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Summary

  1. Top of page
  2. Summary
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References
  8. Supporting Information

We prospectively assessed common clinical endpoints for their usefulness in avoiding hyperinflation of the cuffs of laryngeal mask airways (slight outward movement) and tracheal tubes (disappearance of an audible leak around the cuff during manual ventilation < 20 cmH2O) in 640 children. Cuff pressures were measured at induction and immediately before emergence from anaesthesia. With the laryngeal mask airway (sizes 1–4), the median cuff pressures ranged from 90 to > 120 cmH2O at induction and 105 to > 120 cmH2O before emergence. With tracheal tubes (sizes 3–7 mm), median cuff pressures were 40–60 cmH2O at induction and 45–70 cmH2O at emergence. With the use of nitrous oxide a consistent rise in cuff pressure was observed between the first and second readings whereas cuff pressures remained constant when nitrous oxide was not used. The use of clinical endpoints alone was associated with significant hyperinflation of cuffs with both devices in almost all patients, with an exacerbation when nitrous oxide was used. In order to avoid unnecessary cuff hyperinflation in laryngeal mask airways and tracheal tubes, the routine use of cuff manometers is mandatory in children.

Laryngeal mask airways and tracheal tubes are the most commonly used airway devices in paediatric anaesthesia. Since the invention of the laryngeal mask airway, its use has increased greatly and the vast majority of anaesthetic procedures in children are now facilitated with a laryngeal mask airway [1]. In recent years, cuffed tubes have been increasingly used in young children mainly based on a lower tracheal tube exchange rate compared with uncuffed tubes, a better control of air leakage, a lower rate and better control of anaesthetic gas flow, and a decreased risk of aspiration [2].

High cuff pressures in laryngeal mask airways or tracheal tubes may result in a reduction or complete occlusion of mucosal perfusion. In laryngeal mask airways, a more generalised pattern of pressure is exerted on periglottic and supraglottic structures while tracheal tube cuff hyperinflation predominantly results in subglottic problems. Although side effects are not easily measured in children, studies in adults showed that higher pressures in laryngeal mask airway cuffs are generally associated with increased morbidity such as sore throat, hoarseness and nerve palsies [3–5]. As a child’s airway is more prone to mucosal damage and swelling than that of an adult, the likelihood of airway morbidity related to hyperinflation of laryngeal mask airway cuffs and consequent mucosal malperfusion is presumably higher in children.

Cuff pressures are not routinely monitored in many institutions. Instead, clinical endpoints are applied to determine optimal positioning and adequacy of the cuff’s seal [6]. Our study investigated in vivo cuff pressures that were generated when cuff inflation was guided by clinical endpoints. Additionally, changes in cuff pressure were assessed over time in children undergoing anaesthesia with and without nitrous oxide.

Methods

  1. Top of page
  2. Summary
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References
  8. Supporting Information

This quality assurance study was approved by the Local Research Ethics Committee before the study was begun. As this study was classified as a quality of care audit, the need for parental consent was waived by the Committee. Six hundred and forty consecutive children aged 0–16 years requiring general anaesthesia were included, receiving either a laryngeal mask airway (LMA Classic® or LMA-Unique®, PAC MED Richmond, Victoria, Australia; sizes 1–4; n = 400) or a cuffed tracheal tube (Mallinckrodt, Hazelwood, MO, USA; sizes 3–7 mm; n = 240). Data for tracheal tube size 3.5 mm are missing because this size is not available in Australia. The choice of airway (laryngeal mask airway or cuffed tracheal tube) and its size was left to the discretion of the anaesthetist in charge. Typically, the size of laryngeal mask airway was chosen based on the patient’s weight (Table 1) and tracheal tube size according to the formula (age/4) + 4, with a size adjustment of the tube by 0.5 mm to compensate for the presence of the cuff.

Table 1.   Manufacturer’s recommended sizes and maximal cuff inflation volumes (ambient room air) for laryngeal mask airways.
Size Patient’s weightInflation volumes
1Neonates/infants up to 5 kg4 ml
1.5Infants 5–10 kg7 ml
2Infants/children 10–20 kg10 ml
2.5Children 20–30 kg14 ml
3Children 30–50 kg20 ml
4Adults 50–70 kg30 ml

