Depth of insertion of the ProSealTM laryngeal mask airway

M. S. Stix* and C. J. O’Connor Jr

Department of Anesthesiology, Lahey Clinic, 41 Mall Road, Burlington, MA 01805, USA

Corresponding author. E-mail: michael.s.stix@lahey.org
{dagger}LMA® is the property of Intavent Limited.

Accepted for publication: October 9, 2002


    Abstract
 Top
 Abstract
 Introduction
 Methods and results
 Comment
 References
 
Background. The depth of insertion of the ProSealTM laryngeal mask airway (PLMA{dagger}) is unknown. We measured depth of insertion in satisfactorily positioned PLMAs.

Methods. All women received size 4 masks and men size 5 masks. We measured the position of the integral bite block in relation to the upper incisors documented in patients over a 6-month period. Depth of insertion was scored by dividing the integral bite block into quarters. Satisfactory positioning of the ProSealTM itself was determined by (i) positive ‘suprasternal notch test’, (ii) no venting via the drain tube during maximal lung inflation, and (iii) an unobstructed airway.

Results. We studied 274 patients (147 women and 127 men). The midway point of the bite block was proximal to the incisors (e.g. within the oropharynx) in 78% of women (95% CI 71–85%) and 92% of men (95% CI 87–97). The standard deviation for the depth distribution in women was 0.8 cm and for men was 1.0 cm.

Conclusions. Usually most of the integral bite block lies within the oropharynx. It was never normal for the entire bite block to stick out of the mouth (4 SD from the mean for both men and women). The position of the integral bite block relative to the upper incisors gives valuable information during assessment of PLMA position.

Br J Anaesth 2003; 90: 235–7

Keywords: equipment, masks anaesthesia


    Introduction
 Top
 Abstract
 Introduction
 Methods and results
 Comment
 References
 
There is little information on depth of insertion of the ProSealTM laryngeal mask airway (PLMA{dagger}) (Laryngeal Mask Company, Henley-on-Thames, UK). Early publications concerning the PLMA gave no guidelines concerning how much the airway tube should normally stick out of the patient’s mouth.1 2 Clearly, the integral bite block was designed to conform to normal adult female and male anatomy, but details were absent from the original article.1 We observed that when most of the bite block was outside the patient’s mouth the PLMA was frequently malpositioned. We therefore always examined the relation of the bite block to the upper incisors after an insertion. We present a summary of these observations over a 6-month period. Our data show evidence that the insertion depths for women and men are relatively consistent. The relation of the bite block to the upper incisors gives valuable information during assessment of PLMA position.


    Methods and results
 Top
 Abstract
 Introduction
 Methods and results
 Comment
 References
 
Hospital institutional review board approval was obtained for collection of data from charts and records for this simple observational study. The 6-month review was from September 2001 to March 2002. No patients with known or anticipated difficult airways received PLMAs. All patients were fasted for solids for a minimum of 6 h. Patients with obesity or gastroesophageal reflux were not excluded.

We used a simple method to score insertion depth, as shown in Figure 1. The bite block was divided into quarters and the portion of the bite block lying opposite the upper incisors (or anterior alveolar ridge if edentulous) determined the score. Increasing score indicated increasing depth of insertion. A score of 1 was given to the first quarter, 2 for the second quarter, 3 for the third quarter and 4 for the fourth quarter. A score of 5 indicated that the entire bite block was within the mouth. A score of 0 indicated that the bite block remained entirely outside the mouth.



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Fig 1 Explanation of depth of insertion scoring system and alignment of quartile lines. The halfway point is the flat top of the syringe plunger. The other quartile markers line up with the rounded beginning of the letter ‘d’ in ‘Adult’ and to where the base of the needle attaches to the syringe. An insertion depth corresponding to the star would receive a score of 3.6.

 
Dividing the bite block into quarters was done without having to mark devices. We observed that the bite block was the same dimension for both 4 and 5 PLMAs and that the writing and symbols on the sides of the bite block was uniform. For both sizes 4 and 5 PLMAs the ‘quartile lines’ lined up, within less than 1 mm, as shown in Figure 1. We memorized these markings in order to score the depth of insertion.

We then refined this scoring system and made our best visual estimates to ‘tenths’ accuracy. For example, a depth of insertion of 3.6 meant that six-tenths of zone 3 was inside the mouth. The final scoring system is illustrated in Figure 1 where the star indicates a depth of insertion of 3.6.

For every patient, general i.v. anaesthesia was induced and a neuromuscular blocking agent was administered. The PLMA was then inserted using the fingertip method and the cuff inflated to 60 cm H2O (Portex Inc. Cuff Pressure Indicator 660001, Keene, NH, USA). Women received size 4 PLMAs and men size 5 PLMAs, regardless of height or weight. We assessed the correct location of the PLMA using clinical tests, and did not use a fibrescope. First, we used a ‘suprasternal notch test’ to assess for PLMA location behind the arytenoid or cricoid cartilages.3 Second, we required that enough force be applied pushing the PLMA inwards to ensure complete absence of drain tube leak during a maximal lung inflation manoeuvre squeezing the circuit bag (so that the airway leak was oropharyngeal without any gas escaping from the drain tube).4 Both of these two tests used a soap membrane to test the drain tube.5 Finally, a clinically unobstructed airway was required. Depth of insertion was only recorded after tests were satisfactorily completed. The patient’s head was maintained in a neutral position, with the occiput slightly elevated on a folded blanket, when measuring depth of insertion. The score was recorded with an inward force on the PLMA, but before taping the device.

