Comparison of Mucosal Pressures Induced by Cuffs of Different Airway Devices

Research and ethical committee approval (Medical University Vienna, Vienna, Austria) was obtained. All patients, or their relatives, consented to postmortem research before the cadavers were used. In the morgue, we studied 19 fresh cadavers less than 24 h postmortem. The causes of death were sepsis (n = 4), bronchial carcinoma (n = 3), carcinoma of the colon (n = 3), myocardial infarction (n = 3), pneumonia (n = 2), pulmonary embolism (n = 2), carcinoma of the breast (n = 1), and cerebral hemorrhage (n = 1). None of the patients were intubated or resuscitated before death. Cadavers underwent laryngoscopic examination, and cadavers presenting with upper esophageal or laryngopharyngeal pathology were excluded. The following devices were investigated:

1. Conventional endotracheal tubes (ETT; Tyco Healthcare Nellcor Mallinckrodt, Pleasanton, CA)

2. Esophageal tracheal Combitube® (ETC; Tyco Healthcare Nellcor Mallinckrodt)

3. EasyTube® (EzT; Teleflex Ruesch, Kernen, Germany)

4. Laryngeal Mask Airway ™ (LMA ™; LMA North America, San Diego, CA)

5. Intubating Laryngeal Mask ™ (ILMA ™, Fastrach ®; Lma North America)

6. Proseal ™ laryngeal mask airway (PLMA ™; Lma North America)

7. Laryngeal Tube Airway (LT; Vbm Medizintechnik, Sulz, Germany)

Each device was inserted in each cadaver. The insertion was randomized by a computer-generated code. Two experienced participants of the study (F.H., P.K.) with experience with all devices in at least 10 patients inserted the devices. We left the devices in place for 3 min for each measurement. The airway tips were placed in either the trachea (ETT, ETC, EzT) or the esophagus (ETC, EzT, LT) without inducing mucosal damage. Maximal esophageal pressures of ETC and EzT at a cuff inflation of 12 ml were measured. Mucosal pressures of proximal cuffs (ETC, EzT, LT) as well as of cuffs of the LMA ™, ILMA ™, and PLMA ™ were evaluated by inflation in 10-ml increments from 0 to 60 ml (LMA ™, ILMA ™) and 0 to 100 ml (ETC, EzT, PLMA ™, LT), respectively.

Mucosal pressure was assessed using a 1.2-mm-diameter strain gauge silicon microchip sensor (Codman MicroSensor; Johnson and Johnson Medical Ltd., Bracknell, United Kingdom), which was calibrated before its use. For each position of the respective device, a sensor was fixed to the surface of the proximal balloon and/or distal cuff with a clear adhesive dressing (thickness of 0.45 mm; Tegaderm; 3M, Ontario, Canada). The sensors were attached to the locations described in table 1. The sensing element of the sensor was orientated toward the mucosal surface. The ETT, ETC, EzT, and LT were inserted into the cadavers using a standard Macintosh laryngoscope No. 3. The correct position of the sensors and the integrity of the mucosa were checked before and after measurement by laryngoscopy. The LMA ™, ILMA ™, and PLMA ™ were inserted using the standard methods described by Brain et al.  4The airway sizes as well as the references are shown in table 2.

Tracheal mucosal pressures exerted by the ETT cuff and the distal cuffs of the ETC and EzT were evaluated by cuff inflation from 0 to 12 ml in 2-ml increments. In addition, the mucosal pressures exerted by proximal cuffs were compared with respect to the amount of air recommended for emergency situations: 85 ml with the ETC,5–1080 ml with the EzT,1140 ml with the LMA ™,12–1540 ml with the ILMA ™,16,1740 ml with the PLMA ™,18,19and 80 ml with the LT.20In addition, a cuff volume of 40 ml was chosen for the ETC and EzT because there is a difference in the recommended filling volume between emergency intubation and elective use during general anesthesia: In elective anesthesia, the volume of the upper balloon can be reduced according to the minimal volume technique from 40 to 85 ml.21We compared the maximal tracheal pressures of the ETT, ETC, and EzT at 12 ml; maximal esophageal pressures of the ETC and EzT at 12 ml; and maximal pharyngeal pressures of the ETC, EzT, LMA ™, ILMA ™, PLMA ™, and LT at the respective recommended and highest volumes as well as at 40 ml.

