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Abstract |
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We sought to determine the optimal concentration of nitrous oxide (N2O) for inflating endotracheal tube cuffs, to avoid overinflation and air leaks. Female patients undergoing endotracheal intubation (inner diameter 7.5 mm) during anesthesia with 67% N2O were randomly assigned to five groups of 25 subjects each, in which cuffs were inflated with 0% (Air), 30% (N30), 40% (N40), 50% (N50), or 67% (N67) N2O. The cuff pressure and the N2O concentration in the cuff were measured. In an additional 15 patients (N40-a group), pilot balloons were replaced with metal tubes, and the mouths and noses of the patients were wrapped with tape, to minimize N2O efflux into the air. Postoperative sore throats were evaluated in double-blinded interviews. Cuff pressures increased significantly in the Air and N30 groups but decreased in the N67 group. Cuff pressures were <22 mm Hg in the N40 and N50 groups, but the N50 group had air leaks. The N2O concentration in the cuff in the N40 group was significantly smaller than that in the N40-a group, suggesting N2O rediffusion. The incidence of sore throats (40% in the Air group) was reduced significantly in the N40 and N50 groups. Therefore, 40% N2O is optimal for filling the cuff during anesthesia with 67% N2O.
Implications: Nitrous oxide (N2O) diffuses into the cuff, equilibrating at a smaller concentration than the gas mixture with which patients are ventilated. Our data indicate that inflation of the cuff with 40% N2O is recommended to prevent both excessive endotracheal cuff pressure and air leaks during anesthesia with 67% N2O, reducing postoperative sore throats.
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Introduction |
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Sore throats are common (35%–60%) among surgical patients anesthetized by using endotracheal intubation (1–5). Morbidity associated with endotracheal intubation is attributable to factors such as tube size, lateral wall pressure, movement, hypotension, and the lubricant used. Cuff-related tracheal damage is influenced by both lateral wall pressure and the duration of intubation; of the two factors, pressure is much more important (6). Ideally, the pressure exerted against the tracheal wall by the cuff of an endotracheal tube should be low enough to allow adequate capillary mucous membrane blood flow and high enough to prevent air leaks and aspiration of regurgitated gastric contents. To avoid ischemic damage, the endotracheal tube cuff pressure should be maintained below the mean mucosal capillary perfusion pressure of 22 mm Hg (7). Despite satisfactory initial sealing with air, nitrous oxide (N2O) diffuses into the cuff during anesthesia, increasing the cuff volume and intracuff pressure (8). Automated devices to prevent gas diffusion into the cuff (2,9–13) and cuffs made of new materials with high gas-barrier properties (14) have been developed. Stanley and Liu (15) reported that the ideal procedure is to inflate the endotracheal cuff with the same gas mixture with which patients are to be ventilated. A preliminary study, however, showed that inflation of the cuff with 67% N2O decreased the cuff pressure and induced air leaks during anesthesia with 67% N2O, indicating that the N2O concentration for inflation of the cuff was too high to maintain an air seal. This report describes a randomized study of a range of gas mixtures to determine the optimal concentration of N2O for inflation of cuffs, to prevent excessive tracheal pressure as well as air leaks. In addition, the contribution of the content of the gas mixture to reductions in the incidence of postoperative sore throats was assessed in double-blinded interviews.
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Methods |
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Subjects were female patients (n = 140, 18–68 yr old, ASA physical status I or II) undergoing elective abdominal surgery. Patients who smoked or had symptoms of upper airway irritation were excluded from the study. The investigation was approved by the Ethics Committee of the National Defense Medical College. Informed consent was obtained from the patients, who were randomly allocated to five groups of 25 patients each, i.e., the Air and N67 groups, in which the cuff was inflated with air and 67% N2O, respectively, and the N30, N40, and N50 groups, in which the cuff was inflated with 30%, 40%, and 50% N2O, respectively. In the Air and N67 groups, the cuff was deflated every hour and reinflated with air or 67% N2O, respectively. For an additional 15 patients (the N40-a group), the plastic connection and the pilot balloon were replaced with a metal tube, to avoid diffusion out of the system, and the cuff was inflated with 40% N2O. Furthermore, the mouths and noses of all patients in the N40-a group were covered with adhesive tape, to minimize diffusion of the oropharyngeal gases into the air. The N2O gas mixture to inflate the cuffs was aspirated from the common gas outlet of an anesthesia machine. The N2O concentration of the gas mixture to inflate the cuffs was measured with a multigas monitor (Capnomac Ultima®; Datex, Helsinki, Finland) calibrated with standard gases.
