Cuffed endotracheal tubes are used to prevent aspiration of pharyngeal contents into the lower trachea , thereby reducing the risk of pneumonitis [2,3], and to prevent air leakage around the tube, thereby enhancing the efficacy of artificial ventilation of the lungs. The cuffs of endotracheal tubes tend to seal the trachea better as the cuff pressure is raised . But high cuff pressure compromises mucosal blood flow . A frequently quoted upper limit for cuff pressure is 25-30 cmH2O [4,5], although an exact value is unknown and may differ intra- as well as inter-individually. Pressure-induced morbidity ranges from minor stridor after extubation to severe lesions, such as tracheal stenosis and tracheomalacia .
The recently introduced Microcuff® endotracheal tube (Microcuff GmbH, Weinheim, Germany) features a cuff membrane designed of ultra-thin polyurethane with improved tracheal sealing characteristics and therefore requiring lower cuff pressures than conventional tracheal tubes. The objective of the present study was to compare the efficacy of the Microcuff® endotracheal tube, in preventing air leakage around the tube, with a variety of widely used cuffed endotracheal tubes.
Air leakage around the cuff of the endotracheal tube was tested in five different tubes (internal diameter (ID) 7.5 mm; Table 1, Fig. 2). A vertical polyvinylchloride (PVC) artificial trachea with an ID of 20 mm - which is known to be in between the mean diameter of an adult male and female trachea  - was intubated and the cuff of the tube inflated. Cuff pressure was set to 10, 15, 20, 25, 30 and 60 cmH2O, in random order and kept constant by means of a cuff manometer (Mallinckrodt, Athlone, Ireland). The proximal end of the tube was connected to an anaesthesia lung ventilator (ADU®; Datex-Ohmeda, Helsinki, Finland), which was checked according to the manufacturer's data before usage. The distal end of the trachea model was connected to a test lung (Siemens Medical, Erlangen, Germany), with a maximum capacity of 1 L. A gas probe (disposable anaesthesia sampling line; Datex-Ohmeda, Helsinki, Finland) was fixed in the trachea model proximal to the tracheal tube cuff for assessment of air leakage using a side-stream gas analysing monitoring system (A/S® 5; Datex-Ohmeda, Helsinki, Finland). The ventilator was set to pressure-controlled ventilation mode (PEEP 5 cmH2O, peak inspiratory pressure 20 cmH2O, respiratory rate 10 breaths min−1, and fresh gas (air) flow 4 L min−1) and the test lung ventilated. After 10 breaths, sevoflurane 2% was added to the gas mixture. The time from adding sevoflurane to the breathing system until it was detected (>0.5%) by the gas probe was measured. If no sevoflurane was detected within 10 min the test was terminated.
Each experiment was performed four times using two samples of each of the five brands of tube in random order. Tracheal tube cuffs were inflated and checked by inspection prior to each test. Measurements were performed at room temperature of 22-23°C.
In vivo setup
With approval of the Hospital Ethics Committee we enrolled 50 adolescent patients into the study. Inclusion criteria were elective surgery, requiring general anaesthesia with tracheal intubation with a cuffed tracheal tube of ID 7.0 mm. Exclusion criteria were ASA ≥ III, anticipated or known previous difficult tracheal intubation, airway anomalies, and patients at increased risk for gastric regurgitation. The tube cuffs were inflated and checked by inspection prior to intubation. After induction of general anaesthesia and achievement of muscle paralysis the patients' tracheas were intubated, randomly using one of five endotracheal tubes of ID 7.0 mm (Table 1; 10 patients for each brand of tube). Correct tube position was confirmed by capnography and auscultation of the lungs. Adequate tube size was verified by the presence of air leakage without cuff inflation. If no air leakage was obtained at 20 cmH2O peak inspiratory pressure with the cuff not inflated, the tube was changed. Pressure-controlled ventilation was started with ventilator settings: peak inspiratory pressure 20 cmH2O, PEEP 5 cmH2O, respiratory rate 15 breaths min−1, fresh gas flow 4 L min−1, 30% O2 in 70% N2O, and sevoflurane according to the patients' requirements. Patients were in supine position with their heads in the neutral position.
