Anesth Analg 2006;102:319-321
© 2006 International Anesthesia Research Society
doi: 10.1213/01.ANE.0000180768.81020.C4
GENERAL ARTICLES
Middle Ear Pressure Changes During Anesthesia With or Without Nitrous Oxide are Similar Among Airway Devices
Mathias Hohlrieder, MD*,
Christian Keller, MD, MSc*,
Joseph Brimacombe, MB, ChB, FRCA, MD ,
Stephan Eschertzhuber, MD*,
Günter Luckner, MD*,
Irene Abraham, MD , and
Achim von Goedecke, MD*
*Department of Anaesthesia and Intensive Care Medicine, Leopold-Franzens University, Innsbruck, Austria; James Cook University, Cairns Base Hospital, Australia; Department of Ear, Nose and Throat Surgery, Leopold-Franzens University, Innsbruck, Austria
Address correspondence to Prof. J. Brimacombe, Department of Anaesthesia and Intensive Care, Cairns Base Hospital, The Esplanade, Cairns 4870, Australia. Address e-mail to jbrimaco{at}bigpond.net.au.
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Abstract
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We tested the hypothesis that middle ear pressure (MEP) is influenced by the choice of airway device during anesthesia with or without nitrous oxide (N2O) in the gas mixture. Eighty consecutive anesthetized, paralyzed ventilated patients (ASA physical status I–II, 18–65 yr) were randomly allocated for airway management with the orally inserted tracheal tube, classic laryngeal mask airway, ProSeal laryngeal mask airway, or laryngeal tube suction with or without N2O 66% in the gas mixture. MEP was measured from both ears in random order by a blinded observer before induction of anesthesia and every 10 min for 70 min. In the N2O groups, N2O was changed to air after 40 min. There were no differences in MEP among the airway devices in the N2O or air groups. MEP was unchanged in the air groups but increased in the N2O groups with N2O (P < 0.0001) and decreased with air (P < 0.02). Baseline values for MEP were similar, but MEP was always higher for the N2O groups (P < 0.001). We conclude that the choice of airway device does not influence MEP among orally inserted tracheal tube, classic laryngeal mask airway, ProSeal laryngeal mask airway, and laryngeal tube suction during anesthesia with or without N2O in the gas mixture.
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Introduction
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Middle ear pressure (MEP) changes during nitrous oxide (N2O) anesthesia can impede hearing (1,2), influence surgical outcome (3,4), increase postoperative nausea and vomiting (PONV) (5), and occasionally cause tympanic rupture (6) or facial nerve injury (7). The main factors influencing MEP during N2O anesthesia are duration of anesthesia, the concentration of N2O, and the patency of the Eustachian tube, which functions like a relief valve. There is evidence that the choice of airway device influences MEP, perhaps by affecting the patency of the Eustachian tube. Satoh et al.1 found that MEP takes longer to increase during administration of N2O and longer to decrease after cessation for the laryngeal mask airway (LMA) than the tracheal tube (TT), and Nader et al. (5) noted that MEP was higher with the LMA than the TT during N2O anesthesia. In the following randomized, prospective, single-blind study, we tested the hypothesis that MEP is influenced by the choice of airway device during anesthesia among the orally inserted TT, classic LMA, ProSeal laryngeal mask airway (PLMA) (8), and laryngeal tube suction (LTS) (9) with and without N2O in the gas mixture.
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Methods
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Eighty consecutive patients (ASA physical status I–II, 18–65 yr) undergoing routine general and peripheral musculoskeletal surgery in the supine position were studied in a randomized fashion. Ethical committee approval and written, informed patient consent were obtained. Patients were excluded from the trial if they had a known or predicted difficult airway, oropharyngeal and/or ear pathology, mouth opening <2.0 cm, a body mass index >30 kg/m2, or were at risk of aspiration. The patients were randomly allocated (by opening a sealed opaque envelope) into 8 equal-sized groups: TT, LMA, PLMA, and LTS, each used with N2O or air
A standard anesthesia protocol was followed and routine monitoring was applied. All anesthetics were induced with IV fentanyl 2 µg/kg and propofol 2.5 mg/kg. Maintenance was with IV propofol 6–10 mg · kg–1 · h–1 in O2 33% and N2O or air. Neuromuscular blockade was achieved with IV rocuronium 0.6 mg/kg. Patient's lungs were ventilated via a face mask for 3–5 min. The airway devices (size 7.5 mm inner diameter for all TT and size 4 for all LMA/PLMA/LTS) were used in strict accordance with their respective manufacturer's recommendations and inserted by 4 users with experience using all 4 devices (>50 uses with each of the extraglottic device and >1000 insertions of the TT). All devices were fixed by taping the tube in the midline over the chin. Once an effective airway was obtained, the intracuff pressure was set and held constant at 30 cm H2O for the TT and 60 cm H2O for the extraglottic airway devices using a digital manometer (Mallinckrodt Medical, Athlone, Ireland). During the maintenance phase, patient's lungs were ventilated with a tidal volume at 8–10 mL/kg, a respiratory rate of 12 breaths/min, an inspiratory:expiratory ratio of 1:2, and a fresh gas flow of 3 L/min.
