Management of the pediatric airway can be challenging because of specific anatomical features of the pediatric airway, craniofacial syndromes, low functional residual capacity, and the greater oxygen demand of children.1 Additionally, patient cooperation for techniques such as preoxygenation.2 or awake flexible bronchoscopic (FB) intubation (the standard approach in adults with an anticipated difficult airway)3,4 is limited. Thus, hypoxia and insufficient ventilation during anesthesia are more frequent in children than in adults.5 Supraglottic airway devices (SGAs) are the central rescue tools according to the airway guidelines of the Association of Paediatric Anaesthetists,6 and FB intubation through SGAS is recommended in case of failed intubation.6

Table SGA size with weight and age ranges and the corresponding appropriate endotracheal tube sizes
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Case reports and case series have proven the concept of FB intubation via SGAs for difficult pediatric airway management.7,8,9,10,11,12 Some SGAs have also been used for elective FB-guided intubation.13,14 Passage of the endotracheal tube (ETT) through the lumen of the SGA and removal of the SGA after successful intubation, however, can be impeded by a mismatch of the inner diameter of the SGA and the outer diameter of the ETT or the ETT cuff’s pilot balloon. This problem was mentioned by Jagannathan et al. for the Ambu® Aura-i™ size 1.5.13 The authors chose to cut the cuff’s pilot balloon to remove the SGA after intubation and subsequently repair the cuff with a 22G angiocatheter. Similarly, Weiss et al. reported a size mismatch between the cuff’s pilot balloon and the LMA classic™ sizes 1.0 to 2.0 or 2.5.15

Incompatibilities of pediatric SGAs and ETTs might be known to many pediatric anesthesiologists but not to a broader anesthesia community. Although anesthesiologists need to know which combination of pediatric-sized SGAs and ETTs can safely be used for intubation, data regarding the compatibility of SGAs and ETTs for intubation and SGA removal after intubation are largely missing. This in vitro study evaluates 1) the feasibility of passing ETTs through pediatric SGAs, as performed during intubation, and 2) the feasibility of SGA removal over the ETT. Combinations of several pediatric SGAs and pediatric cuffed and uncuffed ETTs were used. The study also provides a reference table facilitating the choice of appropriate SGA-ETT combinations.

Methods

This in vitro study evaluated the feasibility of passing various ETTS through a variety of pediatric SGAs and removing the SGAs over the ETTs, as performed during intubation with pediatric SGAs and subsequent SGA removal. The study was performed at the Department of Anesthesiology and Pain Medicine at the University Hospital Bern, Switzerland. It did not involve any study subjects, and therefore approval from the local institutional review board or registration in an international trials registry was not required.

Procedures and outcome measurements

After generous lubrication (Lary Phary Spray™, Dr. Panzer GmbH, Potsdam, Germany) of the ETTs and SGAs, each tested ETT was passed through each SGA as would be performed for intubation through the SGA. The feasibility of passing the ETT was noted. The SGA was then removed over the ETT as would be performed clinically during removal of the SGA from the patient’s pharynx, leaving the ETT in the trachea. The feasibility of removing the SGA over the ETT was noted. This evaluation was performed independently by two investigators (MKB and LT), each of whom was experienced in the use of SGAs, intubation, and intubation through SGAs.

For each SGA-ETT combination, the outcome parameters were 1) the feasibility of ETT passage through the SGA, as necessary for intubation, and 2) the feasibility of SGA removal over the ETT (i.e., passage of the ETT and the cuff’s pilot balloon through the SGA, as is necessary for SGA removal). The two investigators independently graded each SGA-ETT combination regarding the two outcome parameters. Whenever resistance was felt, requiring more than virtually no force to pass the ETT through the SGA, the respective combination was again tested by both investigators to establish consensus. Use of only minimal pushing without force was allowed for passing the ETT through the SGA and for removing the SGA over the ETT. Results were compared with manufacturers’ recommendations to evaluate the agreement of their recommendations with our results.

