British Journal of Anaesthesia, 116 (2): 289 94 (2016) doi: 10.1093/bja/aev435 Respiration and the Airway Overinflation of the cuff and pressure on the neck reduce the preventive effect of supraglottic airways on pulmonary aspiration: an experimental study in human cadavers M. Hensel 1,, *, W. Schmidbauer 2,, D. Geppert 3, S. Sehner 4, G. Bogusch 5 and T. Kerner 6 1 Department of Anaesthesiology and Intensive Care Medicine, Chefarzt der Abteilung Anästhesiologie und Intensivmedizin, Park-Klinik-Weissensee, Schönstrasse 80, Berlin 13086, Germany, 2 Department of Anaesthesiology, Intensive Care Medicine, Emergency Medicine, Combat Search and Rescue Bundeswehrkrankenhaus, Berlin 10115, Germany, 3 Department of Anaesthesiology, Intensive Care Medicine, Emergency Medicine and Pain Therapy, Asklepios Klinik Nord Heidberg, Hamburg 22417, Germany, 4 Department of Medical Biometry and Epidemiology, University Medical Center, Hamburg-Eppendorf, Hamburg 20246, Germany, 5 Center for Anatomy, Charité-Universitätsmedizin, Berlin 10117, Germany, and 6 Department of Anaesthesiology, Intensive Care Medicine, Emergency Medicine and Pain Therapy, Asklepios Klinikum Harburg, Hamburg 21075, Germany *Corresponding author. E-mail: hensel@park-klinik.com Abstract Background: The oesophageal leak pressure is defined as the pressure which breaks the seal between the cuff of a supraglottic airway and the peri-cuff mucosa, allowing penetration of fluid into the pharynx and the oral cavity. As a consequence, a decrease in this variable increases the risk of reflux and can lead to pulmonary aspiration. The aim of this study was to analyse the effects of cuff overinflation and pressure on the neck on the oesophageal leak pressure of seven supraglottic airways. Methods: Three laryngeal masks, two laryngeal tubes, and two oesophageal tracheal tubes were tested in an experimental setting. In five human cadavers, we simulated a sudden increase in oesophageal pressure. To measure baseline values (control), we used an intracuff pressure as recommended by the manufacturer. The first intervention included overinflation of the cuff by applying twice the amount of pressure recommended. A second intervention was defined as external pressure on the neck. Results: The oesophageal leak pressure was decreased for laryngeal masks (control, 28 cm H 2 O; overinflation, 9 cm H 2 O; pressure on theneck,8cmh 2 O; P<0.01) and for laryngeal tubes (control, 68 cm H 2 O; overinflation, 37 cm H 2 O; pressure on the neck, 39 cm H 2 O; P<0.01) and was unaffected for oesophageal tracheal tubes (control, 126 cm H 2 O; overinflation/pressure on the neck, 130 cm H 2 O; n.s.). Conclusions: Cuff overinflation and pressure on the neck can enhance the risk of gastro-oesophageal reflux when using supraglottic airways. Therefore, both manoeuvres should be avoided in clinical practice. Key words: laryngeal masks; models, anatomical; respiratory aspiration These authors contributed equally to this work. Accepted: October 30, 2015 The Author 2016. Published by Oxford University Press on behalf of the British Journal of Anaesthesia. All rights reserved. For Permissions, please email: journals.permissions@oup.com 289
290 Hensel et al. Editor s key points Gastropharyngeal reflux can occur during the use of a supraglottic airway, but it is not known whether or not the overinflation of the cuff or pressure on the neck affect the risk of reflux. In a cadaver model, the effect of overinflation of the cuff or pressure on the neck on the sealing effect on gastropharyngeal reflux was assessed during the use of one of seven supraglottic airways. Overinflation of the cuff or pressure on the neck increases the risk of reflux during the use of the laryngeal masks or the laryngeal tube. Using supraglottic airways, there is no complete protection against gastric insufflation, gastro-oesophageal reflux, or subsequent pulmonary aspiration. 1 Given that the risk of aspiration is always present in prehospital airway management and during general anaesthesia, it is of clinical importance to identify factors that impair the sealing capabilities of supraglottic airways. 2 4 Using a cadaver model, our group has shown in previous studies that there are some major differences between the individual supraglottic airways with regard to sealing capabilities during sudden increases in oesophageal pressure. 5 7 It seems that oesophageal tracheal dual-lumen tubes can prevent reflux better than laryngeal tubes or laryngeal masks. 