The volume and pressure of standard air-filled endotracheal tube cuffs gradually increase during anaesthesia with nitrous oxide (N2O) because of diffusion of N2O into the cuff; N2O in cuffs might finally approach an equilibrating concentration of 40–50% during 67% N2O anaesthesia [1–4]. The mechanism underlying an equilibrating concentration that is lower than the gas mixture with which patients' lungs are ventilated is likely to be N2O rediffusion . In fact, filling the cuffs with 40% N2O can preserve stable cuff pressures without excessive cuff pressure or air leaks during 67% N2O anaesthesia [3,4]. In clinical practice, aspiration of standard endotracheal tube cuffs is used to prevent the development of excessive cuff pressure during N2O anaesthesia. When cuffs are repeatedly aspirated to reduce an increase in pressure, an equilibrating concentration of N2O might be achieved. However, there are few reports in which the technique of repeated cuff aspiration has been assessed. The purpose of this randomized study was to assess the time required to achieve a stable cuff pressure after repeated cuff deflation to inhibit the developments of excessive pressure during N2O anaesthesia, and to investigate changes in N2O concentration in the cuff.
Subjects were 44 patients ASA I-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 were administered intramuscularly 1 h preoperatively. Anaesthesia was induced with fentanyl (0.1–0.2 mg) and propofol (2–2.5 mg kg−1) after the patients started breathing 100% oxygen. Vecuronium (0.1 mg kg−1) was administered to relax the muscles and the trachea was intubated with a Hi-Contour© endotracheal tube (Mallinckrodt, Athlone, Ireland) – 7.5 mm i.d. for females, 8.0 mm i.d. for males; no lubricant was used. The pilot balloon of the endotracheal tube was connected to a pressure transducer through a threeway stopcock to monitor cuff pressure (AS3®; Datex, Helsinki, Finland). Immediately after tracheal intubation, the cuff was aspirated as much as possible and then inflated with the lowest volume of air that would seal the airway when the intra-airway plateau pressure was 18 cmH2O. The initial volume of gas to fill the cuff was recorded. The mean intracuff pressure was measured every 10 min in all patients. A circle absorber breathing system was used and anaesthesia maintained with 67% N2O in 33% oxygen, supplemented with isoflurane or sevoflurane. The lungs were ventilated mechanically. The extent of paralysis was constant throughout the study. All patients were supine during the operative procedure. Airway and oropharyngeal suctioning was performed once before tracheal extubation using a flexible suction catheter to remove any secretions.
Assessing cuff pressure after repeated aspiration
Using a random shuffle, patients whose surgery was expected to last at least 7 h were allocated to two groups; gases in the cuff were aspirated to decrease the cuff pressure to the initial pressure every 30 min for the first 3 or 4 h (Groups Def-3 or Def-4, respectively); cuff pressure was monitored for an additional 3 h without further aspiration of the cuff. At the end of each study, the gases in the pilot balloons and cuffs were aspirated as completely as possible. Approximately 15 min later, the volume of aspirated gases was measured at room temperature with a calibrated syringe both after the gases were compressed and decompressed, and the mean was taken to avoid a possible error due to friction between the barrel and piston of the syringe. N2O, N2, O2 and CO2 concentrations were measured by quadrapole mass-spectrometry (AMIS 2000®; INNVISION A/S, Odense, Denmark), which was calibrated with standard gases.
Changes in gas concentration during repeated aspiration
In addition, some patients whose surgery was expected to last at least 4 h were randomly allocated to four groups (n = 6 for each); gases in the cuff were aspirated to decrease the pressure to the initial value every 30 min for the first 1, 2, 3 or 4 h; in each group of time elapsed, the gases were aspirated as completely as possible to measure the volume and the gas composition, and the study was terminated.
Data are the number of patients or mean ± SD. Twoway ANOVA for repeated measurements assessed changes over time within as well as between groups and one-way ANOVA compared raw data between groups. Post hoc analysis to allow for multiple comparisons was performed using a Bonferroni-Dunn correction. A t-test made single comparisons of cuff pressure, volume and concentration of N2O. Proportional data were evaluated using the x2-test. P < 0.05 was considered as significant.
