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Pressures Exerted Against the Cervical Vertebrae by the Standard and Intubating Laryngeal Mask Airways: A Randomized, Controlled, Cross-Over Study in Fresh Cadavers

Keller, Christian MD*; Brimacombe, Joseph MB, ChB, FRCA, MD; Keller, Karoline RN*

doi: 10.1213/00000539-199911000-00042
GENERAL ARTICLES
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In this randomized, controlled, cross-over study, we measured the pressures exerted by the standard laryngeal mask airway (LMA) and the intubating laryngeal mask airway (ILM) against the cervical vertebrae during insertion, intubation, and maneuvers commonly used to facilitate intubation. We also assessed the effect of these pressures on cervical spine (C-spine) movement. Twenty cadavers (6–24 h postmortem) without cervical abnormality were initially studied. Three microchip pressure sensors were implanted into the pharyngeal surface of C2-3. The size 5 ILM and LMA were inserted in random order into each cadaver with the head-neck in the neutral position. Maximal cervical pressures (CPmax) were recorded for both devices during cuff inflation from 0–40 mL and with the intracuff pressure (ICP) at 60 cm H2O, insertion, fiberscope-guided intubation, partial withdrawal/reinsertion (ILM only), elevation/depression of the handle with 20 N of force applied (ILM only). In five additional matched cadavers, the effect of pressure on posterior displacement of C3 was assessed, and CPmax was measured during laryngoscope- and fiberscope-guided oro/nasotracheal intubation (controls). CPmax was higher for the ILM than the LMA over the inflation range (96 vs 15 cm H2O;P < 0.0001) and with the ICP at 60 cm H2O (95 vs 10 cm H2O;P < 0.0001). During cuff inflation, CPmax was generally unchanged for the ILM but was always increased for the LMA. CPmax for the LMA and ILM was similar during insertion (224 vs 273 cm H2O) but higher for the ILM during fiberscope-guided intubation (96 vs 43 cm H2O;P < 0.0001). At 60 cm H2O ICP, CPmax increased during LMA (224 cm H2O;P < 0.0001) and ILM insertion (273 cm H2O;P < 0.0001) and increased for the LMA (43 cm H2O;P < 0.0001) but was unchanged for the ILM (96 cm H2O) during fiberscope-guided intubation. For the ILM, CPmax increased during handle depression (394 cm H2O;P < 0.0001) and partial withdrawal/reinsertion (265 cm H2O;P < 0.0001) but decreased during handle elevation (6 cm H2O;P < 0.00001). CPmax for the controls was usually zero and was always less than the LMA/ILM (P < 0.0001). The mean (range) for posterior displacement of C3 was 0.8 (0–2) mm at 100 cm H2O and 2.8 (1–5) mm at 400 cm H2O. Laryngeal mask devices exert greater pressures against the cervical vertebrae than established intubation techniques and can produce posterior displacement of the C-spine.

Implications Laryngeal mask devices exert greater pressures against the cervical vertebrae than established intubation techniques and can produce posterior displacement of the cervical spine. We recommend that laryngeal mask devices only be used in the unstable cervical spine if difficulties are anticipated or encountered with established techniques, pending the results of studies demonstrating its relative safety.

*Department of Anaesthesia and Intensive Care Medicine, Leopold-Franzens University, Innsbruck, Austria; and †Department of Anaesthesia and Intensive Care, University of Queensland, Cairns Base Hospital, Cairns, Australia

Address correspondence and reprint requests to Dr. J. Brimacombe, Department of Anaesthesia and Intensive Care, Cairns Base Hospital, The Esplanade, Cairns 4870, Australia. Address e-mail to 100236,2343@compuserve.com.

Accepted for publication July 2, 1999.

The standard (LMA) and intubating (ILM) laryngeal mask airways can be inserted and used to facilitate intubation without manipulation of the head-neck (1–6). There have been several reports of successful LMA (7–13) and ILM (14,15) use in the unstable cervical spine (C-spine), but both devices are in direct contact with the mucosa overlying C2-6 (16), and it has been postulated that the C-spine might be displaced posteriorly if sufficient forces are applied (17). A recent study by this group showed that the pressure exerted by the ILM and LMA against the oropharyngeal mucosa was 182 and 26 cm H2O, respectively (17). However, these measurements were made with the LMA and ILM already inserted, and the impact on C-spine movement was not assessed. In this randomized, controlled, cross-over cadaver study, we measured the pressures exerted by the LMA and ILM against the cervical vertebrae during insertion, intubation, and maneuvers commonly used to facilitate intubation. We also assessed the effect of these pressures on C-spine movement.