Evaluated SGA and ETTS

We evaluated eight commonly used single-use, pediatric SGAs (Fig. 1A) and four cuffed and two uncuffed ETTs (Fig. 1B). For all SGA sizes 1.0-2.5 and for all ETTs with an inner diameter of 3.0-5.5 were assessed. We did not physically modify any of the SGAs.

Fig. 1
figure 1

Supraglottic airway devices (SGAs) and endotracheal tubes included in this study. A: From left to right: Air-Q® inflatable, Air-Q®sp, Ambu® AuraGain™, Ambu® Aura-i™ (upper images); Ambu® AuraOnce™, Ambu® AuraStraight™, i-gel®, LMA Unique™ (lower images). Note the different angles of the SGA (all size 1.5). B: From left to right: Mallinckrodt™, Rüschelit™, Microcuff™, Sheridan™ cuffed, Portex™ uncuffed, Sheridan™ uncuffed (all size 3.5)

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The SGAs studied included the following: Air-Q® inflatable and Air-Q®sp (self-pressurized, both Cookgas® LLC, St. Louis, USA, distributed by Mercury Medical, Clearwater, FL, USA); Ambu® AuraGain™, Ambu® Aura-i™, Ambu®, AuraOnce™, Ambu® AuraStraight™ (all from Ambu A/S, Ballerup, Denmark); i-gel® (Intersurgical Ltd., Wokingham, Berkshire, UK); LMA Unique™ (Teleflex Medical Europe Ltd., Athlone, Co Westmeath, Ireland). The Air-Q inflatable, Air-Qsp, Ambu AuraGain, and Ambu Aura-i are SGAs that are specifically marketed for tracheal intubation and are used for this purpose.13,14 The four pediatric standard SGAs - Ambu AuraOnce, Ambu AuraStraight, i-gel, LMA Unique - are not specifically marketed for tracheal intubation but are used for this purpose.12,16

The ETTs studied included the cuffed pediatric ETTs Mallinckrodt™ (Covidien, Mansfield, MA, USA), Rüschelit™ (Teleflex Incorporated, Limerick, PA, USA), Microcuff™ (Microcuff GmbH, Weinheim, Germany), and Sheridan™ (Teleflex Incorporated, Limerick, PA, USA) and the uncuffed ETTs Portex™ (Smiths Medical International Ltd., Hythe, UK) and Sheridan™ (Teleflex Inc., Limerick, PA, USA).

Clinically relevant combinations of SGA and ETTS

According to the manufacturers, the choice of the correctly sized SGA is based on the patient’s weight. In contrast, the rule commonly used to choose the correctly sized ETT is based on the patient’s age. Thus, to identify the clinically relevant SGA-ETT combinations, we translated the patient’s weight to his or her age using growth curves from the World Health Organization (Table).Footnote 1 Clinically relevant ETT sizes were then identified for all SGAs. This conversion was accomplished using the following formulas, according to the Resuscitation Council UK Paediatric Advanced Life Support GuidelinesFootnote 2 and departmental guidelines:

$${\text{Cuffed ETTs}}:{\text{inner diameter}} = {\text{age in years}}/ 4+ 3. 5$$
$${\text{Uncuffed ETTs}}:{\text{inner diameter}} = {\text{age in years}}/ 4+ 4.0$$

Endotracheal tubes within a half-size smaller and a half-size larger than the calculated size were defined as clinically relevant. We then assessed whether passage of the ETT and removal of the SGA were possible with the clinically relevant combinations.

Data analysis and statistics

Results are summarized graphically and are shown in the figures. No statistical tests were necessary or applied. Power calculation was not necessary for this feasibility study.

Results

All possible combinations of the eight SGAs and six ETTs were tested. Results for the combinations recommended by the manufacturers are given in Fig. 2.

