The laryngeal mask airway in fresh cadavers versus paralysed anaesthetized patients: ease of insertion, airway sealing pressure, intracuff pressures and anatomic position

Abstract

We compared the performance of the size 5 laryngeal mask airway in 20 paralysed anaesthetized male patients and 20 male cadavers (6-24 h post-mortem). Groups were matched for height and weight. Airway sealing pressure, in vivo intracuff pressure and anatomical position (judged fibre-optically) were documented at zero volume and after each additional 10 mL up to 40 mL. All laryngeal mask airways were inserted at the first attempt and insertion times were similar. There were no differences in airway sealing pressure, in vivo intracuff pressure or fibre-optic position between the groups. We conclude that the performance of the laryngeal mask airway is similar for fresh cadavers and paralysed anaesthetized patients. This may have implications for laryngeal mask airway research and training.

Introduction

Laryngeal mask airway (LMA) research is generally conducted in anaesthetized patients ^[1], but investigations into the mechanisms of aspiration ^[2], pharyngo-laryngeal trauma ^[3] and the testing of new prototypes ^[4,5] may require the use of cadavers due to increased patient risk ^[6]. Furthermore, training in LMA insertion, particularly by non-anaesthetists, is limited by access to anaesthetized patients and the possibility that multiple insertion attempts may increase pharyngo-laryngeal morbidity ^[7]. The potential value of cadavers for research and training depends on the degree of similarity between the cadaver model and the conditions in living humans. In this study, we compare the performance of the LMA between fresh cadavers and anaesthetized patients in terms of ease of insertion (number of insertion attempts required and time to successful placement), airway sealing pressure, in vivo intracuff pressure and anatomical position (assessed fibre-optically).

Methods

Twenty paralysed, anaesthetized male patients and 20 fresh male cadavers (6-24 h post-mortem) were studied. Ethical committee approval was obtained. Informed consent was obtained from relatives (for the cadaver group) and patients (for the anaesthesia group). Anaesthetized patients were excluded if they were less than 18 years, had laryngopharyngeal pathology or were at risk of aspiration. Cadavers were excluded if they had laryngopharyngeal pathology. Groups were matched for height, weight and sex. A standard protocol was followed for anaesthetized patients and routine monitoring applied. Patients were induced with propofol 2.5 mg kg⁻¹ and anaesthesia was maintained with 100% O₂ and sevoflurane 1-2%. Muscle relaxation was with atracurium 0.5 mg kg⁻¹. A single experienced LMA user (>1500 uses) inserted/fixed the LMA according to the manufacturers instructions ^[8]. A size no. 5 LMA was used for all anaesthetized patients and cadavers ^[9]. The number of insert attempts required was recorded. A failed attempt was defined as removal of the device from the mouth. The time between placement of the device in the mouth to cuff inflation was measured with a stopwatch. The pilot balloon was attached via a three-way tap to a 10-mL syringe and a calibrated pressure transducer with an accuracy of ±5%. The intracuff pressure was reduced to −55 cm H₂O in vitro. Airway sealing pressure, in vivo intracuff pressure and fibreoptic position were documented at zero volume with the cuff fully evacuated and after each additional 10 mL up to 40 mL. Air was used to fill the cuff. The fibreoptic position of the LMA was determined from the mask aperture bars using the following scoring system: 4, only vocal cords visible; 3, vocal cords plus posterior epiglottis visible; 2, vocal cords plus anterior epiglottis visible; 1, vocal cords not seen ^[10]. Measurements were made in the supine position with the head/neck in the neutral position and the occiput on a firm pillow 7 cm in height. The airway sealing pressure was measured by closing the expiratory valve of the circle system at a fixed gas flow of 3 L min⁻¹, and noting the airway pressure at which the dial on the aneroid manometer reached equilibrium ^[11].

An unblinded trained observer collected data. Sample size was based on data from a pilot study of 10 patients in which airway sealing pressure was measured in anaesthetized patients and cadavers for a Type I error of 0.05 and a power of 0.9. The distribution of data was determined using Kolmogorov-Smirnov analysis. Statistical analysis was with paired t-test (normally distributed data), Friedman's two-way analysis of variance (non-normally distributed data) and χ²-test. Data are presented as mean ±SD unless otherwise stated. Significance was taken as P < 0.05.

Results

Anaesthetized patients were significantly younger than cadavers, respectively (36±13 years vs. 76±11 years, P<0.0001), but had similar height (176±6 cm vs. 176±6 cm) and weight (76±8 kg vs. 78 ± 7 kg). Data for airway sealing pressure and intracuff pressure were normally distributed. All LMAs were inserted at the first attempt and insertion times were similar for paralysed, anaesthetized patients (8±5 s) and cadavers (9±6 s). There were no significant differences between airway sealing pressure, in vivo intracuff pressure or fibre-optic position between groups (Table 1).

Table 1: Airway sealing pressure, in-vivo intracuff pressure and fibre-optic score with increasing cuff volume for paralysed anaesthetized male patients and fresh male cadavers using the size 5 LMA

Discussion

Our data show that the performance of the LMA in fresh cadavers and in paralysed anaesthetized patients is similar. Compared with the anaesthetized human, cadavers have a lower temperature, probably more rigid pharyngeal musculature and do not undergo spontaneous or positive pressure ventilation. The finding that in vivo intracuff pressures and airway sealing pressures were similar suggests that pharyngeal compliance was also similar. This may be related to the freshness of the cadavers and agerelated differences in the rigidity of the pharyngeal tissues. We speculate that the effects of rigor mortis on increasing pharyngeal rigidity are offset by lower pharyngeal rigidity in the elderly patient.

Our data suggest that fresh cadavers may be a useful investigative tool for LMA research when the use of living humans would be inappropriate. This might include research into the mechanisms of regurgitation and aspiration, the effects of malposition and its relation with pharyngeal, cranial nerve and glottic injury, unorthodox insertion techniques, radiological studies and placement in the unstable cervical spine. Preliminary research with new prototypes should perhaps also be performed on cadavers before subjecting awake and anaesthetized patients to unknown risks. Finally, the cadaver model could also be used for training purposes when access to anaesthetized patients is limited.

We conclude that the performance of the LMA is similar in fresh cadavers and in paralysed anaesthetized patients. This may have implications for LMA research and training.