Hydroxyzine and atropine sulfate were administered IM 1 h before surgery. Anesthesia was induced with fentanyl and thiopental while the patients breathed 100% oxygen. Vecuronium was administered to relax the muscles, and the trachea was intubated with a polyvinyl chloride tube (7.5-mm inner diameter; Ruschelit, Kernen, Germany) by a skilled anesthetist. Lubricant was not used. Immediately after intubation, the cuff was aspirated as much as possible and then inflated with the smallest volume of the aforementioned gases that would not leak when the intraairway plateau pressure was 20 cm H2O. Air leaks were detected by auscultation. The initial volume used to fill the cuff was recorded. A circle absorber breathing system was used, and anesthesia was maintained with 67% N2O/33% oxygen supplemented with isoflurane or sevoflurane. The lungs were ventilated mechanically. A humidifier (Thermovent® 600; Portex, Kent, UK) was used, but use of a nasogastric tube was avoided. All patients were supine during the surgical procedure. Airway and oropharyngeal suctioning was performed once before extubation by using a flexible suction catheter to remove any secretions.
The pilot balloon of the endotracheal tube or the metal tube (for the N40-a group) was connected to a pressure transducer through a three-way stopcock. The mean intracuff pressure was measured every 4 or 8 min using a monitor (AS3; Datex). If the intracuff pressure exceeded 40 mm Hg in the N30, N40, and N50 groups, the cuff was deflated until the pressure decreased to <22 mm Hg. Endotracheal tube cuffs were monitored for air leaks every 4 or 8 min during anesthesia in all groups. When a leak was noticed by auscultation during intermittent positive ventilation, the cuff was inflated with a minimal volume of the gas allocated to the group, to ensure cuff sealing. The volume and the N2O concentration of the aspirated gas from the cuff were measured with a calibrated syringe and a multigas monitor (Type 1302; Bruel & Kjaer, Naerum, Denmark), respectively. Measurements were performed 1 h after the start of anesthesia in the Air and N67 groups and at the end of anesthesia in the other groups.
Each patient was interviewed postoperatively by an anesthetist unaware of the group to which the patient was assigned. The patient graded her postoperative sore throat as follows: 0 = none (no sore throat at any time after the operation), 1 = mild (scratchy throat), 2 = moderate (similar to that noted with a cold), or 3 = severe (more severe than with a cold).
In vitro measurements of N2O concentrations were performed to evaluate dead space (Vdead) in the cuff and pilot balloon. Immediately after inflation of the cuff with 5 mL of 67% N2O (n = 10), the gas mixture in the cuff was aspirated, for calculation of Vdead with the following formula:
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The cuff resting volume (16), which was defined as the smallest cuff volume beyond which a 0.5-mL increase in volume resulted in a >10-mm Hg increase in cuff pressure, was measured in 10 endotracheal tubes used in the study.
Data were presented as number of patients or mean ± SD. Two-way analysis of variance for repeated measurements was used to assess changes with time within as well as between groups, and one-way analysis of variance was used to compare raw data between groups. Post hoc analysis to allow for multiple comparisons was performed by using a Bonferroni/Dunn correction. Student’s t-test was used to make single comparisons of cuff pressures, volumes, and concentrations of N2O. For evaluation of verbal rating scores for sore throats, the Friedman test with "relative to an identified distribution" analysis (17), allowing multiple comparisons, was performed. Proportional data were evaluated by using the 
2 test. A P value of <0.05 was considered statistically significant.
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Results |
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The six groups of patients were comparable with respect to age, height, weight, and duration of anesthesia (Table 1). In the Air group, the mean cuff pressure increased significantly, to 36.8 ± 12.6 mm Hg, during the first hour of anesthesia (P < 0.001) (Figure 1) and the volume of gas aspirated from the cuffs increased significantly (P < 0.001) (Table 2). In the second and third hours after reinflation of the cuffs, the intracuff pressure increased again, to 36.4 ± 15.0 and 30.1 ± 12.1 mm Hg, respectively (P < 0.001 for each). However, the mean cuff pressure in the N67 group decreased significantly, to 4.6 ± 1.6 mm Hg, during the first hour of anesthesia (P < 0.001, n = 21). In the second and third hours after reinflation of the cuffs, the cuff pressure decreased again, to 5.2 ± 2.6 and 4.3 ± 2.1 mm Hg, respectively (P < 0.001 for each). There were four cases in the N67 group that had air leaks within the first hour and six cases that had air leaks during the course of anesthesia. The cuff volume was significantly decreased in the N67 group (P < 0.001). The changes in cuff pressure during the first hour of anesthesia for the Air, N30, N40, N50, and N67 groups were dependent on the N2O concentrations in the gas mixtures used to inflate the cuffs (P < 0.001).
