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Incidence of gastric insufflation at high compared with low laryngeal mask cuff pressure

A randomised controlled cross-over trial

Hell, Johannes; Pohl, Holger; Spaeth, Johannes; Baar, Wolfgang; Buerkle, Hartmut; Schumann, Stefan; Schmutz, Axel

Author Information
European Journal of Anaesthesiology: February 2021 - Volume 38 - Issue 2 - p 146-156
doi: 10.1097/EJA.0000000000001269

Abstract

Introduction

The laryngeal mask has become the most important airway management tool for general anaesthesia, with the advantage of a lower incidence of upper airway complications compared with the endotracheal tube.1 In current anaesthetic practice, the first-generation laryngeal mask is the most commonly used supraglottic airway device for airway management.2

For adequate function the seal between the laryngeal mask and the periglottic tissue, determined by the laryngeal mask's cuff pressure,3 is crucial. Insufficient cuff pressure can result in air leakage leading to ineffective ventilation.4 While an inadequate seal in the oral cavity may result in oral air leakage and, at its worst hypoventilation, a seal leaking more distally, towards the gastrointestinal tract, may lead to gastric insufflation and subsequently promote pulmonary aspiration, a rare (approximately one in 11 000),5 but severe complication.5,6

The occurrence of gastric insufflation depends on the laryngeal mask's position in the hypopharynx7,8 and the tension of the lower oesophageal sphincter.9,10 The incidence of gastric insufflation rises with increasing cuff volume4 and decreasing oral air leakage.10 However, it is not known whether the laryngeal mask's cuff pressure affects the incidence of gastric insufflation. Taking into account that air might leak via the oral cavity and via the gastrointestinal tract, and that high cuff pressure results in reduced oral leakage, we hypothesised that gastric insufflation might occur less frequently with lower compared with higher cuff pressure, due to the increased likelihood that leaking air passes through the oral cavity.

When a poor seal leaks towards the oral cavity it may be detected easily by audible sounds over the mouth,11 but air leaking towards the gastrointestinal tract is difficult to identify. To date analyses investigating gastric insufflation have used acoustic detection methods such as auscultation with a stethoscope or microphone,10,12 but more recently real-time gastric ultrasonography has been shown to be a highly sensitive method for the detection of gastric insufflation.13 The analysis of gastric insufflation with this new method during facemask ventilation resulted in an unexpectedly high incidence of gastric insufflation even at low inspiratory pressures. With this in mind we hypothesised that the incidence of gastric insufflation, detected with highly sensitive real-time ultrasonography, would be higher than expected even during peak airway pressures of 20 cmH2O or less.

We also postulated that the duration of pressure on the lower oesophageal sphincter14 would affect the probability of gastric insufflation. In theory, continuous pressure on the lower oesophageal sphincter, such as during recruitment manoeuvres or during oral leak pressure measurements, would lead to an increased risk of gastric insufflation. Therefore, we hypothesised a higher incidence of gastric insufflation during continuous positive airway pressure (CPAP) compared with intermittent pressure-controlled ventilation (PCV).

To verify our three hypotheses, we compared gastric insufflation during mechanical ventilation with a first-generation laryngeal mask and a cuff pressure of 20 cmH2O, described as the minimum pressure required to achieve an adequate seal,15 and with a cuff pressure of 60 cmH2O, recommended by the manufacturer as the upper limit, in a randomised controlled double-blind cross-over study. We determined the superiority of the high or low cuff pressure in avoiding gastric insufflation in each patient. We also investigated the incidence of gastric insufflation during increasing peak airway pressures using real-time ultrasonography to detect gastric insufflation with the highest possible sensitivity.13 To verify gastric insufflation, detected by real-time sonography, we additionally calculated the cross-sectional area (CSA) of the gastric antrum13,16 for confirmation. To evaluate the effect of continuous pressure on gastric insufflation we compared gastric insufflation during positive pressure ventilation with 20 s of CPAP at the same peak airway pressure. To evaluate the laryngeal mask's seal, we defined the oropharyngeal leak pressure (OLP) as the pressure at which oral gas leakage occurs17 during continuous monitoring of gastric insufflation.