The sample size calculation was based on a type I error α of 0.008 corrected for multiple testing, a power of 0.8, a mean difference of 25%, and an SD within the treatment groups of σ= 0.2, resulting in a sample size of 17. The data were tested for normal distribution using the Shapiro-Wilk statistic (in case of a sample size less than 51) and the usual Kolmogorov D statistic otherwise.22For normally distributed data, we used the general linear model or otherwise the Kruskal-Wallis test to test for differences between various tubes and regions, respectively. The statistical analysis system was SAS 8.01 (SAS, Cary, North Carolina).

We compared the means of the highest pressures of the three locations (anterior, left lateral, and posterior) of all respective 19 measurements. For each comparison, we took out the maximal pressures of the three locations (anterior, lateral, posterior) of the respective airways. The box plots show the median and the quartiles of the respective pressures. A P  value of less than 0.05 was considered to be significant.

The mean ± SD (range) age, height, and weight of cadavers were 75 ± 11 (48–90) yr, 177 ± 8 (169–191) cm, and 85 ± 9 (72–97) kg, respectively. The male-to-female ratio was 14:5. Tracheal as well as esophageal placement of the airway tips was achieved in all cadavers on the first attempt.

Maximal tracheal pressures at a cuff volume of 12 ml were higher using the ETC compared with the ETT and EzT (fig. 1). Maximal esophageal pressures at a cuff volume of 12 ml were significantly higher using the EzT compared with the ETC (fig. 2). Assessing pharyngeal pressures using the ETC (with 85 ml), EzT (with 80 ml), LMA ™ (with 40 ml), ILMA ™ (with 40 ml), PLMA ™ (with 40 ml), and LT (with 80 ml) at those volumes recommended by the manufacturers, we found significantly higher pressures with the ILMA ™versus  all other devices (fig. 3). At maximal volumes (ETC [with 100 ml], EzT [with 100 ml], LMA ™[with 60 ml], ILMA ™[with 60 ml], PLMA ™[with 100 ml], and LT [with 100 ml]), the LMA ™, ILMA ™, and PLMA ™ induced higher pharyngeal pressures compared with all other devices (fig. 3). At a cuff volume of 40 ml, the ILMA ™ and LMA ™ induced higher pressures compared with all other devices (fig. 3).

Other data are not shown for clearness; however, all results of the respective pressures are available on request. With a few exceptions, directly measured mucosal pressures showed significant differences (P < 0.0001) for the ETC, EzT, LMA ™, ILMA ™, PLMA ™, and LT, respectively, in all pharyngeal locations.

Directly measured mucosal pressures exerted by the EzT (cuff volume = 12 ml) on the esophageal mucosa were significantly different (P < 0.0001) in the anterior versus  lateral as well as anterior versus  posterior region. No differences were found for the ETC.

Directly measured mucosal pressures in the trachea at 12 ml showed significant differences (P < 0.0001) between all three locations with the ETT and EzT, and between anterior versus  lateral and lateral versus  posterior locations with the ETC (P < 0.0001). No mucosal laceration or tear was found.

One of the major advances in ETT design was the introduction of high-volume, low-pressure cuffs intended to minimize the pressure exerted on the tracheal mucosa.1Major attention is laid upon the fact that the mucosal pressure exerted by the cuff remains below the capillary mucosal pressure of approximately 26 mmHg. This concept has been incorporated into the new EzT airway, which has been equipped with a high-volume, low-pressure distal cuff system exhibiting significantly lower tracheal pressures. In our cadaver study, we found that pharyngeal, esophageal, and tracheal mucosal pressures increased with rising cuff volumes in all tested devices.

Regarding the ETC, high tracheal mucosal pressures are observed at the maximal recommended cuff volume of 12 ml (up to 41 cm H²O). In the clinical situation, much lower volumes are usually necessary to obtain an airtight seal. Therefore, blocking of the distal ETC cuff in the trachea without a cuff-pressure gauge is only recommended in emergency use. Pressures in the trachea registered for EzT were not significantly different from those observed with a regular ETT. With respect to location, the highest values were found in the lateral location with the ETT, in the posterior location with the EzT, and in the anterior position with the ETC. Although the findings with the ETC and ETT can be explained by the fact that the posterior membranous tracheal wall is more distensible than the cartilaginous anterolateral wall, we have no explanation for the higher posterior values with the EzT. We do not have data for the long-term use of the EzT up to now.