Fig. 2
figure 2

Overview of the possibility of passing various endotracheal tubes (ETTs) through various SGAs and of subsequently removing the SGAs over the ETTs

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Passage of cuffed ETTs through the SGA

Intubation with the cuffed ETTs was possible with all combinations recommended by the manufacturers, except for the LMA Unique, whose manufacturer does not provide recommendations for endotracheal intubation. Nevertheless, passage of the ETTs was possible with this SGA with a wide range of combinations, similar to the other SGAs. The widest range of possible combinations was found with the Air-Q inflatable and the Air-Qsp, which can be used with a size 5.5 ETT in SGAs size 2.0 or larger. This was followed by Ambu AuraGain and Ambu Aura-i, which are also marketed for intubation. With these SGAs, passage of all sizes of tested ETTs was possible with a size 2.5 SGA. The smallest range of possible combinations was found for Ambu AuraStraight, with which only size 1.0 could be used in conjunction with an ETT size 3.0. For standard pediatric SGAs, Ambu AuraOnce and the LMA Unique provided a better range of possible combinations with an ETT (i.e., for SGA sizes 1.0 and 1.5, intubation with ETTs of sizes 3.5 and 4.0, respectively, were recommended).

There was no difference regarding the feasibility of passing an ETT for intubation through the SGA with the various cuffed ETTs. That is, passage of the ETT was possible with all recommended combinations of the four cuffed ETTs tested. The only exception was the LMA Unique, for which there are no manufacturer’s recommendations regarding intubation, and passage of the ETT through the SGA was possible with different ETT sizes depending on the type of ETT used (Fig. 2).

Supraglottic airway device removal after intubation with cuffed ETTS

In contrast to passage of the ETT through the SGA as performed during intubation, passage of the SGA over the ETT as performed during removal of the SGA after successful intubation was not feasible with all SGA-ETT combinations recommended by the manufacturers because the smaller ETTs had an outer diameter of the cuff’s pilot balloon that proved to be larger than the outer diameter of the actual ETT. Therefore, with some of the smaller ETTs the cuff’s pilot balloon became trapped in the airway lumen of the SGA, mostly at the level of the bite block. The only SGAs for which removal was possible with all recommended SGA sizes were Air-Q inflatable and Air-Qsp. In contrast, SGA removal was not possible with Ambu AuraGain, Ambu AuraOnce, or i-gel sizes 1.0 and 1.5, or with Ambu Aura-i, Ambu AuraStraight, or LMA Unique sizes 1.0, 1.5, and 2.0. Here, it was due to differences in the inner diameter of the SGAs, which was largest in Air-Q inflatable and Air-Qsp (Fig. 3). There were no differences with regard to feasibility of SGA removal with the various ETTS.

Fig. 3
figure 3

Visual comparison of the inner diameter of two SGAs. Ambu® Aura-i™ (left) has a much smaller inner diameter than Air-Q® (right). Both SGAs are size 1

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Passage of uncuffed ETTS through the SGA and removal of the SGA with uncuffed ETTS

Passage of the uncuffed ETTS was possible with all combinations recommended by the manufacturers (Fig. 2). Again, Air-Q inflatable and Air-Qsp offered the broadest range of possibilities. No recommendations regarding intubation are available for LMA Unique, although passage of the ETT was possible with the same combinations as with Ambu AuraGain and Ambu Aura-i (Fig. 2). For all SGAs where passage of the uncuffed ETT for intubation was possible, removal of the SGA was possible as well. For Air-Q and all Ambu SGAs, there were no differences regarding the two uncuffed ETTs. For the LMA Unique size 2.0, passage of the ETT was possible with a size 5.0 Sheridan but not with a size 5.0 Portex.

Comparison of the results with the manufacturers’ recommendations

The manufacturers’ booklets for AirQ inflatable, Air-Qsp, Ambu AuraGain, Ambu Aura-i, Ambu AuraOnce, Ambu AuraStraight, and i-gel included specific instructions regarding intubation with the SGA. The manufacturer’s LMA Unique booklet gives no information about intubation with the SGA. The existing recommendations are shown in Fig. 2 (black: not recommended; all others: recommended). None of the manufacturers specified whether the given recommendations regarding the sizing of the ETTs were for cuffed or uncuffed ETTs.