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Methods

Twenty fresh adult cadavers (6–24 h postmortem) were initially included in this randomized, controlled, cross-over trial. Ethical committee approval was obtained, and all patients or their relatives consented to postmortem research. Cadavers with oropharyngeal or cervical pathology were excluded. A single experienced LMA/ILM user inserted/fixed the LMA/ILM according to the manufacturer's instructions (16,18). A size 5 LMA/ILM was used for all cadavers (19).

Pressure exerted against the cervical vertebrae was measured using three strain gauge microchip sensors (Accurate Plus; Mammendorfer Institut fuer Physik und Medizin GmbH, Hattenhofen, Germany). The sensor had a thickness of 2 mm, a diameter of 8 mm, a functional pressure range of 0–350 mm Hg, a temperature sensitivity of <0.2 mm Hg/°C, a zero drift of <3 mm Hg/100 h, and a sensitivity of 5 μV · V−1 · mm Hg−1 and were accurate to ±1% (manufacturer's specifications). The cable had a diameter of 1.0 mm. The sensing element was 6 mm in diameter and was flat and flush with the surface of the sensor. The cervical vertebrae were approached through the mouth and a mucosal incision made at the C1-2 interspace. Circular holes (10 mm in diameter and 2 mm deep) were drilled into the anterior body of C2, the C2-3 interspace, and the anterior body of C3. The sensors were fixed in position with surgical bone wax so that the sensing element was oriented anteriorly away from the vertebrae and was flush with the bony/ligamentous surface. Care was taken to ensure that the contour of the bony/ligamentous surface was restored and that the sensor cables did not cross the sensing element. The mucosa was then sutured. The sensors were connected to a monitor and zeroed.

The ILM and LMA were inserted in random order into each cadaver. Maximal cervical pressures (CPmax) were recorded during inflation of the cuff from 0–40 mL in 10-mL increments and with the intracuff pressure (ICP) at 60 cm H2O, during insertion, fiberscope-guided intubation, partial withdrawal/reinsertion (ILM only), elevation/depression of the handle with 20 N of force applied (ILM only). CPmax for the LMA and ILM were with the ICP at 60 cm H2O, except for insertion, when the cuff was fully deflated.

All insertions and measurements were made with the head-neck in the neutral position with the occiput on a pillow 5 cm in height. Head-neck stabilization was not attempted. A lubricated 7.0-mm inner diameter polyvinyl chloride tracheal tube (Lo-Contur; Mallinckrot Medical, Athlone, Ireland) was used for all intubations. Elevation and depression of the ILM handle by 20 N was accomplished with a calibrated spring weight that allowed a measured amount of force to be delivered when lifted or depressed manually.

When the measurements for the LMA and ILM were complete, the airway sealing pressure and fiberoptic position were recorded at an ICP of 60 cm H2O to provide general information about LMA/ILM position. Airway sealing pressure was measured by closing the expiratory valve of the circle system at a fixed gas flow of 3 L/min and noting the airway pressure at which the dial on the aneroid manometer reached equilibrium (20). The fiberoptic position was determined from the mask aperture bars (LMA) and epiglottic elevator bar (ILM) using the following scoring system: 4 = only vocal cords visible, 3 = vocal cords plus posterior epiglottis, 2 = vocal cords plus anterior epiglottis, 1 = vocal cords not seen (21,22). The position/orientation of the sensors was checked during removal by visual inspection. The accuracy of the probes was tested before and after use in each cadaver by submerging them in water at 21°C to depths of 10 and 50 cm and noting the pressure readings.