Methods

The study was approved by the local Ethics Committee (Ethics Committee No 246/16) of the University of Freiburg, Germany on 30 June 2016 and registered in the German Clinical Trials Register (DRKS# 00010583) before inclusion of the first patient.

Patients

After obtaining written informed consent, female patients scheduled for minor gynaecological surgery and airway management with laryngeal mask were included in the study. Exclusion criteria were age less than 18 years, physical status more than three according to the classification of the American Society of Anaesthesiologists, contraindications for use of laryngeal mask, such as acute gastrointestinal complaints, chronic gastrointestinal disease or former surgery on the upper gastrointestinal tract, uncontrolled diabetes, end-stage kidney disease, known airway pathology, severe restrictive lung disease, neurodegenerative disorder, BMI more than 50 kg m−2 and pregnancy.

During the preanaesthetic evaluation the patients’ airways were evaluated in terms of inter-incisor gap, neck circumference, thyromental distance (Patil test) and Arné-score.18

Anaesthesia and airway management

After implementation of routine monitoring, patients were preoxygenated until the fraction of expiratory oxygen was at least 0.8. General anaesthesia was induced using remifentanil 0.5 μg kg−1 min−1 and propofol 5.0 to 7.0 μg ml−1 using effect site concentration with a target-controlled infusion according to the Schnider model (Injectomat Agilia; Fresenius Kabi, Bad Homburg, Deutschland, Germany). Depth of anaesthesia was monitored via bispectral index (BIS VISTA Monitoring System; Aspect Medical Systems, Norwood, Massachusetts, USA). A BIS between 40 and 60 was considered an appropriate level of anaesthesia. After induction of anaesthesia the remifentanil infusion was reduced to 0.2 μg kg−1 min−1 and propofol administration was adjusted to maintain the target BIS.

In contrast to standard routine we omitted manual face mask ventilation prior to the insertion of the laryngeal mask to avoid gastric insufflation resulting from this measure. A size four disposable first-generation laryngeal mask (AuraOnce, Ambu, Ballerup, Denmark) was inserted, always by the same experienced senior anaesthesiologist (author AS). The laryngeal mask cuff was inflated to the designated pressure and repeatedly re-adjusted using a pressure gauge (Covidien, Plymouth, Minnesota, USA).

After laryngeal mask insertion, gentle manual ventilation was established with a peak airway pressure of 10 cmH2O and a fresh gas flow of 12 l min−1. Successful laryngeal mask insertion was verified by a square-wave pattern capnography, symmetric chest wall expansion and absence of an audible air leak. If laryngeal mask positioning was unsuccessful two further attempts were allowed before the patient was excluded from further involvement in the study. After successful insertion, the position of the laryngeal mask in the hypopharynx was evaluated by flexible fiberoscopy (KARL STORZ GmbH, Tuttlingen, Germany) through the laryngeal mask. To rate the position of the laryngeal mask we used a scoring system,19 ranging from 0, representing ventilation failure, to 4, representing an optimal position. The laryngeal mask's position was considered appropriate at 3 or more points, otherwise the laryngeal mask was repositioned. Figure 1 shows the study protocol.

F1
Fig. 1:
Study procedure. BIS, bispectral index; BW, body weight; CP, cuff pressure; CPAP, continuous positive airway pressure; CSA, cross-sectional area; etCO2, end-expiratory CO2 concentration; GI, gastric insufflation; LM, laryngeal mask; PAP, peak airway pressure; PCV, pressure-controlled ventilation; VT, tidal volume.

Cuff pressures and ventilation protocol

Patients were randomly assigned to one of two intervention groups according to a computer-generated block randomisation list. The individual assignment was disclosed shortly before induction of anaesthesia. One group received a cuff pressure of 20 cmH2O (CP20) followed by a cuff pressure of 60 cmH2O (CP60); the other group received the cuff pressures in the reverse order. At each cuff pressure we applied a sequence of 8 consecutive breaths with PCV, according to the typical settings for PCV with laryngeal mask in our centre. This was followed by 20 s of CPAP, repeated with a sequence of peak airway pressures from 15, 20, 25 to 30 cmH2O (Fig. 2). Following our hypothesis that CPAP is more likely to generate gastric insufflation, patients received PCV first and CPAP second. End-expiratory pressure was maintained at 5 cmH2O throughout the procedure. Primus Infinity (Dräger, Lübeck, Germany) was used with a single use breathing circuit (Dräger) for the whole study procedure. Each ventilation sequence was terminated by either the occurrence of gastric insufflation, when tidal volume more than 18 ml kg−1, which is the maximally allowed tidal volume during a recruitment manoeuvre,20 when end-tidal carbon dioxide partial pressure less than 4 kPa or after sequence completion. Once measurements were completed, patients were peri-operatively ventilated at the cuff pressure that had initially been used.