Groups Def-3 and Def-4 were comparable in gender, age, weight and height (Table 1). The additional four groups of time elapsed were also comparable in gender, age, weight and height (data not shown). Initial cuff pressures did not differ significantly among the six groups (P < 0.535, data not shown). Figures 1 and 2 show the mean cuff pressure in Groups Def-3 and Def-4, respectively. Cuff pressure in the two groups did not differ significantly until 210 min (P < 0.181). Peak cuff pressure during aspiration intervals significantly decreased time-dependently in Group Def-3 (from 27 ± 6 to 20 ± 3 mmHg, P < 0.0001) and Group Def-4 (from 30 ± 6 to 17 ± 2 mmHg, P < 0.0001). After stopping aspiration, cuff pressure in Group Def-3 was higher than that in the Def-4 group (P < 0.0001); there were significantly more patients in Def-3 group (n = 9, P < 0.0001) whose cuff pressure >22 mmHg after stopping aspiration compared with those in Group Def-4 (n = 0).
The volume of gases aspirated from cuffs increased in all groups of time elapsed (P < 0.05 for each) (Table 2). N2O and CO2 concentrations increased in a time-dependent manner (P < 0.0001) (Table 2), whereas N2 and 02 concentrations decreased in a timedependent manner (P < 0.0001). The N2O concentration at 3 h (39.8 ± 4.7%) increased significantly in Group Def-3 during additional 3 h (44.3 ± 3.8%; P < 0.05) (Fig. 3). In contrast, the N2O concentration at 4 h (43.7 ± 4.5%) was very close to that in Group Def-4 at the end of the study (42.3 ± 4.8%; P = 0.579).
Repeated cuff aspiration every 30 min for 4 h stabilized cuff pressure of the Hi-Contour® standard endotracheal tube. During the additional 3 h, cuff pressure was never >22 mmHg, which is the mean capillary perfusion pressure of the tracheal mucosa and is the recommended pressure limit for avoiding ischaemic tracheal mucosal damage [3–5]. In contrast, repeated deflation for only 3 h did not achieve a stable cuff pressure; there was excessive cuff pressure in 90% of patients during the additional 3 h after ceasing cuff aspiration. In the present study, the N2O concentration in the cuffs increased according to the decrease in peak cuff pressure, approaching >40%. To our knowledge, there are few studies in which gas concentrations in the cuff were measured when cuffs were periodically aspirated. Stanley and Liu  reported that the ideal procedure was to inflate the endotracheal cuff with the same gas mixture with which patients' lungs were to be ventilated, which was supported by Raeder and colleagues . The N2O concentration in the cuff was not measured in their studies. In our previous study [3,4], the equilibrating concentration of N2O was 40–50% during anaesthesia with 67% N2O, which indicates that the equilibrating concentration of N2O is lower than the gas mixture with which the patients' lungs were being ventilated. The basis of this lower equilibrating concentration of N2O is that N2O diffuses from the cuff into the oropharyngeal cavity and from the pilot balloon and tubing into the air; there is rediffusion in the standard endotracheal tubes. The present study therefore demonstrates that an equilibrating concentration of N2O can be achieved by repeated cuff deflation; cuff pressure is stabilized thereafter without cuff aspiration. Furthermore, it takes 4 h to achieve an equilibrating concentration of N2O when cuffs are aspirated every 30 min.
CO2 concentrations in the cuff increased significantly, probably approaching the expiratory concentrations. However, CO2 concentrations were one-tenth of those for N2O; therefore, changes in CO2 have little effect on cuff pressure. On the other hand, N2 and O2 concentrations in the cuff decreased significantly during anaesthesia. Because the increase in cuff volume owing to N2O diffusion was >50%, concentrations of other gases in the cuff might be diluted by N2O. The changes in the N2 and O2 volumes might be small and it is likely that changes in these gases have little effect on cuff pressure although changes in the volumes of the corresponding gases were not estimated in the present study.
Sore throat is common in surgical patients following anaesthesia with endotracheal intubation [3,8–12]. A major factor for minor complications, such as sore throat owing to tracheal intubation, is excessive pressure on the tracheal wall. Numerous devices have been developed to inhibit excessive cuff pressure and are reported to be effective for controlling cuff pressure during anaesthesia [3,4,13–19]. In contrast to these other devices, the method of repeated deflation can be performed in clinical practice without any extra cost. Although we did not assess postoperative sore throat in the present study, the incidence rate of sore throat (40–60%) can be reduced to approximately half by controlling cuff pressure during anaesthesia . Therefore, use of the repeated deflation method is recommended to reduce sore throat if no other method to inhibit excessive cuff pressure is used. In this method, cuffs must be aspirated eight times before a stable cuff pressure is achieved. In clinical practice, therefore, it might be a little troublesome because of the number of cuff deflations required.
In conclusion, a method of repeated deflation of an air-filled cuff of a standard endotracheal tube (Hi-Contour®) every 30 min for 4 h, but not for only 3 h, during N2O anaesthesia can achieve an equilibrating concentration of N2O in the cuff. This provides subsequent stable cuff pressure without further deflation of the cuff.
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