In five additional matched cadavers, a metal rod with a diameter identical to the sensor was placed on the anterior surface of C3, and a calibrated spring weight was used to deliver pressures of 100, 200, 300, 400, and 600 cm H2O perpendicular to the surface. A small metal plate was attached to the anterior surface of C2, and any posterior displacement of the rod (C3) relative to the plate (C2) was measured by using a micrometer (accurate to ±0.1 mm). CPmax was also recorded during laryngoscope- and fiberscope-guided oro/nasotracheal intubation (controls). A size 3 Macintosh laryngoscope was used for laryngoscopy.

An unblinded trained observer collected the data. Sample size was selected to detect a projected difference of 25% between the groups with respect to cervical pressure for a type I error of 0.05 and a power of 0.9. The power analysis was based on data from a previous study of 20 patients in which pharyngeal mucosal pressures were measured with the ILM and LMA (17). The distribution of data was determined by using Kolmogorov-Smirnov analysis. Statistical analysis of cervical pressures and airway sealing pressures was accomplished by using a paired t-test (normally distributed data) and Friedman's two-way analysis of variance (non-normally distributed data). A χ2 test was used to compare fiberoptic scores. Significance was taken as P < 0.05.

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Results

The mean (range) age, height, and weight were 79 (59–93) yr, 169 (151–180) cm, and 70 (50–93) kg, respectively. The male to female ratio was 16:9. There were no demographic differences between the initial and additional groups. All airway management techniques were successful at the first attempt. The position/orientation of the sensors was identical, and the pressures were accurate before and after usage. All data for CPmax and airway sealing pressures were normally distributed. Data for airway sealing pressure, fiberoptic score, and CPmax are presented in Table 1. CPmax over the inflation range was higher (P < 0.0001) for the ILM (96 [95% CI: 80–112] cm H2O) compared with the LMA (15 [12–17] cm H2O). CPmax increased with increasing intracuff volume for the ILM from 0 to 10 mL (P = 0.02) and was unchanged from 10 to 20 mL, 20 to 30 mL, and 30 to 40 mL. CPmax increased with increasing intracuff volume for the LMA from 0 to 10 mL (P = 0.002), 10 to 20 mL (P = 0.0001), 20 to 30 mL (P = 0.001), and 30 to 40 mL (P = 0.001). Compared with CPmax at an ICP of 60 cm H2O, CPmax for the ILM increased during insertion (P < 0.0001), depression of the handle (P < 0.0001), and the up-down maneuver (P < 0.0001) remained unchanged during fiberscope-guided intubation and decreased during handle elevation (P < 0.0001). Compared with CPmax at an ICP of 60 cm H2O, the cervical pressure for the LMA increased during insertion (P < 0.0001) and fiberscope-guided intubation (P < 0.0001). Compared with CPmax during insertion, CPmax for the ILM was higher during handle depression (P < 0.0001) and similar during the up-down maneuver. Compared with CPmax during the up-down maneuver, CPmax for the ILM was higher during handle depression (P < 0.0001). In the control group, CPmax during laryngoscope- and fiberscope-guided oral tracheal intubation was zero in all patients. CPmax during laryngoscope- and fiberscope-guided nasal tracheal intubation was zero in four patients, but a pressure of 10 cm H2O was recorded in one patient. CPmax for the controls was always less than the LMA/ILM (P < 0.0001). The mean (range) for posterior movement of C3 relative to C2 was 0.8 (0–2) mm at 100 cm H2O, 1.6 (0–3) mm at 200 cm H2O, 2.0 (0–4) mm at 300 cm H2O, 2.8 (1–5) mm at 400 cm H2O, and 3.0 (1–5) mm at 600 cm H2O.

Table 1

Table 1

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Discussion

These data show that both the LMA and ILM exert transient pressures of >220 cm H2O against the cervical vertebrae during insertion. By contrast, the pressure generated within the upper esophagus (and probably against C6) during the application of 30 N cricoid pressure is approximately 60 cm H2O (23), and concerns have been expressed about the application of cricoid pressure in the unstable C-spine (24). During insertion, both laryngeal mask devices are pressed into the palatopharyngeal curve: the ILM by using the handle and the LMA by using the index finger. Interestingly, several insertion techniques that do not require use of the index finger for insertion have been described for the LMA (16), and these might be more appropriate in the unstable C-spine. Such alternative techniques are generally not applicable to the ILM. We found that the pressure exerted against the cervical vertebrae during laryngoscope- and fiberscope-guided oro/nasotracheal intubation was zero or low. This is because the tracheal tube did not make contact with anterior body of C2-3, and, when it did, the direction of force was generally away from the surface.