Regarding esophageal placement, both the ETC and the EzT exert similar and relatively low pressures on the esophageal mucosa (maximum around 14 cm H²O). The highest values were found with the EzT when compared with the ETC. Although the same method was used, our data show lower values as compared with the measurements of Keller et al.  1Direct esophageal injury induced by an inflated cuff can therefore be certainly prevented by strictly adhering to the manufacturer’s guidelines. Furthermore, the danger of a permanent injury to the mucosa seems limited, because the investigated devices are not intended for prolonged use. The ETC may be used for up to 8 h.9 

The LMA-Classic ™ exerts pressures of approximately 25–35 cm H²O on the pharyngeal or hypopharyngeal mucosa, equally distributed among anterior, lateral, and posterior locations up to a filling volume of 40 ml. Interestingly, even lower pressures were observed for the PLMA ™, with mucosal pressures mainly below 20 cm H²O, at least when maximally recommended filling volumes are observed. Pharyngeal mucosal perfusion is progressively reduced when mucosal pressures exceed 34 cm H²O.3Therefore, both devices are considered safe with respect to the pharyngeal mucosa. Significantly higher pressures, especially in the anterior location, were observed for the ILMA ™ (up to 41 cm H²O). However, this device is not intended for prolonged use, and its removal is recommended after successful tracheal intubation. Interestingly, the other supraglottic airways (ETC, EzT, and LT) exert pressures lower than that with the ILMA ™ and similar to those obtained for the LMA-Classic ™ (in the range of 23–36 cm H²O). Opposite to the findings of Keller et al. ,1our data show the highest values in the anterior pharyngeal location using the ETC. Similar results were found for the LMA ™, ILMA ™, and LT. In contrast, the highest values were found in the posterior location with the PLMA ™ and EzT. The latter results may be explained because the posterior surface is adjacent to rigid anatomical structures (vertebral bodies), whereas the anterolateral surface is adjacent to compliant anatomical structures (epiglottis, tongue, pharyngeal muscles, and membranous trachea).1We have no explanation for the different behaviors of the devices, except that the PLMA ™ may exert higher pressures in the posterior portion because of the additional cuff on the posterior part of the airway. With respect to pharyngeal mucosal pressures, we could not identify any traumatizing potential of these devices with respect to pharyngeal lacerations. The LMA ™ may be used for more than 3 h.13 

A limitation of the study is that mucosal pressures in awake and anesthetized patients may not be compared with the measurements in cadavers. Therefore, the muscle tone of patients is not taken into account. However, Keller et al.  1found that pharyngeal mucosal pressures were similar for cadavers and awake volunteers, which suggests that muscle tone does not play a major role in mucosal pressure measurements. Although their findings should be interpreted cautiously, they consider them to be applicable to the anesthetized patient for several reasons. First, there is evidence that pharyngeal compliance is similar in fresh cadavers and paralyzed anesthetized patients.3,23Second, the data for tracheal mucosal pressures in cadavers closely match those of a previous ETT study in paralyzed anesthetized patients,24and the data for pharyngeal mucosal pressures closely match those of two previous studies of the cuffed oropharyngeal airway in paralyzed anesthetized patients.25 

In summary, our data demonstrate that increasing cuff volumes are directly transferred into increased mucosal pressures up to levels potentially exceeding mucosal perfusion pressure. Those mucosal pressures are seldom observed by limiting the cuff volumes following the minimal volume technique. Recommended volumes should be strictly followed because slightly overextended cuffs may exert high pressures, e.g. , inflating a size 5 LMA ™, PLMA ™, or ILMA ™ with 60 ml air can result in pharyngeal mucosal pressures of up to 60 cm H²O. The observed differences between the devices do not seem to be clinically relevant and do not preclude the use of any device in clinical routine cases, because the devices investigated (except the ETT) are not intended for prolonged use (for more than 8 h). Esophageal mucosal pressures with the ETC and EzT in the esophageal position remain low when maximal filling volumes recommended by the manufacturers are respected. We therefore recommend adherence to the manufacturers’ guidelines and the use of a pressure gauge for cuffing all devices.