6 Although some information is available regarding the capability of each supraglottic airway to prevent reflux, only little is known about the impact of manoeuvres that are used in clinical conditions to optimize the airway position, such as cuff overinflation or pressure on the neck. Although it has been shown in supraglottic airways that cuff overinflation can lead to postoperative morbidity and to impairment of its primary function, the airway seal, overinflation of the cuff is still a frequent phenomenon in clinical practice. 8 One reason for this might be the incorrect assumption that air leakage around the blocked cuff is avoidable by this manipulation. Several clinical studies have shown that overinflation of the cuff increases, rather than decreases, the air leakage around the device. 910 The application of external pressure on the neck after placement of supraglottic airways has been reported to prevent gastric insufflation. 11 However, it remains an open question as to whether or not the risk of gastro-oesophageal reflux is influenced by these manoeuvres, which are mainly performed by paramedics and emergency physicians. 12 In the present study, therefore, we examined the hypothesis that cuff overinflation and external pressure on the neck can change the sealing capabilities of supraglottic airways during sudden increases in oesophageal pressure. In this regard, differences between individual airway devices were expected. Methods Cadaver model The study was approved by the local Ethics Committee of the Charité University Hospital Universitätsmedizin Berlin (trial registry number: EA1/195/06). Given that it is impossible to determine the protection from aspiration afforded by airway devices in clinical conditions in patients, we used an experimental setting based on investigations in human cadavers. Five unfixed human bodies (three female, two male) were dissected within 24 h after their natural death. The mean age at death was 82 yr (range, 77 88 yr). The bodies were prepared to provide exposure of the trachea and the oesophagus in the neck. The distal end of the trachea was connected to a test lung for respirators and secured by a suture. The distal end of the exposed oesophagus was connected to a vertical flexible tube with a diameter of 2 cm and a height of 130 cm, using a tight suture. By filling this flexible tube with water, orally directed oesophageal pressure was simulated and precisely measured using a centimetre division scale applied on the outside of the tube. The oesophageal leak pressure, or as synonymously mentioned, regurgitation pressure, was first described by Brimacombe and Keller. 13 They have shown that the oesophageal leak pressure is a suitable measure to assess the risk of reflux and pulmonary aspiration. 13 15 Oesophageal leak pressure was defined as the pressure which breaks the seal between cuff and peri-cuff mucosa, allowing penetration of water into the oral cavity and the pharynx. After placement of the supraglottic airway, the tube was completely filled with water to a level of 130 cm (resulting pressure, 130 cm H 2 O) while the oesophageal tube was clamped. After removal of the clamp, the height of the water column remaining after 60 s was measured. The water that penetrated the barrier between the oesophagus and hypopharynx was collected, but we did not differentiate reflux and pulmonary aspiration in this experimental approach. Figure 1 shows a schematic illustration of the experimental procedure. All devices were applied in a randomized sequence using a computerized random-number generator. Airway devices Devices with an oesophageal drainage tube were tested with a closed drainage lumen. The study was carried out using laryngeal mask airways (Classic Laryngeal Mask Airway, Laryngeal Mask Airway ProSeal, and Intubating Laryngeal Mask Airway Fastrach ), laryngeal tubes (Laryngeal Tube and Laryngeal Tube LTS II ), and oesophageal tracheal dual-lumen tubes (Easytube and Combitube ). The sizes of the supraglottic airways were selected for the bodies to establish a representative seal (Classic Laryngeal Mask Airway, Laryngeal Mask Airway ProSeal, Intubating Laryngeal Mask Airway Fastrach, Laryngeal Tube, and Laryngeal Tube LTS II, size 4 or 5; Combitube and Easytube 37 or 41 Charrière). The Combitube and Easytube were inserted into the oesophagus using distal tube placement. All devices