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) and N67 (
) groups during anesthesia with 67% nitrous oxide (N2O). In these groups, the cuff was deflated every hour and reinflated with air or 67% N2O, respectively. Twenty-five patients were assigned to each group. The number of patients included in the statistical analysis is indicated for each point, because data were discarded after air leaks were identified or when anesthesia was finished. The intracuff pressure was significantly increased in the Air group and decreased in the N67 group after inflation with the allocated gas (P < 0.001 for each).
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The intracuff pressure in the N30 group was significantly higher than the pressures in the N40 and N50 groups (P < 0.001 for each) (Figure 2). The cuff pressure in the N30 group increased to >22 mm Hg during anesthesia in 13 cases (Table 3). However, the cuff pressures in the N40 and N50 groups were <22 mm Hg during anesthesia. The cuff pressure in the N40 group slightly but significantly increased during anesthesia (P < 0.001) and was significantly higher than that in the N50 group (P < 0.001). There were two cases in the N50 group with air leaks during anesthesia. At the end of anesthesia, the volume of gas aspirated from the cuff increased in the N30 and N40 groups (P < 0.001 and P < 0.01, respectively) but not in the N50 group.
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), and N50 (
) groups during anesthesia with 67% N2O. Twenty-five patients were assigned to each group. The number of patients included in the statistical analysis is indicated for each point, because data were discarded after air leaks or overinflation (>40 mm Hg) was identified or when anesthesia was finished. The intracuff pressure significantly increased in the N30 and N40 groups during anesthesia (P < 0.001 for each) and was significantly higher than that in the N50 group (P < 0.001 for each).
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In the N40-a group, the cuff pressure (7.9 ± 1.7 mm Hg, n = 15) increased significantly 60 min (15.3 ± 4.5 mm Hg, P < 0.01, n = 15) and 120 min (16.0 ± 3.4 mm Hg, P < 0.01, n = 9) after the start of anesthesia. These pressure values in the N40-a group were significantly higher than those in the N40 group at the corresponding times (P < 0.01 for each). The volume aspirated from the cuff at the end of anesthesia was significantly increased, to 5.1 ± 1.1 mL, in the N40-a group (P < 0.001); this value was significantly higher than that in the N40 group (P < 0.01). The N2O concentration of aspirated gas (53.4 ± 5.1%) in the N40-a group was significantly higher than that in the N40 group (P < 0.001).
There was a significant difference among groups with respect to sore throats (P < 0.001) (Figure 3). The incidences in the N40 and N50 groups were significantly smaller than those in the Air group on Postoperative Days 1 and 2 (P < 0.05 for each). The patients with sore throats improved with each passing day, and there was no claim of sore throat on Postoperative Day 5.
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), or mild (
). There were significant differences between groups (P < 0.001 by the Friedman test), and significant differences, compared with the Air group on the corresponding postoperative day, were noted (*P < 0.05 by "relative to an identified distribution" analysis).Immediately after inflation of the cuff with 5 mL of 67% N2O, the N2O concentration in the cuff was 60.2% ± 1.1%. The dead space of the cuff and pilot balloon, calculated by using Equation 1, was 0.57 ± 0.11 mL. The N2O concentrations, corrected using Equation 2, are shown as the N2O concentrations of the injected gas in Tables 2 and 3. Compared with the N2O concentrations measured during or at the end of anesthesia, these values were significantly different in all groups (P < 0.001 for each) (Tables 2 and 3), except the N50 group (P = 0.14) (Table 3). The resting cuff volume was 9.8 ± 0.3 mL, which was larger than the volume injected or aspirated during anesthesia (Tables 2 and 3).
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Discussion |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Our results indicate that inflation of the cuff of an endotracheal tube with 40% or 50% N2O, rather than 67% N2O, during anesthesia with 67% N2O maintains a stable intracuff pressure (Figures 1 and 2). A N2O concentration of 67% to fill the cuff was too high to maintain stable cuff pressure, because the N2O concentration of the injected gas decreased in the first hour (Table 2).
Our results are inconsistent with those of Stanley and Liu (15), who recommended inflating cuffs with the gas mixtures with which patients are ventilated. The reason for the discrepancy is not clear, because there are many factors that might influence the rate of diffusion. The rates of diffusion of N2O through the material of the cuff should have been similar, because those authors also used endotracheal tubes with polyvinyl cuffs, but the studies differed in that the initial cuff pressure those authors used (53 ± 15 mm Hg) was considerably higher than ours (8.8 ± 1.5 mm Hg). Reader et al. (18) used low-volume, low-pressure cuffs and 67% N2O to fill cuffs in their study, following a protocol very similar to ours, and also achieved stable cuff pressure during anesthesia with 67% N2O. Reader et al. (18) did not measure concentrations of N2O in the intracuff gases.