After pharyngeal suction, an oral artificial airway (Guedel airway, Sims Portex Ltd. Hythe, Kent, UK) was inserted, through which a gas probe was inserted to a depth of 5 cm. The proximal end of the gas probe was connected to the anaesthesia monitoring system for assessment of CO2 appearing in the pharynx. The cuff pressure required to prevent air leakage, as indicated by the disappearing of audible sounds at the mouth and pharyngeal CO2, was assessed in increasing steps of 2 cmH2O using the cuff pressure manometer. If air leakage occurred above 25 cmH2O, cuff pressure was set at 25 cmH2O, or the tube changed, if pressure-controlled ventilation was not efficient. Tests were performed within 5 min after intubation and were repeated 45 min after intubation.
The frequency of in vitro detection of sevoflurane within 10 min from the Microcuff tracheal tube was compared with corresponding values obtained from conventional tracheal tubes at each cuff pressure level using Fisher's exact test. Data are presented as median (range).
Patient characteristics and sealing pressures assessed by auscultation and capnometry (5 and 45 min after intubation) from the Microcuff® tracheal tube group were compared with those from the conventional tracheal tubes using the Mann-Whitney U-test. Cuff pressures at 5 min were compared with those at 45 min and cuff pressures obtained with the audible method were compared with those obtained with pharyngeal capnography using paired t-test. Data are presented as mean ± SD and/or median (range), if appropriate. P < 0.05 was considered as significant for all tests.
In vitro experiments
The onset time for in vitro air leakage with cuff pressure ≤25 cmH2O was less than 60 s for all tested tubes except for the Microcuff® endotracheal tube in which no leakage was detected with cuff pressure ≥15 cmH2O. Times to detection of sevoflurane for the tested endotracheal tubes and cuff pressure levels are presented in Table 2.
In vivo experiments
Fifty patients were enrolled into the study. Patients' ages were (median; range) 14.2 (12.0-17.1) yr, weight 57.5 (40.0-81.9) kg and height 164.9 (146.5-190.0) cm. No significant differences in patients' characteristics were found between the five groups and their characteristics are shown in Table 3.
Auscultatory assessment of air leakage (18.2 ± 7.0 cmH2O) revealed similar results as did capnometric assessment (17.2 ± 7.4 cmH2O, P = 0.55). In 12 cases the capnometric assessment was impossible because secretions blocked the capnometry probe.
Mean cuff pressure (audible) required for adequate air sealing (all tubes) was 18 cmH2O (8-42). Forty-five minute sealing pressures were significantly lower (17.0 ± 6.4 cmH2O) than corresponding 5 min values (19.1 ± 7.9 cmH2O, P = 0.027; Fig. 3) for all tested tubes, except the Microcuff® tube.
The cuff pressures required for air sealing are presented for each of the five tested tubes in Figure 3. The Microcuff® tube required significantly lower sealing pressures (9.5 (8-12) cmH2O) compared to the each of the other four brands, independent from the method or time of assessment (P < 0.05; Mann-Whitney U-test).
This study evaluated the recently introduced Microcuff® endotracheal tube with regard to its in vitro and in vivo ability to prevent air leakage and compared it to four series of conventionally used cuffed endotracheal tubes. The main finding was that, among the tested endotracheal tubes, the Microcuff® endotracheal tube required the lowest in vitro and in vivo sealing pressures to prevent air leakage (P < 0.05; Mann-Whitney U-test).