At our institution, cuff inflation is routinely performed by anaesthetic technicians and the amount of air used is determined using clinical endpoints but never exceeding the recommendations of the airway device’s manufacturer. Depending on the anaesthetist’s preference, the cuff of the laryngeal mask airway was fully deflated, partially inflated or completely inflated before insertion. After insertion of the laryngeal mask airway, the cuff was slowly inflated until a slight outward shift of the device was noted [6]. When the laryngeal mask airway came to a new stationary position, this was assumed to be the optimal position with sufficient cuff inflation and seal [6]. The laryngeal mask airway was then secured in this position with elastic tape. When using an tracheal tube, the cuff was slowly inflated until the audible leak around the tube disappeared during manual ventilation. During this study, all laryngeal mask airways and tracheal tubes were tested (by auscultation) for an audible leak at an airway inflation pressure of 20 cmH2O. Neither the anaesthetist in charge nor the technician inflating the cuff were aware that this study was being conducted nor were any of the manometer readings communicated to them.

Cuff pressures were measured using a calibrated handheld Portex Cuff Inflator Pressure Gauge (Portex Limited, Hythe, Kent, UK) immediately after final positioning of the airway device and at the end of surgery. Anaesthesia management was not standardised but left to the discretion of the anaesthetist in charge. However, the first manometer reading was performed in the absence of nitrous oxide in all cases and the use of nitrous oxide during the operation was recorded.

The distribution of data was tested using the Shapiro–Wilk test. As data were not normally distributed, the Wilcoxon signed rank test was used to assess differences between cuff pressure. A value of p < 0.025 was considered statistically significant to adjust for multiple comparisons. Results were analysed using SigmaStat 3.11 for Windows (Systat Software Inc, San Jose, CA, USA).

Results

  1. Top of page
  2. Summary
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References
  8. Supporting Information

Patients’ characteristics, intraoperative use of nitrous oxide and duration of anaesthesia are shown in Table 2. In the majority of patients, the measured cuff pressures exceeded the manufacturer’s recommendation independent of the type of airway device used (Figs 1–4). There were no differences in cuff pressures related to which type of laryngeal mask airway was used. In the presence of nitrous oxide, cuff pressures consistently increased during surgery (p < 0.001 for laryngeal mask airways and tracheal tubes of all sizes), whereas cuff pressures remained stable in the absence of nitrous oxide (Figs 2 and 4). In addition, the smaller the airway device, the higher the intracuff pressures in laryngeal mask airways and tracheal tubes (Figs 1–4). In none of the airway devices was an audible leak observed at 20 cmH2O following insertion. None of the patients with a laryngeal mask airway had stridor in the recovery area, whereas three children with tracheal tubes presented with stridor following anaesthesia. All three children had tracheal tube intracuff pressures > 60 cmH2O.

Table 2.   Characteristics of children, use of nitrous oxide (N2O) and duration of surgery under anaesthesia using (a) laryngeal mask airways and (b) tracheal tubes. Values are median (IQR [range]).
(a)Size of laryngeal mask airway
1 (n = 40)1.5 (n = 64)2 (n = 108)2.5 (n = 70)3 (n = 63)4 (n = 55)
Sex; M/F23/1740/2456/5235/35 32/31 35/20
Age; months 1 (1–2 [0–4]) 6 (3–9 [1–11])37 (24–50 [8–95])75.5 (61–98 [44–118])122 (111–140 [80–160])172 (163.5–184 [130–205])
Weight; kg 3.9 (3.2–4.4 [2.8–5.1]) 7.2 (5.7–8.6 [4–13])17 (13.4–18 [9.8–21.2])24 (21–27.3 [19–31.9]) 34.7 (32.15–41.3 [30.2–53]) 61.1 (53.2–76.3 [48.7–105])
Use of N2O11 (27.5%)20 (62.5%)49 (45.4%)28 (40%) 30 (47.6%) 39 (70.9%)
Duration of surgery; min25.5 (20–40 [15–60])40 (30–50 [10–100])40 (30–50 [10–120])40 (25–55 [5–130]) 45(31.25–60 [10–130]) 45 (35–76 [10–140])
(b)Size of tracheal tube
3 (n = 30)4 (n = 30)4.5 (n = 30)5 (n = 30)5.5 (n = 30)6 (n = 30)6.5 (n = 30)7 (n = 30)
Sex; M/F21/915/1518/1215/1518/12 14/16 16/14 14/16
Age; months 2 (1–3 [0–8])15.5 (13–26 [7–42])38 (34–43 [27–55])56.5 (50–63 [48–73])81.5 (77–87 [69–120])121 (108–134 [88–166])153.5 (132–171 [104–192])174 (162–186 [144–191])
Weight; kg 4.4 (3.5–5.3 [2.8–7.32])13.1 (9.8–17.3 [7.2–18.4])18.2 (15.6–21 [13.6–22])22 (18.3–23 [15.5–31.8])29.4 (25.6–32.4 [21.4–42.5]) 37.9 (32.4–42.3 [27–67.2]) 48.3 (40–50 [32.7–71.7]) 56.2 (48.3–64.5 [41.4–95.6])
Use of N2O 6 (20%)10 (33.3%)12 (40%)16 (53.3%)11 (36.7%) 11 (36.7%) 11 (36.7%) 14 (46.7%)
Duration of surgery; min40 (30–45 [15–75])42.5 (35–60 [25–120])40 (35–50 [20–120])42.5 (30–60 [20–130])45 (30–60 [20–130]) 42.5 (30–60 [20–140]) 42.5 (30–60 [15–140]) 50 (40–69 [15–150])
image

