Nitrous oxide diffusion into the cuffs of disposable laryngeal mask airways

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1 APPARATUS Nitrous oxide diffusion into the cuffs of disposable laryngeal mask airways P. Maino, 1 A. Dullenkopf, 1 V. Bernet 2 and M. Weiss 1 1 Departments of Anaesthesia and 2 Intensive Care, University Children s Hospital Zurich, Switzerland Summary The aim of this study was to investigate cuff pressure changes found in disposable size 3 laryngeal mask airways (LMAs) from different manufacturers during nitrous oxide exposure and to compare the results with the re-usable LMA. In an in vitro experiment, laryngeal mask airway cuff pressures starting from a baseline pressure of 4 cm H 2 O were recorded using a pressure transducer for min with the laryngeal mask airway cuff exposed to 66% N 2 O in oxygen. pressure increases within 5 min of nitrous oxide exposure were > 25% in the LMA and were not significantly different from those found in the laryngeal mask airway. However, they were significantly greater than those in the Soft Seal, the, the, and laryngeal mask airways, all of which demonstrated stable cuff pressure levels within the first 5 min. The cuff pressure increase following min of nitrous oxide exposure was 13. ± 1.1 and 14.6 ±.7 cm H 2 O in the and laryngeal mask airways, respectively, which was significantly lower than the cuff pressure increase in the Soft Seal and in the Ambu laryngeal mask airways (28.3 ± 2.9 and 3.9 ± 1.2 cm H 2 O, respectively). Unlike the re-usable LMA and the disposable laryngeal mask airway, which have silicone cuffs, the disposable Ambu,, Portex Soft Seal and laryngeal mask airways have cuffs constructed from, which seems to be less susceptible to hyperinflation caused by nitrous oxide diffusion.... Correspondence to: Markus Weiss markus.weiss@kispi.unizh.ch Accepted: 17 September 24 Disposable laryngeal mask airways (LMAs) are being increasingly introduced into the market by manufacturers. Suggested benefits over the standard re-usable LMAs (The Mask Company Ltd, Maidenhead, Berkshire, UK) are reduced logistical workload by recycling and the prevention of infectious protein deposits on LMAs after cleaning, with potential transmission of prion infections associated with multiple use [1 3]. However, there are differences in design, stiffness of the tube and properties of the cuff between disposable laryngeal mask airways. Previous studies have mainly focussed on the ease of insertion, laryngeal seal, morbidity on laryngeal structures and clinical performance [4 6]. An important concern when using the laryngeal mask airway during nitrous oxide supplemented anaesthesia is laryngo-pharyngeal morbidity caused by intracuff pressure increase and cuff overinflation [7 12]. The aim of the present study was to investigate the cuff pressure behaviour of disposable laryngeal mask airways (size 3) from different manufacturers during nitrous oxide exposure and to compare disposable laryngeal mask airways with the re-usable LMA. Methods In an in vitro set-up, pressure volume curves were obtained and cuff pressures (CP) were measured during exposure to nitrous oxide in the re-usable LMA and in five different disposable size 3 laryngeal mask airways (Table 1, Fig. 1). We only investigated size 3 laryngeal mask airways, as Ambu and do not produce smaller laryngeal mask airways. All laryngeal mask airways tested in this study were ordered from the local distributors and were not used in clinical practice prior to this investigation. To establish the laryngeal mask airway cuff pressure volume curves (cuff compliance), the cuffs were inflated using air from a syringe pump and the pressure measured 278 Ó 25 Blackwell Publishing Ltd