Recommendations by Ambu on how to perform intubation through the SGA differs among their various SGA models:

  • Ambu AuraGain recommendations are the most specific: Direct flexible scope-assisted tracheal intubation is recommended by railroading the ETT over a flexible fibrescope. Ambu specifies that the ETT should be fully deflated, implying the use of cuffed ETTs. They do, however, also indicate that “for specific combinations of AuraGain and ETT variants for pediatric patients, it is not possible to remove the AuraGain after the ETT is placed through the mask. They state that both cuffed and uncuffed ETTs are compatible with AuraGain but recommend using uncuffed ETTs in pediatrics if the SGA is to be removed after intubation.

  • Ambu Aura-i removal is possible, with no limitations regarding smaller sizes or cuffed ETTs. This contrasts with our study results, which showed that removal of Ambu Aura-i in sizes 1.0, 1.5, and 2.0 is not possible with cuffed ETTs.

  • For Ambu AuraOnce, the manufacturer recommends inserting an exchange catheter through the SGA with fibreoptic guidance, followed by removing the SGA and railroading the ETT over the exchange catheter.

  • For Ambu AuraStraight, direct intubation with a fully deflated ETT and FB guidance is recommended. It is also recommended that the SGA be left in place.

We noted that the manufacturers’ booklets for specific SGAs are not all the same in different languages, and it can be cumbersome to find the relevant information. For example, the German manufacturer’s booklet for Ambu AuraOnce does not indicate compatible ETT sizes, whereas the English version of the booklet does so. The manufacturer’s booklet of i-gel indicates compatible ETT sizes only. It does not indicate whether they are cuffed or uncuffed. Nor does it give further instructions on its use.

Thus, the manufacturer’s recommendations for Ambu Aura-i and i-gel are in conflict with our results: Its recommendations give a false impression that intubation and SGA removal are possible with all recommended sizes, whereas we found that this is not true for Ambu Aura-i sizes 1.0, 1.5, and 2.0 or i-gel sizes 1.0 and 1.5.

Clinically relevant combinations

Clinically relevant SGA-ETT combinations are shown in the Table. The study results are summarized for the clinically relevant SGA-ETT combinations in Fig. 4. Interestingly, several clinically relevant combinations showed limitations with regard to the possibility of passing the ETT or removing the SGA. The only exceptions were Air-Q inflatable and Air-Qsp, with which passage of the ETT and removal of the SGA were feasible with all clinically relevant combinations. All other SGAs displayed limitations, which even included SGA size 2.5 and uncuffed ETTs, indicating that size mismatches are clinically relevant and not limited to small children.

Fig. 4
figure 4

Overview of the possibility of passing cuffed and uncuffed ETTs through various SGAs and of subsequently removing the SGAs over the ETTs. Only the clinically appropriate SGA-ETT combinations are shown. Clinical appropriateness was determined by correlating the SGA size, corresponding weight and age, and corresponding ETT size, as shown in the Table

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Discussion

This study assessed combinations of eight pediatric SGAs and six ETTS with regard to passing the ETT through the SGA and removing the SGA over the ETT as performed during intubation through an SGA and subsequent SGA removal. Passage of the ETT through the SGA was possible with many SGA-ETT combinations, but SGA removal proved impossible for many combinations with cuffed ETT because the ETT cuff’s pilot balloon was often larger than the inner diameter of the SGA. This problem was not addressed in many manufacturers’ booklets. We also found that intubation through an SGA or SGA removal is impossible with many clinically relevant SGA- ETT combinations.

Intubation through an SGA is an important technique for difficult airway management in children.17,18,19 Whereas it is usually a straightforward procedure in adults, important limitations are noted in children. For example, the relatively short length of the ETTs in relation to the pediatric SGA could result in the cuff lying at or above the level of the glottis or could lead to ETT dislocation during SGA removal.16,20 Another limitation of intubation through pediatric SGAs is the size mismatches of the inner SGA diameter and the ETT (we provide guidance on possible SGA-ETT combinations in the Table). These mismatches, which inhibit tracheal intubation and/or subsequent SGA removal, are not clearly documented in the manufacturers’ booklets. Thus, prior to anesthesia induction, each SGA-ETT combination must be assessed. Also, a reference table, such as the one in Fig. 2, can be used for guidance, which might be particularly useful for emergency airway management and failed intubation. We strongly recommend that each anesthesia department adapt Fig. 2 for their own purpose and check the compatibility of SGA and ETTs before use. For all combinations, generous lubrication is absolutely necessary.