Figure 1.  Cuff pressures following insertion of laryngeal mask airways in children. The box plots indicate the median, IQR and 10th/90th percentiles. The maximal recommended intracuff pressure of 60 cmH2O is indicated by the dashed line.

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image

Figure 2.  Cuff pressures of laryngeal mask airways at the end of surgery in children receiving air or nitrous oxide. The box plots indicate the median, IQR and 10th/90th percentiles. The maximal recommended intracuff pressure of 60 cmH2O is indicated by the dashed line.

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image

Figure 3.  Cuff pressure following insertion of tracheal tubes in children. The box plots indicate the median, IQR and 10th/90th percentiles. The maximal recommended intracuff pressure of 30 cmH2O is indicated by the thick dashed line; the thin dashed line shows a suggested pressure of 20 cmH2O for paediatric patients.

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image

Figure 4.  Cuff pressures of tracheal tubes at the end of surgery in children receiving air or nitrous oxide. The box plots indicate the median, IQR and 10th/90th percentiles. The maximal recommended intracuff pressure of 30 cmH2O is indicated by the thick dashed line, the thin dashed line shows a suggested pressure of 20 cmH2O for paediatric patients.

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Discussion

  1. Top of page
  2. Summary
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References
  8. Supporting Information

The results of the present study demonstrate that hyperinflation of the cuff in laryngeal mask airways and tracheal tubes is present in most children when cuff inflation is guided exclusively on commonly used clinical signs.

In converting laryngeal mask airway intracuff pressure into the pressure acting on the mucosa, it must be taken into account that the relationship is non-linear and varies within the pharyngeal structures [7, 8]. Although mucosal pressures in adult sized laryngeal mask airways rarely exceed mucosal perfusion pressures, the incidence of complications increases significantly with higher cuff pressures [3–5]. Potential morbidities range from sore throat to more serious complications such as laryngeal or tracheal lesions, vocal cord paralysis, hypoglossal and recurrent laryngeal nerve injury and even dislocation of the arytenoid cartilages [5, 9–14]. Many morbidities arise from reduced mucosal blood flow because of the raised intracuff pressure and high filling volumes [3, 4]. Although the laryngeal mask airway sealing pressure is higher in smaller sized devices [15–18], a correlation between sealing pressure and position of the cuff has not been established [19]. Moreover, adequacy of ventilation does not necessarily correlate with ideal positioning based both on fibreoptic and radiological assessment [3, 4, 14]. The recommended maximal laryngeal mask airway intracuff pressure is 60 cmH2O. The manufacturer’s guidelines state that the given inflation volumes (Table 1) are maximum values and lower values can suffice to obtain a seal and/or achieve the 60 cmH2O intracuff pressure [6]. Based on in vitro data, only a small fraction of the maximum recommended cuff filling volume was required to achieve a cuff pressure of 60 cmH2O [20]. Furthermore, even in the in vitro setting, the maximal recommended volumes result in hyperinflation of the cuff for almost all paediatric laryngeal mask airway brands and sizes [20]. In vivo data are not yet available. To maintain the blinded status of the anaesthetist and the technician in charge during this study, inflation volumes were not recorded and these individuals were not informed of the manometer reading. However, during routine clinical management, the technicians are instructed to inflate the cuffs slowly while never exceeding the maximal recommended filling volumes (Table 1).

The aim of this study was to evaluate whether or not judgement based on clinical signs (appropriate position of the laryngeal mask airway and seal) was adequate to meet the manufacturer’s recommendation regarding pressure limit. While it is generally assumed that the clinical endpoint is adequate [6], the results of our study show that utilising this technique alone resulted in hyperinflation of the laryngeal mask airway cuffs in nearly all patients, thus potentially increasing morbidity by exerting unnecessarily high pressures on pharygolaryngeal structures. The observed hyperinflation was more marked in smaller sized devices, thus occurring in a population more prone to pathological consequences of airway injury. As expected, the use of nitrous oxide resulted in an increased intracuff pressure.