2 P. Maino et al. Æ Nitrous oxide diffusion in LMA Table 1 Commercially available disposable paediatric laryngeal mask airways from five different manufacturers and reusable paediatric laryngeal mask airways used for in-vitro evaluation of cuff compliance and N 2 O diffusion characteristics. mask airway Reference Number LMA Ref. SN AMCF LMA- mask airway Single Use Ref Soft Seal Mask Single Use 22 3 Airway Device (LAD) Single Use Ref Mask Ref Tr 1 SOLUS Single Use Ref. 3, polyvinyl chloride. Manufacturer The Mask Company Ltd Maidenhead, Berkshire, UK The Mask Company Ltd Maidenhead, Berkshire, UK Portex Ltd, Hythe, Kent, UK Products Ltd Bath, UK Ambu A S Ballerup, DK Ltd. Wokingham, Berkshire, UK material Silicone Silicone Figure 1 Photograph of tested laryngeal mask airways; all size 3 (A: LMA; B: LMA ; C: Portex Soft Seal; D: ; E: ; F: Ambu). using a pressure transducer (Pressure Monitoring Kit, Baxter BV, Ad Uden, Netherlands) connected to the pilot balloon by means of a three-way stop-cock. The transducer was connected to an anaesthesia monitoring system (A S 5; Datex Ohmeda, Helsinki, Finland). The syringe pump assembly consisted of an Alaris Asena Ò GH syringe pump (IVAC Medical Systems, Hampshire, UK), lowcompliance 5-ml infusion syringes (Single use syringe, 5-ml; CODAN Medical ApS, Rodby, Denmark), and a 5-cm non-compliant infusion line (PE-Infusion Line, Clinico Medical GmbH, Bad Mersfeld, Germany). The laryngeal mask airway cuffs were set to ambient pressure and air was slowly (1 ml.min )1 ) inflated into the cuff using the syringe pump. pressure was measured following increments of.5 ml air injected up to a cuff pressure of cmh 2 O. Measurements were performed with the laryngeal mask airways held in the horizontal position and with the cuff unrestricted. Pressure changes in the laryngeal mask airway cuffs during exposure to nitrous oxide were measured by placing the cuffs in a box continuously flushed with 3 l.min )1 of 66% nitrous oxide in oxygen. The pilot balloons were left outside the box and connected to a pressure transducer. Experiments were started at cuff pressures of 4 cmh 2 O. pressure was noted after 5 and min. Measurements were stopped if cuff pressures exceeded 13 cmh 2 Oin order to avoid over-distension and deformation of the cuff. volume at ambient pressure was determined by complete evacuation of air using a syringe. In addition, the thickness of the cuff membrane was measured using a sliding calliper (precision.2 mm). Each experiment was performed eight times using two examples of each brand (eight measurements). All tests were performed at room temperature (22 C) and at ambient pressure. Statistical analysis Pressure volume curves were drawn for the measurement of cuff compliance. compliance at 4 cmh 2 O and cmh 2 O cuff pressure was taken from the pressure volume curves. Pressure time curves were obtained to assess the time (T 15% T 2% T 25% ) in which the cuff pressure increased from 4 to, and cmh 2 O, respectively. Mean laryngeal mask airway cuff compliance at 4 and cmh 2 O and mean time to 15, 2 and 25% pressure increase during nitrous oxide exposure were taken from Figs 3 and 4, respectively. The cuff pressure increase in the LMA after 5 min was compared to the cuff pressure increase in each of the five disposable LMA cuffs using the Mann Whitney test. The cuff pressure increase after min was analysed using the Mann Whitney test and was compared to the group with the lowest increase. Data are presented as median and range and or mean ± SD. A p-value of <.5 was considered statistically significant for all tests. Results We found that cuff compliance was highest in the Soft Seal and lowest in the LMA (Table 2, Fig. 2). The cuff pressure increase within 5 min of nitrous exposure oxide was > 25% in the LMA and was Ó 25 Blackwell Publishing Ltd 279

3 P. Maino et al. Æ Nitrous oxide diffusion in LMA Anaesthesia, 25,, pages Table 2 Time in which LMA (size 3) cuff pressure increased from 4 to cmh 2 O(T 15% ), cmh 2 O(T 2% ) and cmh 2 O (T 25% ) and cuff pressure increase after exposure to nitrous oxide for 5 and min. Experiments were started at 4 cmh 2 O cuff pressure and were terminated if cuff pressure exceeded cmh 2 O. mask Residual volume (ml) compliance (ml 4 cmh 2 O) compliance (ml cmh 2 O) cmh 2 O (T 15% ) cmh 2 O (T 2% ) Thickness of cuff wall (mm) cmh 2 O (T 25% ) pressure increase 5min (cmh 2 O) pressure increase min (cmh 2 O) Portex Soft Seal Mask Airway Device Ambu Mask 16.5 ± ± ± 7. (72.2; ) 14.3 ± ±.2 )1.6 ± 1.5* ()2.; )3.9.) >** 14.6 ±.7 (14.3; ) 22.1 ± ).8 ± 1.4* ().7; ) 28.3 ± 2.9** (28.6; ) 16.8 ± ± ± 11.2 >** (81.3; ) 15.6 ± ±.2 ).3 ±.6* (.; )1.3.) 