Time-Related Cuff Pressures of the Laryngeal Tube With and Without the Use of Nitrous Oxide

Abstract

The Laryngeal tube (VBM Medizintechnik, Sulz, Germany), a new supraglottic airway, consists of an airway tube, two cuffs, and two distal apertures between the two cuffs. One concern with the use of this device is ischemic change to the oropharyngeal mucosa. We studied the time-course change of the intracuff pressure (which reflects the pharyngeal pressure) of the laryngeal tube during anesthesia with and without nitrous oxide. After insertion of a laryngeal tube, 24 patients were randomly allocated to 1 of 2 groups. In one group (group N or nitrous oxide group), 66% nitrous oxide was used, whereas in the other group nitrous oxide was not used (group A or air group). In both groups, sevoflurane was used to maintain anesthesia. Time-course changes of the intracuff pressure and postoperative airway complications were recorded. In group N, the intracuff pressure significantly increased over time (P <0.001; the maximal pressure: 120 cm H₂O), whereas in group A the intracuff pressure remained stable. The intracuff pressure was significantly higher in group N than in group A (P <0.0001; 95% confidence intervals for difference: 6–20 cm H₂O at 30 min). Post-operatively, two patients in group A and one patient in group N complained of mild sore throat.

In Brief

IMPLICATIONS: Nitrous oxide may increase pharyngeal pressure by the cuffs of the laryngeal tube, and thus it is advisable to monitor and adjust the intracuff pressure of the laryngeal tube during anesthesia to minimize possible ischemic changes to the oropharynx.

The laryngeal tubes were a gift from the manufacturer; no other financial support was obtained.

The Laryngeal tube (VBM Medizintechnik, Sulz, Germany) has been developed to secure a patent airway during either spontaneous breathing or controlled ventilation. It consists of an airway tube with a small balloon cuff attached at the tip (distal cuff) and a larger asymmetric balloon cuff at the middle part of the tube (proximal cuff) (Fig. 1). The cuffs are inflated through a single pilot tube and balloon, through which cuff pressure can be monitored. There are two distal apertures in the tube between the two cuffs through which gas movement may take place. When the device is inserted, it lies along the length of the tongue and the distal tip is positioned in the hypopharynx. The proximal cuff provides a seal in the upper pharynx and the distal cuff seals the esophageal inlet. Several studies have shown that the laryngeal tube has a potential role during anesthesia (^1–5).

Figure 1.: The Laryngeal tube (VBM Medizintechnik, Sulz, Germany), a new supraglottic airway, consists of an airway tube with a small balloon cuff attached at the tip (distal cuff) and a larger asymmetric balloon cuff at the middle part of the tube (proximal cuff). There are two distal apertures in the tube between the two cuffs through which gas movement may take place.

One theoretical concern with the use of the laryngeal tube is ischemic change to the oropharyngeal mucosa, caused by the high pressure that may be exerted by its cuffs (⁶). Keller et al. (⁷) and Brimacombe et al. (^8,9) have studied the relationship between the intracuff pressure and the pressure exerted by the cuff on the pharynx for several different airway devices. They have found that for the laryngeal mask airway there was a poor relationship between the intracuff pressure and pressure exerted on the pharynx (⁷), whereas for the laryngeal tube the pressure exerted on the pharynx increased as the intracuff pressure increased (⁸). Therefore, unlike for the laryngeal mask airway, the intracuff pressure change of the laryngeal tube can be a good indicator for the pressure exerted on the pharynx. There have been no formal reports comparing the time-course change of the intracuff pressure during anesthesia with and without nitrous oxide. Therefore, the main aim of this study was to examine these.

Methods

After obtaining approval from the local Research Ethics Committee and written informed consent from patients, we studied 24 patients (ASA physical status I or II, aged between 18 and 75 yr) undergoing elective surgery (expected to take 30 min to 2 h) for whom the laryngeal tube was indicated. Patients were excluded if they had any pathology of the neck, upper respiratory or alimentary tracts, or were at risk of pulmonary aspiration of gastric contents.

Preoperatively, the view of the oropharynx was classified according to Mallampati et al. (¹⁰) and Samsoon and Young (¹¹). If the faucial pillars, soft palate, and uvula could not be seen (score 4), the patient was excluded from the study. The usual monitors were used. A firm pad (7 cm in height) was placed under the patient's occiput. After the patient breathed oxygen through a face mask for a minimum of 3 min, anesthesia was induced with a sleep dose of propofol 2.0–3.0 mg/kg IV, supplemented with fentanyl 2 μ g/kg. Neuromuscular blockade was produced by vecuronium and was confirmed using a peripheral nerve stimulator. Anesthesia was maintained with sevoflurane in oxygen (without nitrous oxide) until random allocation of the patients was made. If ventilation via a face mask was judged inadequate, the patient was withdrawn from the study.