were inserted by the same experienced anaesthetist (>1000 extraglottic airway device applications). The correct placement of all supraglottic airway devices was evaluated by performing the following tests. Initially, sufficient ventilation of the test lung was checked by delivering 10 consecutive breaths (maximal inspiratory pressure, 15 cm H 2 O). A correct position was assumed when there was no audible air leakage during this manoeuvre and the pressure could be maintained. Thereafter, a constant airway pressure was built up by pressing the ventilation bag with 15 cm H 2 O for 60 s without any pressure loss and no detectable bubbles in the water column, indicating an oesophageal leak. In devices with an incorporated oesophageal drainage tube, the correct position of the oesophageal point was verified by inserting a gastric tube through the oesophageal lumen and moving it forward until it became visible at the oesophageal end. As the last step, the correct position was checked by means of fibreoptic pharyngoscopy. If necessary, the position of the extraglottic airway was corrected or the size changed until all baseline tests performed were passed.
Protection from aspiration by airway devices 291 Supply of water Diameter of 2 cm Height of 130 cm Test lung Extraglottic airway device Clamping/ declamping Trachea Oesophagus Larynx Flexible tube Fig 1 Schematic illustration of the experimental set-up. Interventions Subsequently, two interventions were analysed to evaluate their effect on the oesophageal leak pressure. For each supraglottic airway, three test scenarios were investigated, two respective interventions (overinflation and pressure on the neck) and one control, using the five anatomical models. Each measurement was repeated 10 times. This implies that 150 individual measurements were performed using each device. The primary outcome measure was the difference in oesophageal leak pressure of the individual supraglottic airway between the baseline value (control) and the value after overinflation and after pressure on the neck. The sample size estimation was based on values obtained from previous studies by our group and by conducting a pilot study on a smaller sample size. 5 7 Accordingly, a power analysis was performed to determine the number of experiments required (estimated average value for the difference in oesophageal leak pressure, 20 (SD 11) cm H 2 O). Adifferenceof10cmH 2 O was considered to be clinically relevant. During the control measurements, we applied the intracuff pressure as recommended by the manufacturer (60 cm H 2 O for laryngeal masks, laryngeal tubes, and oesophageal tracheal duallumen tubes). In the first intervention, the intracuff pressure of the device was adjusted to double the recommended intracuff pressure (120 cm H 2 O). In devices with two cuffs, both were inflated up to the same pressure. The intracuff pressure was controlled using a manometer. The second intervention was defined as external manual counterpressure by pushing the extraglottic airway 2 cm further into the hypopharynx for the entire duration of the tests. Statistics The distribution of oesophageal leak pressure measurements was shown with boxplots, separated by intervention for each airway device. To compare the oesophageal leak pressure between devices and interventions, a linear mixed model was used to take into
292 Hensel et al. account the cluster structure resulting from the design of the study. To take into account the fact that with every human cadaver each device was tested with every intervention, these repeated measurements per cadaver were included as a random effect to model the potential variability of human cadavers. Additionally, the interaction between device and intervention was included as a fixed effect. Using the likelihood-ratio test, the significance of this interaction was tested. If this test was nonsignificant (P>0.05), the interaction was excluded, and only the main effects were modelled. With the resulting model tests on individual cadavers contrasts were performed; Scheffe-adjusted P-values (and 95% confidence intervals for differences) were reported to account for multiple comparisons (Supplemental material, Table S1). Results were described and shown graphically using the model-based marginal means and the corresponding 95% confidence intervals for the fixed effects. These analyses were conducted with StataCorp 2015 (Stata Statistical