16.5 ± ±.2 ).2 ± 1.1* (.; ) ) 3.9 ± 1.2** (3.6; ) 13. ± 1.1 (13.; ) Data are presented as mean ± SD. *Statistically significant difference from disposable to the LMA for 5-min values. **Significantly higher cuff pressure compared to the lowest value after min (Mann Whitney test; p <.5). not significantly different from the LMA; this was significantly greater than that in the Soft Seal, the, the Ambu, and laryngeal mask airways, which all demonstrated stable cuff pressure levels within the first 5 min (Table 2, Fig. 3). The cuff pressure increase after min of nitrous oxide exposure was 13. ± 1.1 and 14.6 ±.7 cmh 2 O in the and laryngeal mask airways, respectively, which was significantly lower than the cuff pressure increase in the Soft Seal and Ambu laryngeal mask airways (28.3 ± 2.9 and 3.9 ± 1.2 cmh 2 O, respectively) (Table 2, Fig. 4). Pressure (cmh 2O) 4 CLASSIC UNIQUE 2 SOFT SEAL MARSHALL INTERSURGICAL Volume of Air inflated (ml) Figure 2 Volume pressure curves of tested re-usable and disposable paediatric laryngeal mask airways (size 3). Data are mean ± SD (n = 8 measurements each). Pressure (cmh 2 O) T 25% T 2% T 15% Time (Min) Discussion Portex Figure 3 In vitro cuff pressure behaviour of laryngeal mask airway cuffs (all size 3) during nitrous oxide exposure for 5 min (66% nitrous oxide in oxygen; cuff pressure baseline 4 cm H 2 O). Data presented as mean ± SD. The dashed lines indicate pressures of, and cm H 2 O. The cuff pressure in the LMA and laryngeal mask airway groups after 5 min was significantly higher than in the other groups (Mann-Whitney test; p <.5). In this in vitro study we have investigated cuff compliance and cuff pressure changes during nitrous oxide exposure in disposable size 3 laryngeal mask airways from five different manufacturers and compared them with the standard reusable LMA. The main finding was that there is a significant difference in cuff pressure increase due to nitrous oxide exposure between the different laryngeal mask airways studied which is not dependent on 2 Ó 25 Blackwell Publishing Ltd

4 P. Maino et al. Æ Nitrous oxide diffusion in LMA Pressure (cmh 2 O) Portex Time (Min) Figure 4 In vitro cuff pressure behaviour of laryngeal mask airway cuffs (all size 3) during nitrous oxide exposure for min (66% nitrous oxide in oxygen; cuff pressure baseline 4 cmh 2 O). Data presented as mean ± SD. The dashed lines indicate pressures of, and cmh 2 O. The cuff pressure in the and laryngeal mask airway groups after min was significantly lower than in the other groups (Mann Whitney test; p <.5). cuff compliance but seems dependent on thickness and the material from which the cuff is constructed. pressure increases due to nitrous oxide influx within air filled spaces is dependent on the partial pressure difference across the cuff membrane, the cuff volume, the area provided for gas exchange, and the permeability coefficient of the cuff membrane, which is a result of cuff material and cuff membrane thickness. Our results demonstrate that independently of cuff compliance, laryngeal mask airways made from silicone are more susceptible to diffusion of nitrous oxide into the cuff. The majority of available disposable devices are constructed from, in contrast to the laryngeal mask airway, which is constructed from silicone. Our data clearly demonstrate that the cuff pressure increase in silicone cuffs is more significant when exposed to nitrous oxide. However, the recommended cuff pressure limits may well be reached in cuffs of laryngeal mask airways when exposed to nitrous oxide for a period of time. Our findings are confirmed by the clinical report of van Zundert et al. [4] comparing the laryngeal mask airway with the Soft Seal laryngeal mask airway made from. In this study a similar -min cuff pressure increase was observed from 45 up to.3 ± 23.6 mmhg in the LMA group and was negligible (from 45 up to 46.8 ± 3.6 mmhg) in the