt was considered a failure and PLMA insertion was attempted again. Three attempts were allowed, and if insertion was still unsuccessful, an extra attempt was allowed with the other technique. Insertion time was recorded as the time from inserting the prepared PLMA (introducer or Flexi-slip stylet) into the mouth until successful placement. The reasons for failed insertion and the number of insertion attempts were documented. The oropharyngeal leak pressure (maximum allowed = 40 cm H2O) was measured following successful placement.15 The above data collection and evaluation of successful PLMA placement were performed by the same observer (C-F.C.C.) who was different from the operator (H-S.C.). After successful placement of the PLMA, the patient’s lungs were ventilated with an inspired tidal volume of 8-12 mL·kg−1 and a respiratory rate of 10-12 breaths min−1, maintaining end-tidal carbon dioxide < 45 mmHg. Anesthesia was maintained with 2-3% sevoflurane in 50% oxygen. Cisatracurium 4 mg iv was given for surgical relaxation, if necessary. Adverse events (e.g., hypoxia, aspiration, laryngospasm) were noted.

At the end of the surgical procedure, the PLMA was removed when the patient’s airway reflexes had fully returned. Any visible blood staining on the PLMA was noted at removal by an investigator (J.S. or K-C.H.) blinded to the method of insertion. Severe oropharyngeal injuries (i.e., loss of teeth, lip/tongue laceration) were also recorded. Patients were visited by the investigator who observed blood staining on the PLMA, and they were asked whether they had a sore throat (throat pain at rest or when swallowing) and/or dysphagia (difficulty on swallowing) during the morning after surgery. The primary outcome measurement in this study was the success rate of PLMA insertion at first attempt. Secondary outcome measures were insertion time, oropharyngeal leak pressure, and airway complications.

The sample size was determined on the basis of a published study in which the success rate of PLMA insertion using the introducer technique was 84% at the first attempt.14 Assuming a 15% improvement in success rate (i.e., 99%) with the Flexi-Slip stylet technique, a minimum sample size of 71 patients per group was required, assuming a type 1 error of 0.05 (two-tailed) and a power of 0.9. To allow for possible dropouts, 80 patients were enrolled for each group. Data were presented as mean ± standard deviation except as otherwise noted. The characteristics of patients, insertion time, and oropharyngeal leak pressure were compared using the Student t test. Insertion success rates, the presence of visible blood staining, and the occurrence of airway complications were compared using Chi square analysis or Fisher’s exact test, as appropriate. Fisher’s exact test was used when the expected values in any of the cells of a contingency table were below five. The SPSS® 13.0 software (SPSS Inc., Chicago, IL, USA) was used for data analysis. A value of P < 0.05 was considered statistically significant.

Results

One hundred and sixty patients met the eligibility criteria and were enrolled in the study. The trial protocol was completed in all patients. The characteristics of the patients were similar in both groups (Table 1). Insertion success rates, insertion time, oropharyngeal leak pressure, reasons for attempt failure, and total dose of propofol are presented in Table 2. While the overall success rate was not significantly different between groups (Flexi-Slip group, 100% vs Introducer group, 95%; P = 0.12), the success rate of insertion at the first attempt was higher for the Flexi-Slip stylet technique vs the introducer technique (100% vs 86%, respectively; P = 0.001). There were four patients in the Introducer group in whom the PLMA was not placed successfully within three attempts. The PLMA was inserted successfully in these patients with a single attempt using the alternate Flexi-Slip stylet technique. First attempt and overall insertion times were shorter for the Flexi-Slip stylet technique (Table 2). There was no difference in oropharyngeal leak pressure between groups. A total of 21 failed PLMA insertion attempts were recorded. Malpositioning of the PLMA was the most common reason for attempt failure using the introducer (14 attempts). Eight malpositionings were noted by failed suction catheter insertion; the other six malpositionings were detected by audible air leaks over the mouth, the drainage tube, or the epigastrium. The total dose of propofol was similar in both groups. No episodes of hypoxia, aspiration, or laryngospasm occurred during PLMA insertion.

Table 1 Characteristics of patients
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Table 2 Results of the ProSeal laryngeal mask airway insertion
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Airway complications are shown in Table 3. Visible blood staining on the PLMA occurred less frequently with the Flexi-Slip stylet than with the introducer (4% vs 15%, respectively; P = 0.015). Also, the incidence of postoperative sore throat was lower in the Flexi-Slip stylet group than in the Introducer group (8% vs 23%, respectively; P = 0.008). There was no significant difference in the occurrence of dysphagia between the groups.

