1 Department of Anaesthesia and 2 Department of Otorhinolaryngology, University Childrens Hospital, Steinwiesstrasse 75, CH-8032 Zurich, Switzerland
*Corresponding author. E-mail: markus.weiss@kispi.unizh.ch Declaration of interest. The investigated paediatric cuffed tracheal tubes were ordered from local distributors and partially provided without charges. No financial support was obtained for the presented work. Dr Weiss and Dr Gerber are actually involved in designing a new cuffed paediatric tracheal tube in co-operation with Microcuff GmbH, Weinheim, Germany.
Accepted for publication: July 31, 2003
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Abstract |
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Methods. In 15 series of cuffed (11) and uncuffed (four) paediatric tracheal tubes (ID: 2.57.0 mm) from four different manufacturers the following dimensions were measured: outer diameter of the tube, position and largest diameter of the tube cuff inflated at 20 cm H2O and position of depth markings and compared with age-related dimensions.
Results. Outer diameters for tubes with similar IDs varied markedly between manufacturers and between cuffed and uncuffed tracheal tubes from the same manufacturer. Cuff diameters at 20 cm H2O cuff pressure and cross-sectional cuff area at 20 cm H2O cuff pressure did not always cover maximal internal age-related tracheal diameters and cross-sectional areas. Placing the tube tip in the mid-trachea, the cuffs of cuffed tubes with ID 3.0, 4.0, or 5.0 mm would become positioned within the larynx. If the cuffs were placed 1 cm below the cricoid level, many of the tube tips would be dangerously deep within the trachea. Only five of the 11 cuffed tubes had a depth marking. In many of these tubes the distances from depth marking to tube tip were greater than the age-related minimal tracheal length.
Conclusion. Most cuffed paediatric tracheal tubes are poorly designed, in particular the smaller sizes. A better design of cuffed tubes with a short high-volume, low-pressure cuff, cuff-free subglottic space and adequately placed depth markings are urgently needed.
Br J Anaesth 2004; 92: 7888
Keywords: anaesthesia, paediatric; children; equipment, tubes tracheal, cuffed
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Introduction |
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However, there were concerns that cuff hyperinflation could cause tracheal mucosal injury (oedema, ulcerations, circular necrosis of the subglottic region) with the risk of complications such as stridor after extubation or subglottic stenosis.15 16 The cuff will also reduce the internal diameter available for the tracheal tube and if tubes with a smaller internal diameter must be used, higher airway resistance, increased work of breathing, and difficult tracheo-bronchial suctioning may result.17
The design of cuffed paediatric tracheal tubes, in particular the position and size of the cuff and depth markings is an underestimated issue.16 Data that are available to compare the design of cuffed tracheal tubes for neonates, infants and children in relation to age-related anatomic data are limited.18
Our goal was to evaluate the design of paediatric cuffed tracheal tubes (CPTT) from different manufacturers and to relate them to anatomical airway measures of the trachea from birth to adolescence.
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Methods |
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Anatomical airway measures are calculated from the data of Pettersson and Ringertz to give the lower and/or upper normal limits (2 SD).22 The data were based on normal chest films obtained from 170 children and CT-examinations obtained from 130 children.23 24
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Results |
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The outer diameters of the cuffed tubes varied markedly for a given ID, both between tubes from different manufacturers (00.9 mm) and between cuffed and uncuffed tracheal tubes from the same manufacturer (01.1 mm) (Table 3). In smaller tubes up to ID 4.5 mm, the outer diameters indicated by the manufacturer were the same or larger than the minimum age-related internal tracheal diameter for both the Khine and the Motoyama formula.
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Discussion |
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Outer diameters
Variation in tracheal tube wall thickness is related to the nature of the material (Latex, PCV), risk of kinking, presence of wire reinforcement, and variations as a result of manufacturing. This results in different outer diameters for tubes with identical internal tube diameter (Table 3). Most anaesthetists are probably not aware of differences in outer tube diameters because tracheal tubes are chosen according to the internal diameter. This leads to possible use of oversized, ill-fitting tubes so that the tube may need to be changed or there could be risk of subglottic damage. In addition, the effective outer tracheal tube diameter of cuffed tubes includes the deflated cuff which can be considerably larger than the OD printed on the tube shaft and varies with cuff type and manufacturer.28 29 Because outer tube diameters for a given internal diameter from different manufacturers varies by as much as 0.9 mm, it is not surprising that several formulas to predict proper tube size have been proposed for cuffed and uncuffed tubes in children.2 19 20 21 3032
Cuff diameters
Tracheal tube cuffs seal better if inflated to a higher pressure. To avoid high-cuff pressures, HVLP cuffs are now standard in adult use.33 They are based on the principle that at 20 cm H2O cuff pressure the cross-sectional area of the cuff corresponds to about 150% of the internal cross-sectional area of the trachea. Thus, HVLP cuffs seal the trachea by filling the internal tracheal lumen at low pressure. We found that none of the CPTT that we studied up to an ID of 4.5 mm met the requirements of a HVLP tube cuff, although some did in the larger sizes. In addition, many of the cuff diameters and cross-sectional areas were the same as or even smaller than age-related maximal dimensions (Table 4). Consequently, cuff pressures of more than 20 cm H2O would be needed to seal the tracheal lumen. Although cuff pressures about 2530 cm H2O are accepted as the upper limit of safety for adults, no data exist in children about cuff pressure limits and lower cuff pressures are preferable.34
Cuff position
In most of the cuffed tubes, the upper border of the cuff corresponds to the upper border of the depth marking of the next larger sized uncuffed tracheal tube. Thus, the cuff would lie in the subglottic space, between the vocal cords, or even in the supraglottic space if the tubes are placed either according to radiological criteria or to an age-related formula for predicting depth of tube insertion (Fig. 2).35 36 This is probably why two manufacturers provide HVLP cuffs only from tube size ID 5.0 mm and higher (No. 9 and 12). Rüsch provides a cuffed tube with Murphy eye (No. 14) only from tube size ID 5.0 mm upwards, whereas the corresponding tube in the Magill version is provided from size ID 2.5 mm. The Sheridan Tracheal Tube Cuffed Magill (No. 3) seems to be least likely to allow a laryngeal cuff position. However, the use of this tube is limited by the availability of only integer values of ID up to ID 6.0 mm, which reduces the chance of an adequate seal, because the tubes are used over an age range of 4 yr.