F2
Fig. 2:
Ventilation manoeuvres. Cyclic positive pressure ventilation including eight consecutive breaths over 1 min with an inspiratory to expiratory ratio of 1 : 2 pressure-controlled ventilation was followed by a continuous positive pressure at the respective peak inspiratory pressure for 20 s, referring to continuous positive airway pressure. All manoeuvres were performed at a continuous positive end-expiratory pressure of 5 cmH2O while peak airway pressures were increase from 15 to 30 cmH2O by steps of 5 cmH2O. Please note interruptions of the x-axis.

Gastric insufflation measurement

Real-time sonography was continuously performed throughout the study by two experienced physicians, blinded to the patients’ group allocation (JH and HP), using a curved-array low-frequency transducer (2 to 5 MHz, SonoSite M-Turbo; SonoSite GmbH, Frankfurt, Germany) and standard abdominal settings. A sagittal layer passing through the superior mesenteric vein and the aorta served as the standardised plane in the supine position. The appearances of acoustic shadows or comet-tails were considered to indicate gastric insufflation.13,16,21 To verify the presence of gastric insufflation, we calculated the CSA of the gastric antrum22–24 using sonography. For that purpose, we determined the two-dimensional diameters of the gastric antrum in the supine position as the mean of three consecutive measurements immediately prior to induction of anaesthesia, at the end of each ventilation sequence, or immediately after detection of gastric insufflation. The study protocol dictated the immediate cessation of airway pressure once gastric insufflation was detected by real-time sonography, so we did not assess the CSA for estimation of the volume of insufflation; there is a correlation between CSA and gastric volume of fluids and solid contents25 but not between CSA and gas volume. Patients with a CSA at least 360 mm2 before anaesthesia were excluded from the study due to the increased risk of aspiration.26

Oropharyngeal leakage pressure measurements

The OLP was determined after each ventilation sequence using the method of manometric stability.17 At a fresh gas flow of 3 l min−1 the expiratory valve of the breathing system was closed and airway pressure increased. When oral leakage was in equilibrium with fresh gas flow, airway pressure reached stability. If gastric insufflation occurred before this, the OLP measurement was terminated without documentation of OLP.

Peri-operative and postoperative evaluation

Peri-operative airway complications were recorded. One hour after the intervention and again one week later, patients underwent a structured interview (Suppl. 1, https://links.lww.com/EJA/A334). Seven items were rated as ‘no’, ‘mild’, ‘moderate’ or ‘severe’ complaints. The complaints were compared between the two intra-operative cuff pressure groups, corresponding to the first cuff pressure during the intervention.

Statistical analysis

Data are given as mean ± SD if not indicated otherwise. Sample size calculation and data analyses were performed by an experienced bio-statistician from our Institute for Medical Biometry and Statistics. The sign test was used to evaluate differences in the incidence of gastric insufflation and the peak airway pressures at which it occurred in the two cuff pressure groups to determine the more favourable cuff pressure to avoid gastric insufflation. We chose the sign test due to its high resilience and its ability to discriminate between small differences, also accepting lack of statistical power compared with alternative tests. Hence, a statistically significant result would be a strong indicator for a favourable cuff pressure. Assuming a positive effect of 18% (superiority of the lower cuff pressure), a paradoxical effect of 8% (superiority of the higher cuff pressure) and a drop out quota of 15%, a total sample size of 184 patients was required to achieve a test power of 85% at an alpha error probability of 0.05.

In determining the study's primary endpoint, the occurrence of gastric insufflation at CP20 compared with CP60 only the results of the ventilation sequence and the sign test were used. Ordinal scaled pairs of observations in a nine-step scale (Suppl. 2, https://links.lww.com/EJA/A334) were compared. For statistical reasons we attributed gastric insufflation at peak airway pressures exceeding 30 cmH2O in individuals in which gastric insufflation had not occurred.