Soft Seal laryngeal mask airway group. The authors concluded that the plasticiser added to soften the makes the cuff less permeable to nitrous oxide [4, 13]. However, our findings demonstrate that other laryngeal mask airways ( LMA) made from without softener and with lower cuff compliance than the Soft Seal laryngeal mask airway and similar cuff volume showed less pressure increase after min of nitrous oxide exposure. This implies that silicone and are the main determinants of susceptibility to nitrous oxide diffusion among reusable and disposable laryngeal masks. Secondly, the reduced thickness of the cuff wall seems to be responsible for the higher min cuff pressure increase in the Ambu and the Soft Seal laryngeal mask airway compared to the other two laryngeal mask airways made from silicone (Table 2). The plasticiser seems to be responsible for the high compliance in the Soft Seal laryngeal mask airway (Fig. 2). Lumb & Wrigley demonstrated in vitro in a LMA (size 4) a similar cuff pressure increase of 7 and 25 cmh 2 O, starting at mmhg (14 cmh 2 O) cuff pressure, after 5 and 3 min, respectively [14, 15]. In the clinical part of this trial, however, they found a delayed cuff pressure increase of 4 cmh 2 O after 3 min nitrous oxide exposure. Overinflation of laryngeal mask airway cuffs is associated with the risk of airway morbidity by exerting pressure on laryngeal structures. Related airway morbidity ranges from sore throat 2 h after anaesthesia to more serious complications such as paralysis of vocal cords and the hypoglossal nerve, and dislocation of the arytenoid cartilages [9 11, 16 2]. There are limited data on the relationship between pharyngeal intracuff pressures, pressure exerted by the cuff on the laryngo-pharyngeal mucosa and mucosal perfusion pressure [2, 21]. There is evidence that the diameter of pharyngeal blood vessels may be reduced when the mean mucosal pressure reaches 34 cmh 2 O (CP 4 cmh 2 O) and that complete mucosal blood vessel collapse is present in 9% of patients when the mean mucosal pressure reaches cmh 2 O (CP cmh 2 O) [21]. When laryngeal mask airways are inflated to recommended injection volumes (2 ml for a size 3 laryngeal mask airway), the wall pressures are substantial (92.3 ± 6.9 mmhg for a size 3 laryngeal mask airway) and these are likely to exceed the pharyngeal mucosal capillary perfusion pressure [21]. In children, whose mean arterial pressure is lower than in adults, the mucosal capillary perfusion pressure will be even lower. There are two studies demonstrating that reducing the cuff pressure reduces the incidence of sore throat [11, 22] and in two other studies it has been shown that if cuff pressure is limited to cmh 2 O, this results in a very low incidence (5%) of sore throat [22, 23]. These concerns should be considered when evaluating new disposable laryngeal mask airways. Our findings imply that continuous cuff pressure monitoring and adjustment in laryngeal mask airways with silicone cuff is mandatory shortly after insertion of the laryngeal mask airway. Whether continuous cuff monitoring in clinical practice can be Ó 25 Blackwell Publishing Ltd 281

5 P. Maino et al. Æ Nitrous oxide diffusion in LMA Anaesthesia, 25,, pages omitted in paediatric laryngeal masks with a cuff, as suggested by van Zundert et al. [4], will need to be the subject of future clinical trials. We evaluated the laryngeal mask airways in laboratory conditions to guarantee similar conditions for each type of mask. In clinical practice, diffusion characteristics of nitrous oxide into laryngeal mask airway cuffs may differ with body temperature and partial restriction of the cuffs in the larynx and with uneven partial pressure differences for nitrous oxide over the surface area of the cuff [14, 15]. Nonetheless, our findings clearly demonstrate that disposable laryngeal mask airways constructed from are less susceptible to cuff pressure increases due to nitrous oxide diffusion than those constructed from silicone. In conclusion, we found that the susceptibility to cuff pressure increases due to N 2 O in disposable