A size 4 was used when the patient's height was ≥155 cm, whereas a size 3 was used when the height was <155 cm (¹). Before insertion, cuffs were deflated and a water-soluble lubricant (KY jelly) was applied to the cuffs. The patient's head was extended on the neck (“sniffing position”). The tip of the laryngeal tube was placed against the hard palate behind the upper incisors and the device was slid down in the center of the mouth until a resistance was felt or the second bold black line on the tube had just passed between the upper and lower teeth. The cuff inflator manometer (VBM) was connected to the pilot bulb of the laryngeal tube via a three-way stopcock, and the cuffs were inflated until the intracuff pressure reached 60 cm H₂O (¹). The accuracy of the manometer was confirmed using an arterial line and manometer.

After insertion of the laryngeal tube, we connected the breathing system and assessed adequacy of ventilation by gently squeezing the reservoir bag, observing the presence of end-tidal carbon dioxide waveforms and chest movement. If it was not possible to ventilate the lungs, the position of the device was adjusted by gently pushing or pulling the device. Adequacy of ventilation was reassessed. If it was not possible to insert the device or ventilate through it, one more attempt at insertion of the device was allowed. If insertion had failed after 2 attempts or if gas leaked around the device during manual ventilation with the maximal airway pressure of <16 cm H₂O, the study was abandoned and the airway was maintained either through a face mask or through a tracheal tube. If ventilation was adequate (defined as ventilation without air leak around the device at the peak airway pressure of 16 cm H₂O), a bite block was inserted, the laryngeal tube snagged into its wedge, and both were fixed to the patient using a sticky tape.

Patients were then randomly allocated to one of two groups. The allocation was made by block randomization (blocks of 10). In one group (group N or nitrous oxide group), 66% nitrous oxide was used, whereas in the other group (group A or air group) nitrous oxide was not used. In both groups, sevoflurane was used to maintain anesthesia and inspiratory concentration of oxygen was adjusted to approximately 30%; analgesics were given in accordance with the anesthesiologist's preference.

The intracuff pressure was measured every 10 min during the entire course of anesthesia. The pressure was measured using the cuff inflator manometer, by adjusting the direction of the three-way stopcock toward the manometer. If ischemic change to the tongue or lips was noticed, the anesthesiologist was allowed to adjust the intracuff pressure of the laryngeal tube, or replace it by a face mask, the laryngeal mask airway, or a tracheal tube. If surgery was prolonged >2 h, the anesthesiologist was also allowed either to adjust the intracuff pressure of the laryngeal tube to 60 cm H₂O, or to replace the laryngeal tube with a face mask, the laryngeal tube, or a tracheal tube. If the use of the laryngeal tube was abandoned or the intracuff pressure was adjusted, the data collection was terminated at that point.

At the end of the operation, neuromuscular blockade was antagonized with neostigmine (and atropine was also injected to prevent bradycardia). Administration of anesthetic drugs was then stopped while the device was left in place. The device was removed after the patient had regained consciousness spontaneously and had responded to verbal command to open the mouth. The cuffs of the laryngeal tube were deflated before removal. The patient was asked before return to the ward whether or not there were the following complains: sore throat (constant pain, independent of swallowing), dysphagia (difficulty in, or pain provoked by, swallowing), sore jaw, dysphonia (difficulty in, or pain on, speaking), numbness of the tongue or the oropharynx. The presence or absence of swelling of or ischemic change to the oropharynx was determined.

Our main interest was to compare the intracuff pressures between the groups, and thus the number of patients required was calculated based on this factor. Available data were the increase in the intracuff pressures 30 min after the start of anesthesia using nitrous oxide: 15 (standard deviation [SD]: 4) cm H₂O in one study (¹²), and 12 (SD: 3) cm H₂O in another study (¹³). We considered that the difference would be clinically important if the increase in the intracuff pressure in group A was ≤50% compared with group N. Six patients would be required to detect this difference with a power of 90%. Nevertheless, we decided to study 12 patients for each group to obtain reasonable variability ranges in the intracuff pressure.

Normal plots (plots of normal scores) showed that the intracuff pressure at 30 min was normally distributed in each group, but the F test showed that the variability of the two groups was not similar. Therefore, the Mann-Whitney U-test was used to compare the pressures between groups. For additional information, we examined whether or not the intracuff pressure had changed over time for each group. The two-way analysis of variance was used for the comparison, because the residuals of the data were normally distributed for each group. Only the data from 0 to 30 min were used, because no data at 40 min or afterward were available in some patients. The 95% confidence intervals for the median difference in the intracuff pressures between the 2 groups were also calculated.

