When the cuffs of endotracheal tubes are filled by gases during anesthesia with nitrous oxide (N2O), changes in intracuff pressure depend on N2O concentrations in both the intracuff gases and in the gas mixtures with which patients are ventilated during anesthesia (1–4). The cuff volume and intracuff pressure increase via N2O diffusion into the cuff when air is used to fill the cuff for satisfactory initial sealing. N2O in cuffs, however, equilibrates at a smaller concentration than the gas mixture with which patients are ventilated because N2O in the cuff rediffuses out through the pharyngeal side of the cuff and the pilot balloon during anesthesia (3,4). Furthermore, inflating cuffs with 40% N2O maintains stable cuff pressure without excessive cuff pressure or air leaks during anesthesia with 67% N2O (4). However, after reducing N2O-inspired concentrations during anesthesia or after cessation of anesthesia with N2O, intracuff pressure might decrease because of N2O rediffusion into the air, resulting in a high risk of air leaks or aspiration. Among a number of methods and devices to inhibit an increase in cuff pressure caused by diffusion of N2O during anesthesia (1–13), filling cuffs with N2O is one of the most reliable methods. However, there are only a few reports assessing the limitations of N2O-filled cuffs when N2O-inspired concentrations are reduced. Therefore, in the present randomized study we assessed the decrease in cuff pressure after substituting oxygen for N2O in patients in whom we achieved stable cuff pressure by inflating cuffs with 40% N2O during anesthesia with 67% N2O.
Subjects were patients (n = 50, aged 19–84 yr, ASA physical status I or II) undergoing elective surgery. The investigation was approved by the Ethics Committee of the National Defense Medical College. Informed consent was obtained from the patients. Hydroxyzine and atropine sulfate were administered IM 1 h before induction of anesthesia. Anesthesia was induced with fentanyl (0.05–0.1 mg) and propofol (1.5–2.5 mg/kg) while the patients breathed 100% oxygen. Vecuronium was administered to relax the muscles. Patients were randomly allocated to five groups of 10 each. The trachea was intubated (8.0 mm inner diameter for men and 7.5 mm inner diameter for women) with the Hi-Contour (Mallinckrodt, Athlone, Ireland), Sheridan (Kendall, Mansfield, MA), Rush (Ruschelit, Kernen, Germany), Reinforce (Mallinckrodt), or Profile Soft-Seal Cuff (Sims Portex, Kent, UK) endotracheal tube. Lubricant was not used. The pilot balloon of the endotracheal tube was connected to a pressure transducer (UK802; Baxter, Tokyo, Japan) through a three-way stopcock. The mean intracuff pressure was measured every 10 min with a monitor (AS3; Datex, Helsinki, Finland). Immediately after intubation, the cuff was aspirated as much as possible and then inflated with the smallest volume of 40% N2O and 60% oxygen that would not leak when the intraairway plateau pressure was 18 cm H2O. The initial volume used to fill the cuff was recorded. The N2O gas mixture to fill cuffs was aspirated from the common gas outlet of an anesthetic machine. The N2O concentration of the gas mixture for filling cuffs was measured with a multigas monitor (Capnomac Ultima; Datex) that was calibrated with standard gases. A circle absorber breathing system was used, and anesthesia was maintained with 67% N2O and 33% oxygen, supplemented with isoflurane or sevoflurane. The lungs were mechanically ventilated. Endotracheal tube cuffs were monitored for air leaks every 10 min for 120 min in all groups during intermittent positive ventilation. The extent of paralysis was constant throughout the study periods.
Oxygen was substituted for N2O 120 min after the start of anesthesia. The mean intracuff pressure and air leaks through the cuffs were then monitored every 5 min. When intracuff pressure decreased to approximately one-third or when intracuff pressure decreased to less than half and air leaks were noticed, the gases in the cuff were aspirated as completely as possible, and the study was terminated. Approximately 15 min later, the volume of the aspirated gases from the cuff was measured at room temperature with a calibrated syringe. The volumes were measured both after the gases were compressed and after decompression, and the mean volume was taken to avoid a possible error caused by friction between a barrel and a piston of the syringe. The N2O concentration of the aspirated gases was measured with a calibrated multigas monitor (Type 1302; Bruel & Kjaer, Naerum, Denmark). If air leaks were noticed before the intracuff pressure decreased by more than half, the cuff pressure after cessation of N2O administration, volume of aspirated gases, and N2O concentration data were discarded from the analysis. A straight line was computed from the time versus the logarithm of percentage of pressure by using the least-squares method. The time required for intracuff pressure to decrease by half (T1/2) was estimated.
Data were presented as the number of patients or mean ± sd. Two-way analysis of variance (ANOVA) for repeated measurements was used to assess changes over time within, as well as among, groups, and one-way ANOVA was performed to compare raw data among groups. Post hoc analysis to allow for multiple comparisons was performed with a Bonferroni-Dunn correction. Student’s t-test was used to make single comparisons of cuff pressure, volume, and concentration of N2O. Proportional data were evaluated with the χ2 test. A P value of <0.05 was considered statistically significant.
