A New Endotracheal Tube with a Cuff Impervious to Nitrous Oxide

Nitrous oxide will diffuse into the cuff of an endotracheal tube during anesthesia, causing an increase of cuff volume and intracuff pressure. Several devices have been proposed to maintain low intracuff pressure during operation [1-4]. Recently, a new endotracheal tube with a cuff made of a material with high gas barrier properties (Trachelon Trademark, gas barrier type; Terumo Co., Ltd., Tokyo, Japan) has been developed. In this study, we investigated whether the nitrous oxide-induced increase in endotracheal tube cuff pressure could be prevented by using the gas barrier type tube. Methods After obtaining institutional approval and patients' informed consent, 28 male ASA physical status I or II patients undergoing elective abdominal or extremity operation were studied. Patients were excluded from the study if surgical positioning other than supine was necessary. Patients were randomized into two groups: Group G (n = 14), tracheal intubation with a gas barrier type tube, and Group S (n = 14) tracheal intubation with a standard tube (Trachelon Trademark, soft type). The tubes were identical in size (9.0 mm internal diameter) and composition except the cuff Figure 1.Figure 1: A gas barrier type tube (top) and a standard tube (bottom).After the patients breathed 100% oxygen, a rapid sequence induction of anesthesia was achieved with thiamylal 5 mg/kg and succinylcholine 1 mg/kg intravenously and the trachea was intubated with either tube in the usual manner. Immediately after intubation, the endotracheal tube cuff was filled with air just sufficient to prevent a tracheal gas leak at an end-inspiratory plateau pressure of 20 cm H2 O. The initial air volume was recorded. The pilot balloon of the endotracheal tube was then connected through a three-way stopcock to a pressure transducer tester (Uniflow Trademark model 59-UCAL; Baxter, Santa Ana, CA) that displayed intracuff pressure continuously. Neuromuscular block was achieved by intravenous administration of vecuronium and its magnitude was monitored by a train-of-four stimulation. After neuromuscular block was achieved and ventilation was controlled, the baseline end-expiratory pressure in the endotracheal tube cuff was recorded. General anesthesia was maintained with 50% nitrous oxide in oxygen and either isoflurane or sevoflurane. Muscle relaxation was maintained with vecuronium throughout the study. In every case, end-expiratory intracuff pressures were recorded every 10 min during the first hour and every 15 min thereafter. At the end of the operation, final cuff volume was measured. All pressure measurements were presented in millimeters of mercury as mean +/- SD. Data were analyzed using Student's unpaired t-test for the intergroup comparisons and liner regression for the intracuff pressure changes from the baseline value over time. P < 0.05 was considered significant. Results There were no significant differences between the groups with respect to age, weight, height, and nitrous oxide inhaled time. The baseline intracuff pressures and maximum cuff pressures were significantly lower in Group G than in Group S. No significant differences in initial cuff volumes were observed between the groups. The final cuff volume and volume change in Group G showed significantly smaller increases compared with those in Group S Table 1.Table 1: Cuff Volumes and Pressures Before and After Exposure to Nitrous OxideThe results for the pressure measurements over time are shown in Figure 2. The mean intracuff pressure was significantly lower in Group G throughout the study. The regression lines calculated from intracuff pressure increases from the baseline value are shown in Figure 3. The curves show confidence bands of the lines. The slope for Group G was significantly smaller than that of Group S (0.018 vs 0.133).Figure 2: Intraoperative changes in intracuff pressure of the gas barrier type tube (open circles) and a standard tube (solid circles). The mean intracuff pressure is significantly lower in the gas barrier group throughout the study. Vertical bars indicate SD of the mean. The number of tubes studied is shown at the end of each vertical bar.Figure 3: Increases of cuff pressure from baseline over time. The slope for Group G is significantly smaller than that for Group S (0.018 and 0.133, respectively). The curves depict 99% confidence bands of the lines. G = gas barrier group; S = standard group.Discussion The principal finding of this study was that the gas barrier type tube effectively prevented an increase in endotracheal tube cuff pressure. This finding confirms the previous study by Kitamura et al. [5], who also found that the gas barrier cuff prevented high cuff pressures resulting from diffusion of nitrous oxide. It was thought necessary to confirm this previous finding in a different setting because Kitamura et al. [5] used a Portex tube for comparison. Not only a cuff but a pilot balloon and an inflation line could also affect intracuff pressure. To evaluate the cuff itself, the tubes studied should be identical except for the cuff. Nitrous oxide-induced increase in endotracheal tube cuff pressure is a problem that will remain as long as we use nitrous oxide as a clinical anesthesic. Several devices have been proposed to limit excessive intracuff pressures intraoperatively [1-4]. These devices are based on the idea that the expanded volume in the cuff caused by diffusion of nitrous oxide should leave the cuff through the device. The gas barrier type tube is unique in this respect. It has a newly developed cuff made of flexible fluoro plastic (graft copolymer including fluoro rubber and crystalline fluoro plastic segments), which has high gas barrier properties. In an in vitro study, nitrous oxide permeability through fluorine elastomer was 30 times less than that of polyvinyl chloride (Terumo Co. Ltd., unpublished data). The gas barrier cuff is not as thick as a standard cuff (0.1-0.15 mm vs 0.15-0.18 mm) (Terumo Co. Ltd., personal communication, 1993). As the rate of nitrous oxide diffusion varies inversely with cuff thickness, lower intracuff pressure in the gas barrier tube with a thinner cuff wall may verify the potency of the new material. Physical characteristics of the cuff affect intracuff pressure. Diameter, thickness, and compliance are thought to be the most important cuff features affecting intracuff pressure [6]. In the present study, the initial cuff volume was not significantly different between the two groups, whereas the gas barrier cuff had lower baseline intracuff pressure. This finding suggests that the compliance of the cuff is higher in the gas barrier cuff. The higher compliance of the gas barrier cuff may also contribute to low intracuff pressure. Cost considerations are mandatory when new techniques are introduced into general clinical practice. Although a gas barrier tube is expensive compared with a standard tube, other comparable tubes, e.g., the Hi-Lo Registered Trademark endotracheal tube with Lanz Registered Trademark pressure-regulating valve (Mallinckrodt, St. Louis, MO), and the Brandt Registered Trademark anesthesia tube with rediffusion system (Mallinckrodt), are more expensive. In conclusion, the gas barrier type tube effectively prevented intracuff pressure increases during general anesthesia with nitrous oxide.