Department of Anesthesiology, Lahey Clinic, 41 Mall Road, Burlington, MA 01805, USA
Corresponding author. E-mail: michael.s.stix@lahey.org LMA® is the property of Intavent Limited.
Accepted for publication: October 9, 2002
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Abstract |
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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 7185%) and 92% of men (95% CI 8797). 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: 2357
Keywords: equipment, masks anaesthesia
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Introduction |
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Methods and results |
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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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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 patients 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 1889 years [55 (16)], height 145183 cm [162 (7)], weight 42212 kg [77 (22)], and body mass index (BMI) 1867 kg m2 [29 (8)]. There were 127 men age 1986 years [57 (16)], height 160193 cm [177 (7)], weight 44168 kg [88 (20)], and BMI 1752 kg m2 [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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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 18.418.7 cm). For men, mean depth was 20.9 cm (95% CI 20.721.1 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.
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Comment |
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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.
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References |
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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: 1049[ISI][Medline]
3 OConnor CJ jr, Borromeo CJ, Stix MS. Assessing ProSeal laryngeal mask positioning: the suprasternal notch test. Anesth Analg 2002; 94: 13745
4 OConnor CJ jr, Stix MS. Bubble solution diagnoses ProSeal insertion into the glottis. Anesth Analg 2002; 94: 1671
5 OConnor CJ jr, Stix MS. Place the bubble solution with your fingertip. Anesth Analg 2002; 94: 7634
6 Brimacombe J, Keller C, Berry A. Gastric insufflation with the ProSeal laryngeal mask. Anesth Analg 2001; 92: 161415