In a second approach we analysed the superiority of the cuff pressure according to gastric insufflation by comparing the peak airway pressures at which gastric insufflation occurred during the ventilation sequence and during OLP measurement. If peak airway pressures were equal for both cuff pressures during the ventilation sequence, gastric insufflation pressures during OLP measurements were compared. If there was also no difference, no favourable condition was determined. On this basis, we used also the sign-test.

The effect sizes were calculated with z-distribution of the sign test and transformed to Cohen's d for comparability purposes. Cohen's d more than 0.8 to 1.2 was suggestive of a large effect size.27

To evaluate the influence of the order of the two applied cuff pressures on gastric insufflation we used Cochran–Mantel–Haenszel statistics.

For evaluating the modes of ventilation, CPAP vs. PCV, we compared the frequencies of gastric insufflation with the same peak airway pressures using a χ2 test.

Data from the postoperative evaluation was divided into airway-related (four Questions) and gastrointestinal or aspiration-related complaints (three Questions). The two peri-operative cuff pressures were compared using the Mann–Whitney U test.

Sample size calculation and the randomisation list were created with SAS (version 9.4; Cary, North Carolina, USA). Data were analysed using SPSS (version 25.0; Chicago, Illinois, USA).

Results

Patients’ characteristics and airway management

Between July 2016 and December 2017 184 consecutive patients were assessed for eligibility (Fig. 3). After excluding 20 due to preinterventional CSA at least 360 mm2, 164 were left for analysis. Their characteristics and airway evaluation were comparable between groups (Table 1).

F3
Fig. 3:
CONSORT flow-chart. CP, cuff pressure; CSA, antral cross-sectional area before anaesthesia; LM, laryngeal mask.
Table 1 - Patient characteristics
CP20/CP60, n=83 CP60/CP20, n=81
Age (years) 43.4 ± 13.0 44.1 ± 15.2
Weight (kg) 65.4 ± 11.5 67.1 ± 11.8
Height (cm) 166.1 ± 6.0 167.8 ± 5.9
BMI (kg m−2) 23.65 ± 3.9 23.8 ± 3.9
ASA 1/2/3 36 (43.4%)/46 (55.4%)/1 (1.2%) 37 (45.7%)/40 (49.4%)/4 (4.9%)
Inter-incisor gap (cm) 4.5 ± 0.6 4.5 ± 0.6
Neck circumference (cm) 34.2 ± 3.0 34.3 ± 2.7
Thyromental distance (cm) (7.2 ± 0.9) 7.2 ± 0.8
Arné-Score 2 [0 to 2] 0 [0 to 2]
Brimacombe-Score 3/4 44 (53%)/39 (47%) 35 (43.2%)/46 (56.8%)
Attempts at insertion 1/2/3 75 (90.4%)/5 (6%)/3 (3.6%) 71 (87.6%)/5 (6.2%)/5 (6.2%)
CP20/CP60, first cuff pressure 20 cmH2O, second cuff pressure 60 cmH2O; CP60/CP20: first cuff pressure 60 cmH2O, second cuff pressure 20 cmH2O. Data are given as mean ± SD, median [IQR] or number (%).

Gastric insufflation

A cuff pressure of 20 cmH2O was superior to a cuff pressure of 60 cmH2O in avoiding gastric insufflation during increasing peak airway pressure. This applied when considering the ventilation sequences alone (P < 0.0001, Cohen's d = 0.818) and also in the analysis of the combination of ventilation sequences and OLP measurements (P < 0.0001, Cohen's d = 1.113). Considering the ventilation sequence alone, in 59 patients (36%) the peak airway pressures at which gastric insufflation occurred were higher with CP20 compared with CP60. In 16 patients (10%) CP60 was favourable for avoiding gastric insufflation. In the remaining 89 cases (54%) the peak airway pressure at which gastric insufflation occurred did not depend on cuff pressure (Fig. 4). Analysis of gastric insufflation during the ventilation sequence and oral leak pressure measurement revealed that in 79 patients (48%) CP20 was better for avoiding gastric insufflation. In 68 patients (41%) there was no difference between the two cuff pressures. In 17 patients (10%) CP60 was preferable. The order of the cuff pressure application did not affect these results (P = 0.2876).