Our study showed that, for some combinations, intubation was possible but SGA removal was not. The latter could be explained by the inner diameter of the SGA being too small to allow passage of the ETT cuff’s pilot balloon, which often became stuck at the level of the bite block. AirQ® and AirQ®sp had the largest inner diameters (Fig. 3). All other SGAs showed mismatches with the ETTs. This point is particularly important for Ambu® Aura-i, for which the user manual gives misleading information regarding possible combinations. Despite the fact that Ambu Aura-i is specifically designed for intubation, SGA removal was not possible with it in size 1.0, 1.5, or 2.0. In contrast, SGA removal was possible with Ambu AuraOnce size 2.0, which is not specifically marketed as an intubation aid. The problem of the cuff’s pilot balloon not fitting through Ambu Aura-i13 and the classic LMA15 has been mentioned previously but had not been systematically assessed.

For a difficult pediatric airway, the Association of Paediatric Anaesthetists guidelines recommend attempting fibreoptic intubation through an SGA and to leave the SGA in place.6 Deflation of the cuff could possibly avoid pressure necrosis, and the SGA might stabilize the ETT, thereby avoiding accidental extubation. In some cases, SGA removal is needed for longer-term ventilation or for surgical purposes. Even when the cuff’s pilot balloon fits through the SGA, accidental extubation could occur during SGA removal. In this context, a crucial point is the length of the ETT relative to the length of the SGA.20 Because larger (and longer) uncuffed ETTs fit through a specific SGA, the use of uncuffed ETTs for intubation through SGA has been proposed.16 It is known, however, that when using uncuffed ETTs (compared with cuffed ETTs) a higher proportion requires tube exchange.21 This situation is particularly cumbersome in children with a difficult airway. The use of cuffed ETTs in children has become standard and is currently preferred over the use of uncuffed ETTS for most children.21,22,23 The use of a Cook Airway Exchange Catheter (Cook Medical Inc, Bloomington, IN, USA) has been suggested to avoid accidental extubation when removing the SGA.17,19 This technique could also be used in cases in which the cuff’s pilot balloon does not fit through the SGA. In this case, after SGA insertion and intubation with an uncuffed or cuffed ETT, both ETT and SGA are removed over an airway exchange catheter over which a cuffed ETT is then placed. This technique, however, involves several airway manipulations that increase the risk of perforation with the airway exchange catheter.24 An SGA such as Air-Q - which does not require these maneuvers for intubation because its inner diameter is large enough to allow direct intubation with a cuffed ETT - might be advantageous. As an alternative in case of a mismatched SGA and ETT cuff’s pilot balloon, the balloon could be torn off for SGA removal. It has been proposed to then re-inflate the balloon and block the tube by means of an angiocatheter.13 Special repair kits for torn cuff pilot balloon lines exist, but they might be unavailable in an emergency situation. In addition, data on whether they provide a good, long-term seal are limited.

Because this study was performed in vitro, the usefulness of the various SGAs for intubation must be further evaluated in vivo. Each combination must be tested prior to use. Particularly, the question about the best way to remove an SGA after successful intubation remains unanswered. Direct SGA removal with the help of a stabilizer rod and SGA removal with an airway exchange catheter followed by reintubation over the same catheter are options. Furthermore, as already shown,16 the ETT must be long enough to protrude sufficiently from the SGA into the trachea, which depends on an SGA-ETT match and the airway anatomy.

Conclusions

We showed that passage of ETTs through pediatric supraglottic airways as performed during intubation was possible with all recommended SGA-ETT combinations. Removing the SGA, however, was not possible for the smaller SGA sizes when using cuffed ETTs. This limitation of endotracheal intubation via pediatric SGA is not clearly described in all manufacturers’ booklets, and anesthesiologists should be aware of this lack of information. Our study also showed that there are limitations regarding the passage of an ETT and removal of the SGA for clinically relevant SGA-ETT combinations for all SGAs up to size 2.5, except for Air-Q inflatable and Air-Qsp. We provide a reference table for use in clinical practice.