Once inserted, the ILM exerts more pressure than the LMA against the cervical vertebrae. This is probably because the ILM is more rigid and bulky and cannot readily adapt its shape to the variable pharyngeal anatomy. The cervical pressures exerted by the LMA were similar to those obtained against the distal oropharyngeal mucosa in our previous study (17) but were approximately 50% lower for the ILM. These differences may be related to dissipation of force by the intervening tissues or to variations in the anatomic position of the sensors between studies because they were attached to the devices in the earlier study and fixed to the cadaver in the current study.

We found that inflation of the cuff increased the cervical pressure exerted by the LMA, but that this was less apparent for the ILM. These differences may be related to the higher initial pressure for the ILM or different force vectors acting between the cuff and the tube due to structural and positional differences. These data support our previous recommendation that the ILM should always be removed after use as an airway intubator (17).

During fiberscope-guided intubation, cervical pressures were unchanged for the ILM but were increased for the LMA. We postulate that some of the force used to push the tracheal tube through the LMA is transferred to the LMA tube because of friction. This effect might be compounded by distortion of the semirigid LMA tube. Frictional forces are lower with the ILM because it has a wide bore metallic tube that cannot be distorted. High cervical pressures are also generated by the ILM when it is partially withdrawn and reinserted. This maneuver is used in approximately 25% of patients being tracheally intubated through the ILM and is intended to reposition a downfolded epiglottis (5). Posterior pressure against the handle is not a recommended maneuver with the ILM, but it is common when ILM insertion is difficult and when adjusting the position of the ILM to obtain the best seal (unpublished observations). It is possible that much greater forces could be generated if difficulty is experienced during insertion or if the patient coughs or retches.

Cadaver studies should always be interpreted cautiously because they do not mimic the precise conditions found in patients. Compared with the anesthetized human, our cadaver model had a lower temperature and more rigid pharyngeal musculature. However, there has been a reasonable match between cadaver and noncadaver studies of cricoid pressure (25). Furthermore, we found that airway sealing pressures, fiberoptic position, and first-time insertion success rates were similar to those in paralyzed, anesthetized humans for the LMA (26) and ILM (17). A recent study suggested that ease of insertion, airway sealing pressure, and fiberoptic score for the flexible LMA are the same for fresh cadavers and paralyzed anesthetized patients (27). We found more epiglottic downfolding with the ILM. This has been previously reported in paralyzed anesthetized patients and may be related to the increased antero-posterior diameter of the ILM (17). It is uncertain whether suboptimal anatomic positioning contributed to higher cervical pressures with the ILM.

These data suggest that the cervical pressures generated by laryngeal mask devices can produce posterior displacement of the normal C-spine. Caution must be used when extrapolating these findings to the unstable C-spine, but it is likely that the posterior displacement would be greater in the injured C-spine. There are few objective data guiding the clinician in the appropriate airway management of the patient with the unstable C-spine, but direct laryngoscopic and fiberoptic techniques are rarely associated with neurologic deterioration (28). It has been suggested that laryngeal mask devices are suitable for the unstable C-spine because they do not require head-neck movement for insertion (1–15), but the effect of LMA and ILM insertion on C-spine movement has not been determined, and no outcome data are available. The clinical importance of our findings for use of the LMA and ILM in the unstable C-spine is unknown. C-spine movement studies in anesthetized patients with normal necks and cadavers with unstable necks are required. Meanwhile, we recommend that laryngeal mask devices should only be used in the unstable C-spine if difficulties are anticipated or encountered with established techniques.

We conclude that laryngeal mask devices exert greater pressures against the cervical vertebrae than established intubation techniques, and they can produce posterior displacement of the C-spine. We recommend that laryngeal mask devices only be used in the unstable C-spine if difficulties are anticipated or encountered with established techniques, pending the results of studies demonstrating its relative safety.

We thank Dr. Alison Berry for advice with the manuscript.

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