A Prospective Evaluation of Clinical Tests for Placement of Laryngeal Mask Airways 

The institutional review board for human subjects at the University of Texas Medical Branch, Galveston, approved the study. Informed consent was waived for inclusion into the study. This prospective double blinded observational study was conducted during 150 LMA insertions in adult day-surgery patients classified as American Society of Anesthesiologists physical status I and II. Persons with upper airway disease and those at risk for aspiration were excluded. Patients were anesthetized with 1 [micro sign]g/kg fentanyl and 1.5-2.5 mg/kg propofol. Anesthesia was supplemented with oxygen (33%), nitrous oxide (67%), and isoflurane. Fresh gas flow was set at 3 1/min based on the anticipated flow requirements across a range of patient body weights. Muscle relaxants were not used. A size 3 LMA was used in women weighing <70 kg, and size 4 was used in men and in women who weighed >70 kg. The LMAs were inserted after the loss of eyelash reflex and the relaxation of the jaw muscles with the head extended and the neck flexed. [11-13]The anesthesiologists (with experience of > 200 insertions) used the technique described by Brain et at. [14]Once placed, size 3 and 4 LMAs were inflated with 20 and 30 ml air, respectively. [14,15]The anesthesiologist placing the LMA determined whether the placement was satisfactory. The attempt was to achieve a clinically satisfactory LMA placement as judged by the nine clinical tests evaluated in the study. Although effective gas exchange (ability to ventilate and presence of a capnographic trace) were usually achieved without difficulty, additional attempts were required to achieve satisfactory placement when judged by other tests. The number of attempts required before satisfactory placement of the LMA was noted. The outcome of clinical tests was recorded only for the final attempt. The anesthesiologist who inserted the LMA assessed all except two of the nine clinical tests of LMA placement. Two signs of LMA placement were observed by a second anesthesiologist. The second anesthesiologist, who was positioned by the side of the patient, observed the outward movement of the LMA and the anterior movement of the larynx. Each test was scored as positive (i.e., satisfactory) or negative (i.e., unsatisfactory), for clinical use of the LMA. The results were recorded on a standard form before fiberoptic evaluation. Once placed, the LMA was taped to the upper jaw to avoid displacement before and during fiberoptic examination. The head position, which influences leak pressure, [16]was not allowed to change after the device was inserted.

The following tests were used to evaluate placement of the LMA.

1. Sign at placement: Presence of resistance at the end of insertion

2. Signs on inflating of the cuff: Outward movement of the LMA, anterior displacement of the larynx, inability to advance the LMA further once inflated

3. Signs after placement of the LMA: Black line on the LMA in the midline at the incisors; ability to generate airway pressure of 20 cm water during manual inspiration; ability to ventilate manually as judged by movement of the chest, condensation of expired gases, adequacy of expired gas volume, and the feel of the bag; carbon dioxide evident on capnography, with a minimal alveolar plateau phase during exhalation during manual ventilation; and movement of the anesthesia bag with return of spontaneous breathing.

The fiberoptic examination with a flexible bronchoscope was done within approximately 5 min of final placement by an anesthesiologist who was not present during placement of the LMA. The fiberoptic observations were made with the tip of the bronchoscope located at the inner aperture of the LMA. Anatomic placement was graded on a four-point scale (1 to 4), which is similar to one described by Brimacombe and Berry [6,17](Table 1): grade 4, only vocal cords seen; grade 3, vocal cords and posterior epiglottis seen; grade 2, vocal cords and anterior epiglottis seen; and grade 1, vocal cords not seen. Brimacombe and Berry assigned a grade of 0 to mask failure; i.e., failure to insert in 10 s or failure of the mask to function with cords not seen fiberoptically. We did not assign a 0 grade during our fiberoptic evaluation because we believed it had little to contribute to the final anatomic placement of the mask. We also recorded the occurrence of laryngospasm at placement of LMA, visibility of the esophagus or transient glottis closure (< 30 s) during fiberoptic evaluation, and presence of blood on removal of the LMA. Blinding in this study was achieved by independently observing the clinical and fiberoptic assessments.