Results

Characteristics of the patients and anesthesia time were generally similar between the groups (Table 1).

Table 1: Patients' Age, Height, Weight, Body Mass Index, and Anesthesia Time

The laryngeal tube was successfully placed and adequate ventilation obtained in 23 of 24 patients at the first attempt and in the remaining 1 patient at the second attempt. Anesthesiologists did not judge it necessary to replace the laryngeal tube with a face mask, the laryngeal mask airway, or a tracheal tube in any patient. In addition, in no patient was there any apparent ischemic change to the tongue or the lips while adjusting the intracuff pressure of the laryngeal tube during anesthesia.

When nitrous oxide was used (group N), the intracuff pressure always increased over time and the increase was significant (P < 0.001) (Table 2). The maximal intracuff pressure recorded within 2 h was 120 cm H₂O. In contrast, when nitrous oxide was not used to maintain anesthesia (group A), the intracuff pressure remained stable. The intracuff pressure was significantly higher in group N than in group A (P < 0.0001; median difference: 9 cm H₂O; 95% confidence interval: 6–20 cm H₂O at 30 min) (Table 2).

Table 2: Time Course Changes of the Intracuff Pressure of the Laryngeal Tube, With (Group N) and Without (Group A) the Use of Nitrous Oxide

Postoperatively, two patients in group A and one patient in group N complained of mild sore throat. No patients complained of dysphagia, sore jaw, dysphonia, or numbness of the tongue or the oropharynx. Swelling of or ischemic change to the oropharynx was not detected in any patient.

Discussion

We have shown that, when nitrous oxide was used to maintain anesthesia, the intracuff pressure of the laryngeal tube progressively increased over time, whereas when nitrous oxide was not used, the intracuff pressure did not change significantly.

There have been two reports regarding the pressure exerted on the pharyngeal tissues by the cuffs of the laryngeal tube. In one report, Asai and Kawachi (¹⁴) studied the exerted pressure by calculating the difference between intracuff pressures measured with the device in place in the patient (P in vivo: adjusted to be 60 cm H₂O) and held in air (P ex vivo), with the cuffs inflated with the same volume of gas (P = P in vivo − P ex vivo). The exerted pressure was 29 (range, 24–36) cm H₂O (¹⁴). Brimacombe et al. (⁸) directly measured the pressure exerted on the pharynx (using gauge microchip sensors) in cadavers and in three awake volunteers, and found that as the intracuff volume was increased the exerted pressure also increased. They inflated the cuffs to certain cuff volumes, and provided the intracuff pressures only in volunteers. In their study of 3 volunteers, when the mean intracuff pressure was 70 (range, 55–93) cm H₂O, the pressure exerted on the posterior pharynx was 37 (26–60) cm H₂O. This value is in a similar range to the calculated exerted pressure in the former study at the intracuff pressure of 60 cm H₂O (¹⁴). When the intracuff pressure was 124 (97–158) cm H₂O, the exerted pressure was 46 (22–78) cm H₂O (⁸). Using the cuffed oropharyngeal airway and a fiberoptic bronchoscope, Brimacombe et al. (⁹) found that blood vessels in the pharyngeal mucosa started to be compressed when the exerted pressure on the pharynx exceeded 34 cm H₂O, and collapsed when the exerted pressure reached 73 cm H₂O.

In our study, when nitrous oxide was not used to maintain anesthesia, the intracuff pressure remained approximately 60 cm H₂O in all the patients. When the above reported results are applied, the exerted pressure would be approximately 30–35 cm H₂O, indicating that the perfusion of the pharynx would not be markedly reduced. In contrast, when nitrous oxide was used, the intracuff pressure increased up to 120 cm H₂O during 2 hours of anesthesia, indicating that the pressure on the pharynx may be high enough to compress, if not collapse, the blood vessels in the pharynx. Therefore, there is a theoretical risk of ischemic change to the pharyngeal tissues.

There have been several reports of the use of the laryngeal tube and the presence or absence of postoperative airway complications (^1–5). The reported incidence ranges from 0% to 34%, which includes the incidence in our study (13%). This range of the incidence is also similar to that after the use of the classic laryngeal mask airway (³) or the ProSeal laryngeal mask airway (^5,15). Nevertheless, in one report (³), in 2 of 36 patients in whom the laryngeal tube was used, apparent ischemic change to the tongue was observed, and reduction of the intracuff pressure relieved the congestion.

In conclusion, it is advisable to monitor and adjust the intracuff pressure of the laryngeal tube during anesthesia to minimize possible ischemic changes to the oropharynx.