Table 3 Airway trauma during insertion and postoperative airway morbidity
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Discussion

This study shows that insertion success at first attempt is more likely with the Flexi-Slip stylet than with the introducer. The higher success rate at first attempt and the shorter insertion time for the Flexi-Slip stylet technique implies that this technique may be performed more easily than the introducer technique. In addition to reducing the number of insertion attempts, use of the Flexi-Slip stylet reduced the occurrence of blood staining and airway complications. The main reason for failure using the introducer in this study was malpositioning of the PLMA with the inability to insert a suction catheter; this is consistent with the findings of Brimacombe et al. 14 They reported that the principal cause of failed PLMA placement was impaction of the PLMA at the back of the mouth, resulting in failed pharyngeal passage or folding over of the cuff. To answer this problem, the Flexi-Slip stylet provides a supporting structure to prevent the cuff from folding over and also to reduce impaction of the device at the back of the mouth during insertion. After removal of the stylet, the PLMA regains its flexibility, allowing it to conform to the anatomy of the patient.

The dedicated introducer for PLMA insertion was first introduced in 2000.1 Insertion of the PLMA was considered easier with the introducer technique than with the digital technique because the introducer occupies less space than the finger, directs the cuff around the oropharyngeal inlet, and facilitates full depth of insertion.4 However, even with the introducer, the soft cuff of the PLMA still lacks support and tends to fold back on itself.14,16 As a result, the success rates of PLMA insertion and the reasons for failed insertion are similar between the introducer technique and the digital technique.17 Compared with the introducer, the Flexi-Slip stylet not only provides an angle similar to the introducer tool but also extends support from the airway tube to the middle portion of the cuff. The data presented in this study indicate that these advantages allow easier and faster insertion with fewer insertion attempts required when using the Flexi-Slip stylet technique.

Several techniques involving priming the drainage tube with a guiding device have been reported to facilitate PLMA insertion.6-8 Some of these guiding devices (e.g., gastric tube, suction catheter) are soft and may fail to guide the PLMA around the oropharyngeal inlet.6,7 The gum elastic bougie is rigid enough to guide the distal cuff directly into the hypopharynx.8 However, because of concerns regarding potential complications from blind insertion of the gum elastic bougie,18 it is recommended that the gum elastic bougie be placed into the proximal esophagus using a laryngoscope.14 This technique is suggested as a backup technique when the digital and introducer techniques fail.14 Compared with the gum elastic bougie technique, a potential advantage of the Flexi-Slip stylet technique is that the soft tip of the stylet is kept within the drainage tube of the PLMA, which may reduce the incidence of airway trauma during blind insertion. Also, an assistant or laryngoscope is not required for the Flexi-Slip stylet technique. Therefore, the Flexi-Slip stylet technique seems a likely candidate for routine PLMA insertion.

The Flexi-Slip stylet is made of a malleable coated wire with a soft atraumatic tip designed originally to facilitate endotracheal intubation. In order to reduce possible distortion of the stylet during insertion, this study employs a relatively large Rüsch Flexi-Slip stylet (5.6-mm diameter) to obtain better insertion stability. Slight lubrication of the stylet prior to use assists smooth stylet removal. The malleable wire of the stylet is inserted only 5-6 cm into the cuff, and as a result, the rigid portion of the cuff (Fig. 1B) is commonly shorter than the distance between the upper incisors and posterior pharyngeal wall. This may prevent the distal part of the cuff from sticking to the pharyngeal wall when the PLMA is advanced into the pharynx. While distortion of the flexible portion of the distal cuff is possible during insertion, in this study, this problem did not cause failed insertions with the Flexi-Slip stylet technique. We speculate that the Flexi-Slip stylet increases the stiffness of the middle portion of the cuff so the PLMA can pass under the dropped tongue without the cuff folding over. Further, if the cuff does not fold over on itself, cuff inflation helps to adjust the PLMA to the optimal position.

There are several limitations to this study. First, all insertions were performed by an experienced anesthesiologist who was not blinded to the method of insertion. This is a possible source of bias, and the results may not apply to other users. Additional evaluation of the Flexi-Slip stylet technique by other operators is required to improve the generalizability of our findings. Second, the hemodynamic changes during insertion of the PLMA were not recorded. Thus, a greater effect of the Flexi-Slip stylet technique on hemodynamics due to increased stiffness of the cuff cannot be ruled out. Third, patients with anticipated difficult airways were excluded from this study, so the present results may not apply to patients with a difficult airway. Fourth, this study did not assess the exact position of the PLMA with a fibrescope. Malpositioning of the PLMA was evaluated only by clinical assessment.

In conclusion, when the Flexi-Slip stylet was used for PLMA insertion by a single experienced operator in patients with a normal appearing airway, the technique demonstrated a higher success rate at first attempt, lower insertion time, and fewer airway complications compared with the introducer technique. It is our view that the Flexi-Slip stylet technique is a likely candidate for routine PLMA insertion.