In this study, gas mixtures with five different N2O concentrations were used to inflate the cuffs. Few studies have investigated the effects on intracuff pressure of using various concentrations of N2O to inflate cuffs. The cuff pressure in the Air and N30 groups became too high during anesthesia with 67% N2O (Tables 2 and 3). Although the mean cuff pressure in the N50 group was stable (Figure 2), 50% N2O was slightly too high a concentration to maintain the intracuff pressure, because air leaks were noted for some patients in the N50 group (Table 3). The N67 group could not avoid decreases in the intracuff pressure. Therefore, our results indicate that inflation of the cuff with 40% N2O is optimal to avoid significant increases in intracuff pressure and air leaks, resulting in less severe postoperative sore throats (Figure 3).
In the N40-a group, we investigated the underlying mechanism of the equilibrium with a smaller concentration of N2O than that with which the patients were being ventilated. During anesthesia with 67% N2O, the intracuff pressure, cuff volume, and N2O concentration of aspirated gas for the N40-a group increased more than those for the N40 group, indicating that there was rediffusion of N2O into the air through the pilot balloon or oropharyngeal cavity in the N40 group. These findings are consistent with those of Fill et al. (19), who reported that N2O can be found near the pilot balloon of a conventional endotracheal tube. A large pilot balloon, as proposed by Brandt and Pokar (12), might rediffuse more N2O than standard-sized pilot balloons, and this may be why air can be used to inflate the cuffs of the rediffusion system, to maintain stable cuff pressure.
The initial N2O concentration in the cuff, which is a key factor influencing intracuff volume and pressure during anesthesia, was corrected by using Equation 2, resulting in a decrease to approximately 88% (Tables 2 and 3). Consequently, the N2O concentration in the cuff increased slightly but significantly in the N40 group and did not change significantly in the N50 group (Table 3), consistent with changes in the intracuff pressure in the N40 and N50 groups (Figure 2). Furthermore, a decrease in the initial concentration of N2O might, in part, account for the inconsistency of our results with those reported by Stanley and Liu (15), because the final cuff N2O concentration (55.3 ± 3.9%, Group 3 in Table 2) in their study was very close to 88% of the injected gas (60% N2O).
It has been reported that tracheal lateral wall pressure is equal to the intracuff pressure when high-volume cuffs are used (20). We measured the mean intracuff pressure to determine tracheal lateral wall pressure, because the cuff volume injected or aspirated (from 3.3 to 7.6 mL) was less than the resting cuff volume (9.8 ± 0.3 mL). Lubricant was not used, because it has been shown to increase the incidence of postoperative sore throats and hoarseness (21,22). When the cuff was inflated with 40% N2O, the incidence of sore throats was reduced to that of the Air group. Repeated deflation in the Air and N67 groups might affect the incidence of sore throats, but the incidence in the N67 group was significantly smaller than that in the Air group and was very similar to those in the N40 and N50 groups. The incidence of sore throats was 48% in our control group (Air group), consistent with previous reports (1–5). Therefore, it is likely that repeated inflation and deflation had little effect on the incidence of sore throats.
In a comparison of numerous methods to prevent postoperative complications of endotracheal intubation (1–3,9–13,21), inflation of the cuff with 40% N2O is one of the most effective techniques. Furthermore, the method involves no additional cost, because the N2O mixture is available from the common gas outlet of anesthesia machines. Therefore, this technique is recommended for all patients subjected to tracheal intubation and anesthesia with 67% N2O. Although the present method is effective, it is limited to N2O diffusion-induced increases in cuff pressure during anesthesia. It is also prudent to maintain a higher cuff pressure to prevent aspiration, because the lateral wall pressure should exceed the summation of the hydrostatic pressure that can be generated by a column of liquid above the cuff and the inspiratory pressure that is generated in a negative direction by patient breathing (23).
In conclusion, these data indicate that inflating an endotracheal tube cuff with the same gas mixture with which the patient is being ventilated is an unreliable method for maintaining stable cuff pressure. However, inflating the cuff of the endotracheal tube with 40% N2O is optimal for preserving cuff pressure during anesthesia with 67% N2O and is effective in reducing the incidence of postoperative sore throats. The basis of this recommendation is that N2O efflux from the cuff into the oropharyngeal cavity and from the pilot balloon and tubing into the air results in equilibration at a smaller concentration than the gas mixture with which the patient is being ventilated.
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Acknowledgments |
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The authors thank Dr. D. P. Crankshaw, Anesthesia Research and Education Unit, Department of Pharmacology, University of Melbourne, Australia, for his assistance in editing this manuscript, and all members of the Department of Anesthesiology, National Defense Medical College, for their help and cooperation during this study.
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References |
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