The five groups of patients were comparable in sex, age, weight, and height (Table 1). Initial intracuff pressure did not differ significantly among the five groups (P = 0.06;Table 2). Cuff pressure did not differ significantly among the five groups during the 120 min of anesthesia with N2O (P = 0.15), but the cuff pressure in the Reinforce group increased significantly during anesthesia (P < 0.05;Fig. 1). Cuff pressure did not exceed 22 mm Hg, and there were no air leaks. The cuff pressure at 120 min did not differ among groups (P = 0.098).
The volume of aspirated gases was significantly smaller than the initial filling volume in all groups (P < 0.001 for each;Table 2). The volume change in the Rush group was significantly larger than in the Hi-Contour and the Profile Soft-Seal Cuff groups (P < 0.01 for each). There were no significant differences among the five groups in the N2O concentration of aspirated gases. T1/2 was significantly different between groups (P = 0.017), and T1/2 in the Hi-Contour group was 27.8 ± 8.5 min, which was significantly shorter than in the Profile Soft-Seal Cuff group (49.7 ± 18.5 min;P < 0.01;Table 2). The number of patients who had air leaks was not significantly different among groups (Table 2).
We found that the intracuff pressure of the standard endotracheal tubes decreases relatively quickly after administration of N2O is discontinued, despite maintenance of a stable cuff pressure during anesthesia. Although this deflationary phenomenon of N2O-filled cuffs might be expected, our results comprise the first evidence suggesting the possibility of air leaks and aspiration during prolonged emergence from anesthesia or during transportation of a patient with tracheal intubation immediately after anesthesia. We recently developed a method to inhibit an increase in cuff pressure (4), and this method enables us to assess decreases in intracuff pressure after achieving a stable pressure. N2O in cuffs equilibrates at a smaller concentration than the gas mixture with which patients are ventilated, and 40% N2O is optimal for inflating cuffs to preserve stable cuff pressure during anesthesia with 67% N2O (4). By using this method in the present study, stable cuff pressure was achieved during anesthesia with N2O. After substituting oxygen for 67% N2O, the cuff pressure of each endotracheal tube decreased, and the volume aspirated from cuffs decreased at the end of the study. Therefore, when N2O-inspired concentrations are reduced the N2O-filled cuff has limitations, although filling cuffs with N2O is reliable for preserving stable cuff pressure during anesthesia with N2O. Furthermore, the mean concentration of N2O in the cuff became <30% (Table 2), strongly suggesting that a decrease in cuff pressure after anesthesia was caused by N2O efflux through the cuff and pilot balloon (3,4).
In this study, T1/2 was assessed to compare decreases in intracuff pressure in five different endotracheal tubes. The T1/2 in the Profile Soft-Seal Cuff group (49.7 ± 18.5 min) was significantly longer than in the Hi-Contour group (27.8 ± 8.5 min). The precise mechanism of this difference is not known. In the Profile Soft-Seal Cuff (a cuff with increased N2O-gas barrier properties), an increase in intracuff pressure was less than that in the standard endotracheal tubes (the Lo-Contour; Mallinckrodt) (8,9). A cuff impervious to N2O might be the underlying mechanism of a longer T1/2 in the Profile Soft-Seal Cuff. However, we recently reported that the Profile Soft-Seal Cuff inhibits an increase in intracuff pressure through high compliance of the cuff rather than low diffusion of N2O (14). Therefore, these properties of the Profile Soft-Seal Cuff might make cuff pressure decrease more slowly than in other standard endotracheal tubes. Because the volume change and the concentration of N2O had no significant differences among the groups (Table 2), the high compliance of the Profile Soft-Seal Cuff, rather than limited efflux of N2O, may contribute, at least in part, to a longer T1/2 of the Profile Soft-Seal Cuff in this study.
During anesthesia, anesthetists might sometimes decrease the concentration of N2O to avoid hypoxemia. After anesthesia with N2O, it is also necessary to administer pure oxygen to patients with tracheal intubation when emergence from anesthesia is prolonged or when patients are transported to another unit for respiratory care management or radiological examinations. Our results demonstrate that the pressure of N2O-filled cuffs will quickly decrease after oxygen is substituted for N2O inspired. However, cuffs filled with air will equilibrate with the N2O containing inspired gas during the course of the anesthesia, resulting in an increase in cuff volume and pressure. Because N2O concentration in the air-filled cuff becomes about 27% one hour after the start of anesthesia with N2O (4), we speculate that this deflationary phenomenon would occur in the air-filled cuff if the cuff is aspirated to avoid excessive cuff pressure during anesthesia. Furthermore, much higher cuff pressure is needed to avoid aspiration because the lateral wall pressure should exceed the summation of hydrostatic pressure that can be generated by a column of liquid above the cuff and inspiratory pressure that is generated in a negative direction by the patients’ breathing (15).
In conclusion, our results have confirmed that the N2O-filled cuff has limitations because intracuff pressure decreases so quickly after substituting oxygen for N2O, although stable cuff pressure can be achieved during anesthesia with N2O. Therefore, it is suggested that the cuff pressure be checked frequently to avoid air leaks and aspiration of gastric contents during prolonged emergence from anesthesia or during transportation of patients with tracheal intubation. Our data also suggest that the endotracheal tube with the N2O gas-barrier type of cuff might be beneficial because of the longer time required to decrease cuff pressure after cessation of N2O administration and that the highly compliant cuff might be the mechanism of slow changes of intracuff pressure in the Profile Soft-Seal Cuff.
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