Software, release 14; StataCorp LP, College Station, TX, USA). A P-value <0.05 was considered to be significant. Results Each device was inserted into all five anatomical models, and the functional check was successfully conducted on all bodies. Figure 2 provides an overview of the oesophageal leak pressure of all supraglottic airway devices, depending on the intervention and the respective baseline value (control group). The highest oesophageal leak pressure baseline values before the interventions were seen in the oesophageal tracheal doublelumen tubes, which reached the maximal measurable value of 130 cm H 2 O. Both were significantly higher than Intubating Laryngeal Mask Airway Fastrach (P<0.001), which in turn was significantly higher than all other devices (each P<0.001). No significant differences were seen between Laryngeal tubes and LMA ProSeal, whereas the data generated using LMA Classic were significantly lower compared with all other devices. Compared with the control group, after both interventions (overinflation and pressure on the neck), a significant reduction in oesophageal leak pressure was seen in Classic Laryngeal Mask Airway ( 66.5 and 70.9%, respectively), Laryngeal Mask Airway ProSeal ( 35.4 and 41.2%, respectively), Intubating Laryngeal Mask Airway Fastrach ( 27.7 and 60.6%, respectively), Laryngeal Tube ( 45.5 and 45.9%, respectively), and Laryngeal Tube LTS II ( 32.2 and 40.6%, respectively; all P<0.01). In oesophageal tracheal dual-lumen tubes, the above-mentioned effects were not observed. In both Easytube (+2.9%) and Combitube (+3.5%), the maximal oesophageal leak pressure of 130 cm H 2 O was maintained as a result of cuff overinflation and external pressure on the neck (P=1.00). Figure 3 shows the adjusted estimates of mean values, including the associated 95% confidence intervals, after mixed model analysis. Discussion It can be concluded from our results that cuff overinflation and external pressure on the neck can impair the sealing capabilities 140 Oesophageal leak pressure (cm H 2 O) 120 100 80 60 40 20 0 LMA classic LMA ProSeal Laryngeal tube LTSII Laryngeal tube LT LMA fastrach Combitube Easytube Control group Overinflation Pressure on the neck Fig 2 Box-and-whisker plots of the data for oesophageal leak pressure: observed distribution of the measurements separated by intervention for each device. The box indicates the 25th and 75th percentile, and the central line is the median. The bars at the end of the whiskers are 2.5 and 97.5% values. A circle represents an outlier.
Protection from aspiration by airway devices 293 Oesophageal leak pressure (cm H 2 O) 140 120 100 80 60 40 20 LMA classis LMA ProSeal Laryngeal tube LTSII Laryngeal tube LT LMA fastrach Combitube Easytube 0 Control group Overinflation Pressure on the neck Fig 3 Adjusted mean and 95% confidence intervals of the oesophageal leak pressure separated by device over intervention. of supraglottic airways during sudden increases in oesophageal pressure. Although we did not study pulmonary aspiration, our findings permit the conclusion that both manoeuvres can substantially increase the risk of reflux, particularly with laryngeal masks and laryngeal tubes. It has been shown in clinical studies using methylene blue or intra-oesophageal ph electrodes that the incidence of aspiration is much lower than the incidence of reflux. 16 18 Occurrence of reflux, in turn, represents a risk factor for pulmonary aspiration. However, it is impossible for ethical reasons to provoke a sudden increase in oesophageal pressure in clinical conditions in patients in order to test the sealing capabilities of supraglottic airways. Therefore, we consider the results of our experimental study as clinically very meaningful. The significant differences that were seen between control conditions and pressure on the neck and between control conditions and cuff overinflation of up to 50 cm H 2 O, which implies a reduction by 70% of the baseline value, underline this statement. Reviewing the literature regarding tightness characteristics of supraglottic airways, a distinction must be made between the oesophageal leak pressure on the one hand and the oropharyngeal leak pressure on the other. While the oesophageal leak pressure describes the pressure which breaks the seal between the cuff and peri-cuff mucosa, allowing retrograde passage of gastric content into the pharynx, the oropharyngeal leak pressure is a measure of sealing the upper airway by a cuffed supraglottic airway device. 