Fiberoptic evaluation determined if the clinical test result was true- or false-positive or negative. Fiberoptic grades 2, 3, and 4 were considered anatomically acceptable placements, whereas grade 1 was considered a malplacement. Thus a true-positive clinical test result implied a positive clinical test result that coincided with fiberoptic grades 2, 3, or 4, and false-positive coincided with fiberoptic grade 1. Conversely, a true-negative result coincided with fiberoptic grade 1, whereas a false-negative coincided with fiberoptic grades 2, 3 or 4 (Table 1). The specificity, sensitivity, and overall accuracy of nine tests to detect malplacement were determined (Table 2). To determine if any clinical tests could detect the failure of LMA to support the epiglottis, the clinical results with fiberoptic grades 3 and 4 placements were combined and compared with grade 2 placements.

Statistical evaluation of the association of the outcome of clinical tests and fiberoptic grade was performed using Cochran Mantel-Haenszel statistics. The association between clinical tests and fiberoptic grade was assessed by chi-squared analysis with a continuity correction. Logistic regression analysis was used to assess a correlation between the outcome of clinical tests for combined fiberoptic grades 3 and 4 (i.e., the epiglottis is supported by the LMA) placements versus grade 2 placements (the epiglottis is not supported by the LMA). Statistical significance was accepted at P < 0.05.

One patient developed laryngospasm during induction, which required intubation before fiberoptic assessment and thus was excluded from the study. Data were available from the remaining 149 patients, 72 men and 77 women whose ages ranged from 19 to 79 yr. The LMA was placed during the first, second, and third attempts in 75.8% 20.1% and 4% of the patients, respectively. The chance of better anatomic placement was higher on the first than on the subsequent attempts (P = 0.0015). There were 14 fiberoptic grade 1 placements, but clinical tests did not detect malplacement in four of these (Table 3, Table 4).

Six clinical tests correlated best with fiberoptic grade: anterior movement of the larynx, LMA moves out on cuff inflation, black line midline on the LMA, ability to ventilate manually, ability to generate an airway pressure of 20 cm of water, and movements of the reservoir bag on return of spontaneous breathing (P < 0.02 to 0.001;Table 5). Three clinical tests were poorly correlated with the fiberoptic grade: presence of resistance at the end of insertion, inability to advance after inflation of the cuff, and presence of a capnographic trace (Table 5). All nine clinical tests for LMA placement had a higher sensitivity (91%-100%) than specificity (0%-71%) rate. The overall accuracy ranged from 86-94%.

When data from fiberoptic grades 3 and 4 (the epiglottis was supported by the LMA) were combined and compared with grade 2 fiberoptic placement (the epiglottis was not supported by the LMA), two tests showed a correlation with fiberoptic grade. These tests were the ability to ventilate manually (P = 0.0019) and the ability to generate an airway pressure of 20 cm water (P = 0.001).

Besides the anatomic location of the LMA, fiberoptic examination revealed the esophagus in five cases. Laryngospasm occurred in one case that required intubation and was excluded from the study. Transient glottis closure was not observed. Blood was seen during removal of the LMA in eight cases. There was a correlation between the number of attempts and the presence of blood on the LMA (P = 0.046).

Various tests of the placement of the LMA have been recommended, yet their accuracy in predicting the correct anatomic placement of the device has not been evaluated. Our results suggest that clinical tests of LMA placement correlate with anatomic placement of the device as assessed by fiberoptic evaluation. It can be debated if correct anatomic placement of the LMA is the valid end point for assessing the satisfactory placement of the device. Studies using fiberoptic examination, computed tomography, or magnetic resonance imaging show that the airway can be functionally patent and clinically acceptable even though anatomic placement is less than perfect. [6,9,10]Although there is a paucity of hard data, we suspect that a malplaced LMA is more likely to be associated with complications, such as airway trauma and obstruction, gastric distention with mechanical ventilation, postoperative paralysis of vocal cords, and probably regurgitation. [18-20] 

The most convenient method to assess anatomic placement available for large clinical studies is fiberoptic examination through the LMA. When fiberoptically evaluated, a well-placed LMA should reveal a direct view of the cords. Because the epiglottis has been lifted anteriorly, only the base of the epiglottis should be visible at best. The fiberoptic scale described by Brimacombe and Berry [17]provides a convenient tool to assess the placement of the LMA and has been used by other investigators. [21]However, limitations of such a scale must be recognized. For example, Asai [22]commented that differences in fiberoptic assessment could be due to differences in the relative size of the larynx and the LMA. A relatively large LMA may not support the epiglottis, [7]which thus affects the fiberoptic grade. [22]This may be particularly true in pediatric case series, which have reported a high incidence of down-folded epiglottis. In adult patients, in whom there is less variation in the size of the larynx, the concerns raised by Asai may be less applicable.