F4
Fig. 4:
Favoured cuff pressure. Favoured cuff pressure to avoid gastric insufflation according to the study population. Results of the combined approach of analysis of ventilation sequence and oral leak pressure measurement (sign test: P < 0.0001).

In total, gastric insufflation occurred in 82.3% (CP60) and 74.5% (CP20) of patients with increasing peak airway pressure to 30 cmH2O. The cumulative incidence of gastric insufflation was higher with CP60 than with CP20 (P = 0.036 log-rank test Fig. 5). CP20 reduced the relative risk of gastric insufflation by 30% compared with CP60.

F5
Fig. 5:
Cumulative incidence of gastric insufflation. Cumulative incidence of gastric insufflation in dependence of 20 or 60 cmH2O cuff pressure during increasing peak airway pressures and different ventilation modes (P = 0.036 log-rank test). CPAP, continuous positive airway pressure; PCV, pressure-controlled ventilation.

Gastric insufflation was detected in 35% of the patients with CP20 and in 48% of the patients with CP60 at a peak airway pressure of 20 cmH2O or less, respectively.

At all peak airway pressures, gastric insufflation occurred more often during CPAP than during PCV (15 cmH2O: P = 0.002; 20 cmH2O: P < 0.001; 25 cmH2O: P < 0.001; 30 cmH2O: P = 0.009, Suppl. 3, https://links.lww.com/EJA/A334).

In patients with observed gastric insufflation the change in CSA was higher (+209 ± 216 mm2) than in patients without observed gastric insufflation (−9 ± 95 mm2, P < 0.0001).

A sonographic image of the gastric antrum before and after gastric insufflation (SDC1.ppt, https://links.lww.com/EJA/A335) and also a video of real-time sonography during gastric insufflation (SDC2.mp4, https://links.lww.com/EJA/A336) are shown in the Supplemental Digital Content.

Oropharyngeal leak pressure

OLP was lower with CP20 (19.9 ± 4.76 cmH2O) than with CP60 (23.7 ± 5.94 cmH2O, P = 0.0002). The incidence of gastric insufflation at an airway pressure below OLP was 66% with CP60 (n=104) and 38% with CP20 (n=60, P < 0.0001). In seven patients, OLP measurement was not feasible due to poor visualisation of the antrum caused by large amounts of insufflated air.

Peri-operative and postoperative follow-up

No adverse intra-operative events occurred. 11.6% of the patients reported oropharyngeal discomfort in the first interview and 12.8% in the second. Gastrointestinal discomfort was reported by 4.2% of the patients in the first and by 10.3% in the second interview. There was no significant difference between the groups (Suppl. 4, https://links.lww.com/EJA/A334).

Discussion

The main findings of our study are that a low laryngeal mask cuff pressure is superior to a high cuff pressure in avoiding gastric insufflation during increasing airway pressures. The incidence of gastric insufflation, even at low peak airway pressures 20 cmH2O or less, was higher than expected and gastric insufflation was dependent on the duration of the pressure acting on the oesophageal sphincter. As a result gastric insufflation was a more common occurence during continuous airway pressure than during PCV for the same peak airway pressure.

Taking into consideration other negative effects of cuff overinflation28,29 our findings reinforce the notion that lower cuff pressures are more preferable than higher ones. The increase of CSA in our study with immediate interruption of airflow after detection suggests that gastric insufflation may have clinical effects especially if gastric insufflation passes unnoticed during a longer period of PCV or repeated manoeuvres with continuous airway pressures. Although the incidence of aspiration during laryngeal mask ventilation is quite low, 2 : 10 000 to 2 : 11 000,5,6 our results indicate that low cuff pressures reduce the incidence of gastric insufflation and may reduce the subsequent risk of pulmonary aspiration with its often severe consequences. While the relationship between the amount of gastric insufflation and the risk of pulmonary aspiration remains unclear, prolonged gastric distension triggers transient relaxations of the lower oesophageal sphincter30 with possible negative effects. Furthermore, gastric distension could promote postoperative nausea and vomiting31 and massive gastric insufflation which can lead to haemodynamic and ventilatory compromise.32