The PLMA was clinically positioned satisfactorily in 274 patients. Values are given as mean (SD). There were 147 women age 18–89 years [55 (16)], height 145–183 cm [162 (7)], weight 42–212 kg [77 (22)], and body mass index (BMI) 18–67 kg m–2 [29 (8)]. There were 127 men age 19–86 years [57 (16)], height 160–193 cm [177 (7)], weight 44–168 kg [88 (20)], and BMI 17–52 kg m–2 [28 (6)]. A total of 39 patients (26 women and 13 men) had a BMI >35.

Integer values of the scores are presented in Table 1. The scores were rounded down to the nearest integer so, for example, a score of 3.6 was tabulated simply as a 3. Grouped data are given in Table 1.


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Table 1 Depths of insertion. Data are actual number (%)
 
As depicted in Figure 1, the midway point of the bite block divides the depth scores of 2 and 3. Patients in whom the incisors lay beyond the midway point had scores of 3, 4, or 5. For women, such scores were noted in 78% of patients (95% CI 71–85%). For men this occurred in 92% of patients (95% CI 87–97%).

The length of the bite block is 5.6 cm and is the same for both sizes 4 and 5 devices. The middle of the bite block is located 18.0 and 19.5 cm from the bevelled tip of the drain tube in sizes 4 and 5 PLMAs, respectively. From this we computed average depths and standard deviations. For women, mean depth was 18.6 cm (95% CI 18.4–18.7 cm). For men, mean depth was 20.9 cm (95% CI 20.7–21.1 cm). Standard deviation for depth in women was 0.8 cm and for men was 1.0 cm.

Depth of insertion was related to age, height, weight, and body mass index using Excel 1997. Using a 0.05 significance level and a two-tailed test, only three statistically significant correlations were obtained. For adult women depth of insertion was weakly correlated with height r=0.41 and age r=–0.38. For adult men depth of insertion was very slightly correlated with height r=0.22.


    Comment
 Top
 Abstract
 Introduction
 Methods and results
 Comment
 References
 
Much has been written about the remarkable ability of the ClassicTM LMA to be able to provide a clear airway despite a wide variety of malpositions. ‘Suboptimal insertion or fixation of the LMA may allow its distal end to lie anywhere from the nasopharynx to the rima glottidis, yet often the airway remains clinically clear’.1 The PLMA, on the other hand, has a different LMA design, to allow positive pressure ventilation. Its use needs attention to proper positioning of the device tip within the upper oesophageal inlet, behind the cricoid cartilage. In order to use the PLMA in controversial cases a consistent placement is needed and all means should be used to verify tip location. Depth of insertion is one measure that can give immediate information.

One of the principal aims of this study was to substantiate an earlier claim that ‘Usually most of the bite block is within the mouth’.3 In our sample of women this occurred in 78% of patients. Likewise, in men this occurred in 92% of patients. We conclude that anaesthetists must suspect malposition if less than half of the bite block is within the mouth. When this happens, other tests should then be used to confirm the positioning.

It was never normal for the bite block to remain completely outside the mouth (score of 0). The circumstance ‘none of the bite block in the mouth’ corresponds to a depth that is 4 SD from the mean for both women and men (sizes 4 and 5 devices, respectively). We found that when the bite block is outside the mouth, the tip of the PLMA is almost always folded backwards behind the bowl.6 After an insertion attempt, when it is observed that none of the bite block lies within the mouth, it is prudent to remove the PLMA and start again.

In summary, we have found depth of insertion to be consistent in adult women and men without significant anatomical abnormality or pathology. Usually most of the bite block lies within the mouth. When the bite block remains entirely outside of the mouth the PLMA is almost certainly malpositioned. We suggest that the PLMA, like most airway devices, showed markings indicating centimeter depth.


    References
 Top
 Abstract
 Introduction
 Methods and results
 Comment
 References
 
1 Brain AIJ, Verghese C, Strube PJ. The LMA ‘ProSeal’—a laryngeal mask with an oesophageal vent. Br J Anaesth 2000; 84: 650–4[Abstract]

2 Brimacombe J, Keller C. The ProSeal laryngeal mask airway: a randomized, crossover study with the standard laryngeal mask airway in paralyzed, anesthetized patients. Anesthesiology 2000; 93: 104–9[ISI][Medline]

3 O’Connor CJ jr, Borromeo CJ, Stix MS. Assessing ProSeal laryngeal mask positioning: the suprasternal notch test. Anesth Analg 2002; 94: 1374–5[Free Full Text]

4 O’Connor CJ jr, Stix MS. Bubble solution diagnoses ProSeal insertion into the glottis. Anesth Analg 2002; 94: 1671[Free Full Text]

5 O’Connor CJ jr, Stix MS. Place the bubble solution with your fingertip. Anesth Analg 2002; 94: 763–4[Free Full Text]

6 Brimacombe J, Keller C, Berry A. Gastric insufflation with the ProSeal laryngeal mask. Anesth Analg 2001; 92: 1614–15[Free Full Text]