Excessive subglottic pressure can cause mucosal ischaemia, fibrosis, and lead to stenosis, by compression of the mucosa against the non-expandable cricoid cartilage. For the same reason, the cuff must not be intra-laryngeal. In addition, sharp folds and edges of the cuff membrane, particularly if the cuff is deliberately deflated, can damage the airway by cutting the mucosa when the tube moves during the respiratory cycle. This leads to granulation tissue formation, fibrosis, and intra-laryngeal web formation around the tracheal tube.37 The cuff should be located below the cricoid ring, at the level of the tracheal rings, which are able to expand. Secondly, a tracheal tube cuff within the larynx can cause vocal cord palsy, perhaps by compression of the recurrent laryngeal nerve between the cuff and the thyroid lamina.38
If the cuff of the tube is placed below the cricoid or if external cuff palpation is used to locate the tube,39 then some tube tips will be too far down the trachea, particularly in tubes with a Murphy eye and a long cuff (Fig. 3). The reduced margin of safety of cuffed paediatric tracheal tubes even with the cuff placed within the larynx has been noted by Ho and colleagues13 and is a serious problem with current cuffed tubes. Endobronchial intubation can occur with head-neck flexion or cranial migration of the carina in laparoscopic surgery or the Trendelenburg position.4044
Depth markings
Depth markings on CPTT are essential to allow a cuff-free distance below the vocal cords to the cricoid level for the above reasons and to avoid over-insertion. 4547 Only five of 11 of the cuffed tubes that we investigated had a depth marking (Fig. 2). As the upper border of the depth marking is placed at the level of the vocal cords, almost all of these depth markings were too high up the tube shaft and corresponded to the minimal tracheal length for that age-group (Table 5 and Fig. 3). Generally, appropriate placement of cuffed tubes in the trachea and safety during head flexion will need shorter cuffs and adequate depth markings to guarantee a cuff position below the cricoid and a tip far enough above the tracheal carina. The length of the cuff and the presence of a Murphy eye are important determinants of final cuff position in CPTT. Thus short tube cuffs should be used and a Murphy eye must be avoided to allow placing the tube cuff more distally on the tube shaft.
Formulae for selection of cuffed paediatric tracheal tubes
A satisfactory cuffed tube size in children depends on the size of outer tube and cuff diameter so that an air leak around the tube at 20 cm H2O can be established with the cuff not inflated (fit) and no air leakage occurs at a cuff pressure of 20 cm H2O (seal). The two age-related predictors of size for cuffed tracheal tubes in children older than 2 yr are limited both generally and specifically for different tube brands. Using the Motoyama formula19 (ID=[age/4]+3.5) will give an adequate seal more often but more tubes will be too large (according to Table 2). On the other hand using the Khine-formula4 (ID=[age/4]+ 3) fewer tubes will be too large but many will not adequately seal the trachea (Tables 3 and 4). Such facts may reflect the inadequacies of the formulae for appropriate tube choice as much as poor design. However, as indicated by Tables 3 and 4, using a larger tube gives a better seal but less fit and inadequate long cuffs, and a smaller tube would result in a better fit but worse seal. Thus, rather than trying to change these formulae, the tubes should be adapted to meet expected anatomical data and the manufacturer should indicate for the appropriate age group for a specific sized tracheal tube.
Table 3 shows that in small children even with the Khine formula, using a cuffed tube 1 mm smaller than an uncuffed tube, there remains a risk that the tube will be too large for the trachea. The internal diameter of the cricoid is even slightly smaller than the trachea in children below 810 yr. However, further reduction of ID below the size than recommended by Khine is not suitable, because it further increases tube resistance, makes IPPV more necessary and restricts suctioning. In addition, cuffed tubes with an ID 2 and 2.5 mm are not widely available, if at all. Thus, in very small children (<3 kg) uncuffed tubes are still required.
We did not assess all commercially available cuffed and uncuffed paediatric tracheal tubes nor did we study special preformed tubes such as oral or nasal RAE tubes. However, we studied a representative set of widely used paediatric tubes. Good cuff position in preformed oral and nasal tracheal tubes is of particular interest but this can only assessed in vivo. We did not compare our measurements with the dimensions claimed by the manufacturers, nor variations between different lots, although within one brand, dimensions can vary considerably between production lots and manufacturing sites.
We found differences in outer tube diameters of current commercially available cuffed paediatric tracheal tubes. The position and size of the tracheal tube cuff, and the absence of an adequate depth marking are major limitations for correct tracheal tube placement. Ideally, a cuffed paediatric tracheal tube should have a HVLP with a short cuff length, adequate depth markings and not allow the cuff to be inflated in the subglottic region.
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