The superiority of low cuff pressure in avoiding gastric insufflation was even more pronounced in the combined analysis of ventilation sequence and gastric insufflation during oral leak pressure measurement. Gastric insufflation during oral leak pressure measurements allows for the identification of favourable cuff pressures on a more precise airway pressure scale, due to the continuous increase in airway pressure. Thus, combining both approaches resulted in a more distinct preference for lower cuff pressures. To achieve peak airway pressures, exceeding oral leak pressure, measured with a constant fresh gas flow of 3 l min−1, we increased the fresh gas flow to 12 l min−1 during the ventilation sequence. In this way, we achieved peak airway pressures above oral leak pressure.

We assume that the correlation between lower cuff pressures and the incidence of gastric insufflation is caused by the lower oral leakage pressure, reducing the maximal pressure on the lower oesophageal sphincter. It was previously shown that oral leakage reduces the risk of gastric insufflation.10 The threshold for oral leakage in our study, measured in terms of the oropharyngeal leakage pressures, lay within the ranges of previous investigations.33,34 However, in the majority of cases with high cuff pressure, gastric insufflation occurred before oral leak pressure was achieved. These observations strengthen our assumption that oral leakage accounts for the benefits of the low cuff pressure. Consequently, anaesthesiologists may have to accept oral leakage to reduce the risk of gastric insufflation. This, however, must not be achieved at the expense of insufficient oxygenation and ventilation.

Surprisingly, we found gastric insufflation in almost half of the patients even at peak airway pressures of 20 cmH2O or less. This was considerably more frequent than is typically reported7,10,12,35,36 and questions the generally accepted advice to consider peak airway pressures of 20 cmH2O or less as safe.12,37,38 Moreover, the incidence of gastric insufflation during oral leak pressure measurement was more than three times higher than previously reported.39 We attribute the higher rate of gastric insufflation in our trial to our approach for detecting gastric insufflation. Ultrasonography is considered the most sensitive method for this purpose and is particularly more sensitive than auscultation, which was used in most studies to date. In this regard, our results are in accordance with a recent study that found an increased incidence of gastric insufflation during face mask ventilation when observed with real-time ultrasound.13 The higher detection rate of gastric insufflation with ultrasound is attributed to the greater sensitivity of the ultrasound-image to even small amounts of air. In addition, there is a smaller rate of false-positive detections due to gastric motility because of the clear distinction between gastric motility and gastric insufflation with an ultrasound-image.

Gastric insufflation detected in real-time ultrasonography was associated with a relevant increase in CSA whereas an absence of gastric insufflation in real-time ultrasound resulted in an almost unchanged CSA. These results strengthen the reliability of this ultrasound-based method of detection of gastric insufflation. Furthermore, our results suggest that even patients with no obvious risk of pulmonary aspiration may benefit from measures to limit the risk of gastric insufflation because even short episodes of gastric insufflation could lead to a large increase in CSA. Considering the recommended maximum peak airway pressure of 20 cmH2O during ventilation with laryngeal mask,12,37,38 the reduced incidence of gastric insufflation with low cuff pressure strengthens the superiority of a low cuff pressure in this regard in daily practice.

In addition to adjusting cuff pressure, the use of second-generation laryngeal masks may be considered, as these are equipped with a gastric drainage channel intended to reduce the risk of aspiration.40 Even if the superiority of second-generation laryngeal masks with regard to pulmonary aspiration is difficult to demonstrate, due to the low incidence of pulmonary aspiration itself, if safety is the major concern, second-generation laryngeal masks appear preferable, especially when considering the advanced uses of the laryngeal mask.40 Promoting this culture of safety we recommend the application of low cuff pressures if first-generation laryngeal masks are used.