Pressure exerted by the tube cuff on the tracheal wall compromises mucosal blood flow, leading to erosion, ulceration and subsequently tracheal stenosis or tracheomalacia [7,8]. Whereas tracheal stenosis is a rare complication, pressure-related tracheal morbidity is a well known phenomenon [4,9-12]. A widely quoted limit for cuff pressure is 25-30 cmH2O [4,13,14]. The thresholds for impairment of tracheal mucosa blood and lymph flow may vary inter- and intra-individually, depending on systemic arterial pressure and perfusion. Schaffranietz and colleagues therefore proposed the cuff pressure-to-mean arterial pressure index as a criterion for adequacy of tracheal microcirculation, which should remain less than 0.2 . This makes lowest possible sealing pressures desirable.
The improved sealing characteristics of the Microcuff® HVLP tube cuff found in our study may be related to the following factors: first, the Microcuff® 7 μm polyurethane cuff membrane is several times thinner compared to conventional tubes with PVC cuff membranes of approximately 50 μm or thicker. The ultra-thin membrane prevents longitudinal fold and channel formation when the cuff is inflated, which is known to be the primary source of fluid leakage [15-17]. Sealing pressures 45 min after intubation were significantly lower than corresponding values after 5 min in all tubes, except for the Microcuff®. This indicates better sealing characteristics of warmed, lubricated tube cuffs [16,18], when any folds in the cuff membrane may be partially filled with secretions. In the wrinkle-free Microcuff® tube cuff this is not the case. Second, the cylindrical shape of the cuff provides a large contact area and therefore a better tracheal seal, which is less obvious in tubes B and C, than those with the highest sealing pressures (Fig. 3).
Whereas fluid leakage past tracheal tube cuffs has been well studied [15-24], the ability to preventing air leakage has not. Guyton and colleagues found minimal occlusion pressures of 2-40 mmHg, revealing a linear relationship to peak inspiratory pressure . Various alternative cuff designs were presented to enhance sealing characteristics and showed promising results in first evaluations [16,21]. Also different techniques to improve sealing characteristics were presented, such as gel lubrication of tube cuffs [16,18] or preparation with silicon spray. However, none of these designs and techniques are yet established in clinical practice.
Our findings suggest that with the Microcuff® HVLP tube cuff significantly lower cuff pressures can be used during anaesthesia with endotracheal intubation to prevent air leakage than with conventional cuff membranes. These results are in accordance with preliminary data showing the polyurethane cuff prevents fluid leakage at lower cuff pressures . The lower cuff pressures required are especially beneficial in situations with a high risk of mucosal damage, e.g. long-term intubation of haemodynamically unstable patients, or those with tracheotomy, and infants and children.
Some limitations of our study need to be considered. In the in vitro experiments we assessed the cuff pressures to prevent air leakage in a PVC trachea at room temperature. These laboratory conditions are different from the in vivo situation. Warming of the tube and cuff and the presence of mucous and saliva improve the sealing characteristics of cuffs in the in vivo conditions, as indicated by our data. However, in the in vitro experiments the size and shape of the trachea was the same for all tests performed, therefore allowing standardized testing. This study assessed sealing pressure only at standard ventilator settings. Further studies are needed in intensive care patients with high inspiratory pressure to confirm the superior features of the new Microcuff® HVLP to prevent air leakage. At present the main disadvantage of the Microcuff® tube is the higher price (€ 9.50) compared to the other tubes we tested (approximately € 1.60-7.00). This is expected to change, should the tube be produced in grater quantities.
In conclusion, the new Microcuff® HVLP endotracheal tube with an ultra-thin cuff membrane required the lowest sealing cuff pressures to prevent air leakage of all the tubes tested in this study. This characteristic may become interesting for long-term intubated patients, for tracheotomy tubes and for cuffed endotracheal tubes in infants and children, allowing tracheal sealing at lower cuff pressures implicating less damage to tracheal mucosa.
The investigated paediatric cuffed endotracheal tubes were ordered from local distributors and partially provided without charge. No financial support was obtained for the presented work. Dr. Weiss and Dr. Gerber are involved in designing a new paediatric cuffed endotracheal tube in co-operation with Microcuff GmbH, Weinheim, Germany.
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