laryngeal mask airways is mainly influenced by the type of material from which the cuffs are constructed, with considerable advantages seen with cuffs constructed from compared to silicone. Financial disclosure The laryngeal mask airways studied were ordered from local distributors. No financial support was obtained from any manufacturer for the present study. References 1 Tordoff SG, Scott S. Blood contamination of the laryngeal mask airways and laryngoscopes what do we tell our patients? Anaesthesia 22; 57: Lowe PR, Engelhardt T. Prion-related diseases and anaesthesia. Anaesthesia 21; 56: Jung MJ, Pistolesi D, Pana A. Prions, prion diseases and decontamination. Ig Sanita Pubbl 23; 59: van Zundert AA, Fonck K, Al-Shaikh B, Mortier EP. Comparison of cuff-pressure changes in LMA- and the new Soft Seal laryngeal masks during nitrous oxide anaesthesia in spontaneous breathing patients. European Journal of Anaesthesiology 24; 21: Brimacombe J, Keller C, Morris R, Mecklem D. A comparison of the disposable versus the reusable laryngeal mask airway in paralyzed adult patients. Anesthesia and Analgesia 1998; 87: Verghese C, Berlet J, Kapila A, Pollard R. Clinical assessment of the single use laryngeal mask airway the LMAunique. British Journal of Anaesthesia 1998; : Lumb AB, Wrigley MW. The effect of nitrous oxide on laryngeal mask cuff pressure. In vitro and in vivo studies. Anaesthesia 1992; 47: Marjot R. Trauma to the posterior pharyngeal wall caused by a laryngeal mask airway. Anaesthesia 1991; 46: Inomata S, Nishikawa T, Suga A, Yamashita S. Transient bilateral vocal cord paralysis after insertion of a laryngeal mask airway. Anesthesiology 1995; 82: Lloyd Jones FR, Hegab A. Case report. Recurrent laryngeal nerve palsy after laryngeal mask airway insertion. Anaesthesia 1996; 51: Burgard G, Mollhoff T, Prien T. The effect of laryngeal mask cuff pressure on postoperative sore throat incidence. Journal of Clinical Anesthesia 1996; 8: Nagai K, Sakuramoto C, Goto F. Unilateral hypoglossal nerve paralysis following the use of the laryngeal mask airway. Anaesthesia 1994; 49: Al-Shaikh B, Jones M, Baldwin F. Evaluation of pressure changes in a new design tracheal tube cuff, the Portex Soft Seal, during nitrous oxide anaesthesia. British Journal of Anaesthesia 1999; 83: Brimacombe J, Berry A. mask airway cuff pressure and position during anaesthesia lasting one to two hours. Canadian Journal of Anaesthesia 1994; 41: Martins RH, Braz JR, Defaveri J, Gregorio EA, Abud TM. Effect of high laryngeal mask airway intracuff pressure on the laryngopharyngeal mucosa of dogs. Laryngoscope 2; 11: Cros AM, Pitti R, Conil C, Giraud D, Verhulst J. Severe dysphonia after use of a laryngeal mask airway. Anesthesiology 1997; 86: Daya H, Fawcett WJ, Weir N. Vocal fold palsy after use of the laryngeal mask airway. Journal of Laryngology and Otology 1996; 11: Fawcett WJ, Daya H, Weir N. Recurrent laryngeal nerve palsy and the laryngeal mask airway. Anaesthesia 1996; 51: Keller C, Brimacombe J. Mucosal pressure, mechanism of seal, airway sealing pressure, and anatomic position for the disposable versus reusable laryngeal mask airways. Anesthesia and Analgesia 1999; 88: Marjot R. Pressure exerted by the laryngeal mask airway cuff upon the pharyngeal mucosa. British Journal of Anaesthesia 1993; 7: Brimacombe J, Keller C, Puhringer F. Pharyngeal mucosal pressure and perfusion: a fiberoptic evaluation of the posterior pharynx in anesthetized adult patients with a modified cuffed oropharyngeal airway. Anesthesiology 1999; 91: Keller C, Sparr HJ, Brimacombe JR. Positive pressure ventilation with the laryngeal mask airway in non-paralysed patients: comparison of sevoflurane and propofol maintenance techniques. British Journal of Anaesthesia 1998; : Keller C, Sparr HJ, Brimacombe JR. mask lubrication. A comparative study of saline versus 2% lignocaine gel with cuff pressure control. Anaesthesia 1997; 52: Ó 25 Blackwell Publishing Ltd

J. Brimacombe, 1 L. Holyoake, 2 C. Keller, 3 J. Barry, 4 D. Mecklem, 4 A. Blinco 5 and K. Weidmann 5

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