The duration of the pressure acting on the lower oesophageal sphincter appears to affect the probability of gastric insufflation. Our data demonstrated that CPAP caused more gastric insufflation than PCV at the same peak airway pressure. This finding strengthens a mathematical model of the oesophagus14 that contains a time invariant, nonlinear network of impedances and predicts a positive correlation between the application time of pressure on the lower oesophageal sphincter and the risk of gastric insufflation.14 In an earlier study no correlation between ventilation mode and pulmonary aspiration was found.6 However, the only comparison involved volume-controlled ventilation vs. spontaneous or assisted spontaneous breathing. The effects of high continuous airway pressures, for example during recruitment manoeuvres or oral leak pressure measurement with regard to sonographic detection of gastric insufflation, have not been investigated so far. In our opinion, the duration of pressure acting on the lower oesophagus sphincter is an important determinant of gastric insufflation. In this context, we would advise against oral leak pressure measurements and recruitment manoeuvres as standard procedures in daily practice, given the high incidence of gastric insufflation during continuous airway pressure.

We could not detect a difference in postoperative oropharyngeal discomfort between the two cuff pressures. This finding may be associated with the comparatively short operating time. Nevertheless, it is well described that a low cuff pressure reduces adverse events related to the oropharynx.28 We attribute the increased incidence of gastrointestinal discomfort in the second interview to the effects of pain therapy with opioids.

In considering limitations of our study, we have to address that cross-over studies come generally with the inherent risk of carry-over effects. Particularly, undetected trapped air in the distal oesophagus could have influenced the results of the second ventilation sequence. To minimise the potential effects of this, pressure was released for at least 5 s between the different ventilation modes and peak airway pressures. According to previous studies,14 the oesophagus should have deflated while the laryngeal mask was disconnected from the breathing circuit during the CSA measurements between first and second ventilation sequence, but we cannot fully exclude some influence of the first ventilation sequence on the second. However, we found no significant effects of the chronological order of the intervention. Therefore, we assume that such effects did not play a relevant role in our study.

The seal obtained by the laryngeal mask results from many interacting factors. The laryngeal mask's shape in particular and its material contribute significantly to sealing.41 As the observed effects were functional, we expect comparable results with other types of first-generation laryngeal mask s. However, a one-to-one generalisation of our results is not possible for other laryngeal mask types.

For the sake of consistency, we used the same laryngeal mask size 4 for all patients. Using one size for all might be grounds for discussion regarding an optimal fit of the laryngeal mask, but the question of optimal laryngeal mask size is still being debated, and the effects of different sizes seem to be quite small.42,43 Furthermore, some authors recommend a laryngeal mask size 4 as standard use for female patients.44 In this context, we have to acknowledge that we examined only female patients and transposing our results to male patients should be done with care. However, to our knowledge, no differences between incidences of gastric insufflation in men and women have been reported so far,10,13 suggesting that our findings might also be valid for men.

In this study a first-generation laryngeal mask, widely used at our centre, was examined. However, second-generation laryngeal mask s will be increasingly favoured in clinical practice, making sonographic evaluation of gastric insufflation with second-generation laryngeal masks desirable, although due to cost issues and user habits, first-generation laryngeal masks are still in widespread use. Therefore, we want to emphasise the importance of adjusting cuff pressure to reduce gastric insufflation in first-generation laryngeal masks.

In line with other reasons for preferring low cuff pressures when using laryngeal masks,45,46 our results point to advantages of low cuff pressures and avoidance of continuous high airway pressures if gastric insufflation is to be reduced. Furthermore, this study questions the safety of a maximum peak airway pressure of 20 cmH2O and would recommend the lowest possible peak airway pressure to achieve adequate ventilation.

Conclusion

Our highly sensitive ultrasound analyses revealed an unexpectedly high incidence of gastric insufflation during ventilation with laryngeal masks, even at comparatively low airway pressures. Using first-generation laryngeal masks, adjusting the cuff pressure to a low level reduces the risk of gastric insufflation and may reduce the subsequent risk of pulmonary aspiration. We found an increased incidence of gastric insufflation during prolonged positive airway pressure application. As a consequence, we would recommend against oral leak pressure measurements and recruitment manoeuvres as standard procedures in daily practice when using laryngeal masks.

Acknowledgements relating to this article

Assistance with the study: the authors thank Beate Berger for her technical support and Helen Engelstaedter for proofreading the article.

Financial support and sponsorship: the study was financed by the Department of Anaesthesiology and Critical Care, Medical Centre, University of Freiburg.

Conflicts of interest: none.

Presentation: preliminary data were presented as a short presentation at the HAI 2018, Berlin, Germany.

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