Otoscopy was performed preoperatively by a qualified otoscopist (IA) on both ears using a standard halogen bulb otoscope. Tympanometry was performed by an observer blinded as to the airway device using an impedance audiometer AT235 (Interacoustics, Assens, DK). This has a pressure measurement accuracy of 10 mm H2O and a pressure range between –600 mm H2O and +300 mm H2O. A baseline MEP (baseline) was obtained before induction of general anesthesia. After induction, the MEPs were measured every 10 min from each ear in random order (by opening a sealed opaque envelope) throughout the procedure (P10, P20, P30, P40, P50, P60, P70). The MEP was the average of the readings from the right and left ears. In the N2O groups, N2O was changed to O2 33% and air after 40 min (P40).
Sample size was based on a previous study (5) reporting a difference of MEP between the TT and the LMA of 33% and a type I error of 0.05 and a power of 0.9. The distribution of data was determined using Kolmogorov-Smirnov analysis (10). All intragroup and intergroup comparisons of parametric data were made with one-way analysis of variance with post hoc Bonferroni test. Significance was taken as P < 0.05.
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Results
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There were no demographic or surgical differences among groups (Table 1). There were no differences in MEP among the airway devices in the N2O (Table 2) or air (Table 3) groups. MEP was unchanged in the air groups but increased in the N2O groups with N2O (P < 0.0001) and decreased with air (P < 0.02). Baseline values for MEP were similar, but MEP was always higher for the N2O groups (P < 0.001).
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Discussion
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We found that the choice of airway device does not influence MEP during anesthesia among the orally inserted TT, classic LMA, PLMA, and LTS during anesthesia with or without N2O in the gas mixture. Our findings contrast with those of Satoh et al., who, in a 1998 preliminary study of 11 adults whose anesthesia was maintained with sevoflurane 2% and N2O 66% in O2, found that MEP takes longer to increase to a maximum level (42 min versus 16 min) and longer to return to normal after cessation of N2O (68 min versus 33 min) for the LMA than the TT. The authors postulated that this was a result of compression of the Eustachian tube by the cuff of the LMA. Our findings also contrast with those of Nader et al. (5), who, in a 2004 study of PONV in 27 adults, noted that MEP was higher with the LMA than the TT. These differences are probably related to the smaller sample sizes of Satoh et al. and Nader et al. (5) but might possibly be related to more frequent malpositioning, as the LMA might compress the Eustachian cushion if misplaced into the nasopharyngeal cavity. In principle, nasally placed airway devices are more likely to increase MEP as they cross through the nasopharynx.
We found that MEP increases for all airway devices during N2O anesthesia and decreases after cessation of N2O. Breathing gas mixtures containing N2O causes predictable and quantifiable changes in MEP (5,11–13). These changes are related to diffusion of N2O into and out of the middle ear, which, in turn, is influenced by mucosal thickness, the surface area of the middle ear, and the partial pressure gradient (11). Interestingly, it has been shown that N2O is associated with PONV (14) and that MEP decreases to less than atmospheric pressure after cessation of N2O (13). Nader et al. (5) have suggested that these fluctuations from high to low MEP trigger PONV. A limitation of our study is that we did not document MEP during emergence or PONV.
We conclude that the choice of airway device does not influence MEP among the oral TT, classic LMA, ProSeal LMA, or LTS during anesthesia with or without N2O.
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Footnotes
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1Satoh K, Wakusawa R, Okada K, Kawamura T, Murai K, Tiba R. The change of intratympanic pressure during general anesthesia with laryngeal mask [abstract]. Anesth Analg 1998;86:S230. 
This project was supported solely by departmental resources. Dr Brimacombe and Keller have worked as consultants for the laryngeal mask company.
Accepted for publication July 15, 2005.
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References
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Abstract
Introduction