19 Accordingly, both leak pressures differ, because their force vector is different, reflecting a pressure from above or below the cuff of the supraglottic airway. A lower oropharyngeal leak pressure can be associated with an incomplete mask seal, which in turn can cause air leakage and insufflation of air into the stomach. 20 If gastric air insufflation occurs, the risk of gastro-oesophageal reflux is increased. Thus, there is a close interaction between oesophageal leak pressure and oropharyngeal leak pressure, but the two measures are not identical. Most publications relating to leak pressure of supraglottic airways have focused on oropharyngeal leak pressure. 21 23 Those studies were mostly performed in clinical conditions in patients. In contrast, studies dealing with oesophageal leak pressure are based on experimental investigations. 5 7 131424 Apart from our investigation, there is only one other experimental study with an intubating laryngeal mask reflecting the relationship between oesophageal leak pressure and intracuff pressure. 25 The results of that study are in line with our findings. However, our study is the only one to test other supraglottic airways in this field and the first one to have investigated the influence of pressure on the neck. In order to interpret the results of our study, it might be helpful to take the design and function of each airway device into account. With regard to oesophageal tracheal dual-lumen tubes, the positioning of a blocked cuff directly inside the oesophagus seems to provide a certain amount of protection against reflux and aspiration even in the event of cuff overinflation or pressure on the neck. For laryngeal masks, it is conceivable that the overinflation results in a changed anatomical position of the cuff. An orally directed movement of the laryngeal mask and an impaired function of the upper oesophageal sphincter could be the result. For laryngeal tubes, a redistribution of air volume between the proximal and the distal cuff is possible as a result of the intervention. The upwardly directed movement of the proximal cuff can prevent a better seal of the oesophageal cuff. Another explanation has been provided by Licina and colleagues 9 using paediatric laryngeal mask airways. They assumed that an increased cuff volume is associated with a growing rigidity of the cuff, a decreased compliance, and subsequently, with a reduced barrier between mucosa and cuff. As this is an experimental study, some limitations have to be taken into consideration when evaluating the results. 5 7 Although unfixed human bodies which sufficiently represent patient anatomy were used, they cannot carry out possible swallowing and retching movements or respond to influences associated with respiration. Also, pharyngeal soft tissue of anatomical models might differ from that of patients. Besides, it should be considered that protection from aspiration using secondgeneration devices, such as Laryngeal Mask Airway ProSeal, is a dynamic process. In clinical practice, this would depend on a variety of factors, including the alignment of the leading edge of the device onto the upper oesophageal sphincter and the ability to have a properly functioning gastric tube inserted into the stomach. With pressure on the neck or overinflation of the cuff, although the peri-laryngeal seal of supraglottic airways may be altered, aero-digestive separation may still be possible because the gastric tube may allow for suctioning of liquid from the stomach. As cuff overinflation and pressure on the neck are typically performed by clinicians in the prehospital setting and are unlikely to be performed by anaesthetists, it should be considered that these manipulations are ineffective and can enhance the risk of aspiration, particularly in the field of preclinical emergency medicine. 12 26 28 That is why these manoeuvres should generally not be used, principally in view of possible oesophageal injury. Authors contributions Conception and design of the study: M.H., T.K. Anatomical preparation: G.B. Acquisition of data: W.S., D.G. Statistical consulting and analysis: S.S. Analysis and interpretation of data: M.H., W.S., D.G. Preparation of the first draft of the paper: M.H. Revision of the article: W.S., D.G., S.S., T.K. Declaration of interest None declared.
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