The tracheal tube cuff should ideally seal the airway without compromising mucosal perfusion. Excessive pressure exerted on the tracheal mucosa is an avoidable factor that can cause damage after intubation with cuffed tubes [21]. It is recommended that tracheal muscosal pressures should be less than 30 cmH2O in adults [22]. No mucosal perfusion values have been reported for children. However, given the lower perfusion pressures in children’s tracheal mucosae, safety limits should be set lower [23]. In adults, an intracuff pressure > 30 cmH2O for 15 min is sufficient to induce histological evidence of tracheal mucosal lesions and impair mucosal blood flow [24–26]. Total occlusion of mucosal blood flow occurs at a pressure of 50 cmH2O [25]. The incidence and severity of tracheal mucosal lesions observed with fibreoptic tracheoscopy is higher in patients with a cuff pressure greater than 30 cmH20 compared with cuff pressures controlled between 20 and 30 cmH2O [27]. In addition, the incidence of sore throat decreases significantly at lower cuff pressures compared with higher pressures, even while pressures remain within the recommended pressure range [28, 29]. In adults, up to 80% of cuffed tracheal tubes have intracuff pressures > 27 cmH2O after intubation [30, 31]. The commonly used endpoint of no leakage led to an increase in cuff pressure beyond the recommended pressure of 30 cmH2O, with even higher pressures observed in the presence of nitrous oxide. This clearly demonstrates the importance of cuff pressure monitoring for all patients.

To avoid the negative effects of high pressures on the mucosa, uncuffed tubes have traditionally been used in children, particularly those under 8–10 years old. This approach was based on the cricoid being a circular structure [32]; thus a tube fitting snugly through the cricoid would leave an air leak at approximately 25 cmH2O of positive pressure, while at the same time providing an adequate seal without a cuff. This also allows a tube of larger internal diameter to be used, thus lowering resistance to airflow and reducing the work of breathing. Cuffed tracheal tubes were avoided as they were thought more likely to produce subglottic trauma, since the cricoid is the narrowest part of the paediatric airway. However, recent studies show that the paediatric cricoid is primarily an ellipsoid structure [33]. This means that if a circular uncuffed tube is inserted into a non-circular cricoid lumen, a reasonable seal can only be attained by significant pressure on the lateroposterior walls of the cricoid. An air leak at 25 cmH2O inspiratory pressure can still be detected in this situation, with the leak occurring solely from the anterior part of the cricoid lumen. The end result is a reasonable seal of the trachea, with a clinically discernable air leak in an uncuffed tube; however, pressure exerted on certain parts of the mucosa is still present and can be excessive and lead to airway trauma. In addition, subglottic trauma can occur from insertion of oversized tubes, whether cuffed or uncuffed. However, when compared to an uncuffed tube, a smaller diameter tube that produces adequate sealing with a high volume low pressure cuff does not wedge within the delicate cricoid and could provide superior protection against aspiration, and better control over anaesthetic gas flows [34]. A recent study showed no significant increase in airway morbidity related to cuffed tubes in infants and children [35] and this is consistent with other reports although morbidity can still be related to inappropriate handling or equipment [36, 37]. Furthermore, the tube exchange rate is lower with the use of cuffed tubes, thus avoiding repeated instrumentation [23]. As there are clear advantages to the use of cuffed, as opposed to uncuffed, tubes in children, the results of this study should lead not to a decreased use in cuffed tubes but to a strict monitoring of cuff pressures during their use [38].

In conclusion, our results indicate that clinical endpoints should not be used as the sole guide for determining cuff inflation in airway devices and a cuff manometer should always be used. Where nitrous oxide is used, especially over a longer period of time, continuous monitoring should be performed in all children receiving cuffed airway devices.

Acknowledgements

  1. Top of page
  2. Summary
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References
  8. Supporting Information

BvUS is supported by a grant of the Swiss Foundation for Grants in Biology and Medicine (SFGBM) in cooperation with the Swiss National Science Foundation SNSF and by the Voluntary Academic Society Basel, Switzerland. The authors thank Joan Etlinger, B.A., Basel, Switzerland, for editorial assistance.

References

  1. Top of page
  2. Summary
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References
  8. Supporting Information

Supporting Information

  1. Top of page
  2. Summary
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References
  8. Supporting Information

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