Our study differs from others because the LMA were inserted by anesthesiologists with experience of > 200 insertions, although at first glance the number of attempts required for satisfactory placement seems to be unacceptably high. There was only one failure to place the LMA in 150 placements, which compares with large series in which successful placement was achieved in >99% of the patients. [5,23]The incidence of grade 1 placement in the current study was 9.4%, which is well within range of other studies. [20,24]The apparent high failure rate in the first attempt is explained by the fact that the anesthesiologists evaluated placement by several tests, not merely by the ability of the device to permit gas exchange. It is possible that the reported improvement in fiberoptic grades with increasing experience (> 750 placements)[5]is due in part to a similar increase in sophistication in evaluating LMA placement.

There are no uniform guidelines to evaluate satisfactory placement of LMAs. The 1996 LMA instruction manual for the United States recommends three tests to check for correct placement of the device [1]: slight outward movement of the LMA on inflation of the cuff, [2]smooth oval swelling extending behind the thyroid and the cricoid cartilage, [3]and no cuff visible in the oral cavity. [15]However, other clinical tests are frequently used to assess LMA placement. These include the presence of resistance to further downward movement, [2]an anterior bulge of the larynx on inflation of the cuff, [2]chest wall movement with manual ventilation, [7]the ability to ventilate effectively [25]or to generate a specified airway pressure, [26]or a specified end-tidal carbon dioxide level. [27] 

The clinical test of LMA placement we used may differ from those of other investigators. For example, Pennant and Walker [27]considered an expired carbon dioxide level of 25 mmHg an indicator of correct placement. In our study, we considered the presence of capnographic trace with minimal alveolar plateau to be the end point of satisfactory placement because it was less likely to be affected by hemodynamic changes at induction or by changes in minute ventilation volume. Similarly, there are several different methods to assess an effective seal around the larynx by the LMA, such as listening to the escape of gas from the mouth, measuring expired gas volume, or measuring nitrous oxide concentrations at a specified distance from the LMA. [26-28]We assessed the seal around the larynx by determining if we could generate an airway pressure of 20 cm water while squeezing the reservoir bag at a fresh gas flow of 6 1/min. It can be argued that such a test could have overlooked significant leak because fresh gas flow was sufficiently high. We selected the fresh gas flow because it was clinically appropriate and an initial high flow is required to prime the disconnected breathing system. To make the test clinically relevant, we did not specify the ventilatory parameters because ventilators are rarely used during LMA placement. The anesthesiologist could vary the ventilatory parameters to achieve the peak airway pressure.

To avoid any controversy, we defined anatomic malplacement as the inability to see the vocal cords. Using this definition could determine the sensitivity, specificity, and overall accuracy of each test (Table 2and Table 5). The most specific test to detect malplacement was the ability to generate airway pressure of 20 cm water (71%), whereas the ability to ventilate manually had the highest overall accuracy (94%). When grade 2 placements (i.e., the epiglottis was not supported by the LMA) were compared with 3 and 4 placements combined (i.e., epiglottis supported by the LMA) to determine if clinical tests could show whether the device was supporting the epiglottis, the outcome of only two clinical tests correlated with the fiberoptic grade. These tests were the ability to generate airway pressure of 20 cm water and the ability to ventilate manually, suggesting that only these two tests are likely to detect that the epiglottis is supported by the LMA. Therefore we consider the ability to generate 20 cm water pressure and the ability to ventilate manually to be the best clinical tests of correct anatomic placement of the LMA.

This question arises: should a fiberoptic bronchoscope be used routinely to confirm correct placement of the LMA? The strong statistical correlation in our study between the outcome of some clinical tests with the fiberoptic grade suggests that fiberoptic evaluation is unnecessary for routine use of the LMA. However, in four of the 14 grade 1 placements, the tests failed to demonstrate malplacement, suggesting that the tests evaluated are not sufficiently reliable to permit blind airway manipulation. The success rate with blind placement of an endotracheal tube through a blindly placed LMA ranges from 30% to 90%. [29,30]Even when a battery of clinical tests suggests satisfactory placement of the device, our results do not recommend blind placements through the LMA.

We conclude that clinical tests of LMA placement, particularly the ability to generate an airway pressure of 20 cm water and the ability to ventilate manually, correlate with the correct anatomic placement of the device as judged by fiberoptic examination.

The authors thank Steven Marshall, B.S., and Joyce Ouchi for help in preparing the manuscript. This article is dedicated in memory of Michael R. Phillips, M.D.