Equipment for paediatric anaesthesia

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Chapter 12 Equipment for paediatric anaesthesia

Children neither look nor behave like small adults. Their requirements in the perioperative setting differ, including those of anaesthetic technique and equipment. In the UK, surgery for children constitutes less than 10% of the total surgery performed and, for financial reasons, development of equipment centres on the market for adult patients. Despite this, there is a rich history of innovation in paediatric anaesthesia. Some of the items we take for granted appear too simple to have been the subject of invention, an example of this being the T-piece breathing system developed from Magill’s system by Dr Phillip Ayre.1 It may not be the easiest to use,2 but lightly modified, remains popular with paediatric anaesthetists the world over (Fig. 12.1).

The differences, both between adults and children and within children of different ages, affect the design of equipment. This is particularly so for those items relating to control of the airway and breathing. Small pieces of equipment designed for use on small patients, must be handled by unwieldy adult hands, and be compatible with international standard fittings. Bulky equipment increases the chance of technical complications, particularly accidental extubation.

Anatomical and physiological differences between adults and children

The magnitude of these differences relate to age. Neonates and infants present the largest variation, the older child increasingly approximates to adult parameters. For the purposes of this chapter, the most important variation is found in the anatomy and physiology of the respiratory system.

Equipment

Regulation of equipment manufacture

The development and testing of new apparatus, and its ease of use, have been reviewed.5,6 Medical devices sold in the European economic area carry a CE mark (European mark of conformity assessment, Conformité Européene) placed by the manufacturer. To achieve this, the manufacturer provides details of risk analyses, performance in standard tests and technical data relating to manufacture of an item of equipment. The Competent Authority, for the UK, the Medicines and Healthcare Products Regulatory Agency, oversees this procedure (see Chapter 28). Medical devices are classified and tested according to potential risk of injury, e.g. a facemask is class 1 (low risk), a cardiac catheter class 3 (high risk).7 The CE marking process does not imply specific clinical testing; most pre-use testing is so-called bench testing, demonstrating equivalence or better function than existing similar equipment. Under these rules, scaling down of adult equipment to paediatric size is acceptable, but may not produce the most effective devices in use. An urge to release a new device meeting minimum standards onto the market is balanced against the need for commercial success; this provides manufacturers with an incentive to produce equipment with demonstrable clinical value. As an example, the laryngeal mask whilst scaled down from adult versions was still subject to specific testing to confirm it retained anatomical suitability for paediatric use.8 Further versions of this device have been subject to post marketing tests of performance in the clinical environment.9,10 A small number of devices without a CE mark are used on patients; these may be custom built or those requiring more clinical testing before a CE mark can be authorized. A device made within a hospital, for use in that hospital, does not require a CE mark, and may be provided with an exemption certificate for specified use elsewhere. In summary, excepting a small number of unique devices, all medical equipment used on children in the UK has a CE mark. This mark provides reassurance of manufacturing standards but does not imply clinical effectiveness, for which independent evaluation should be sought.

Equipment for management of the airway

Apparatus for management of the paediatric airway, from facemasks through to tracheostomy tubes, is outwardly similar to the adult equivalent. Management of the airway in both adult and paediatric practice has been revolutionized by the introduction of the laryngeal mask airway. Similar airway management devices introduced following the laryngeal mask have not so far enjoyed the same level of success.

Facemasks

These should be available in a range of appropriate sizes and form a good seal at the edges, with minimal dead space. Clear plastic masks are less frightening to awake children and can even be scented, though matching the overpowering bouquet of volatile anaesthetic agents presents a major challenge for any perfumier.

A variety of paediatric facemasks exist (Fig. 12.3). To reduce dead space, the Rendell-Baker-Soucek mask was designed anatomically, from casts of children’s faces in the same way as a dental plate is made.11 This mask achieves a seal by virtue of its close approximation to the contours of the face. Other masks require some form of flexible lip or air filled cushion. The lipped round silicone mask (Fig. 12.3E) is easy to apply, providing an excellent seal for infant use. Disposable masks generally employ a cushion seal, the rest of the mask being of rigid construction. Whichever is chosen, it must be easy to hold and seal on the face, and this may well be a matter of trial and error. Attempts to reduce facemask anatomical dead space may be less important than previously thought, the actual increase in physiological dead space with anaesthesia being less than predicted.12

Tracheal tubes

Tracheal tubes are available in sizes and shapes to suit different patients and surgical procedures (Fig. 12.4). The internal diameter of the tracheal tube is the major determinant in airway resistance and hence the size by which tubes are measured and selected. The fit of the tube to each patient is determined by external diameter which varies with tube wall thickness. This is itself determined by the type of tube (Table 12.1), but can also vary for the same tube from different manufacturers.13 Other factors affect tube resistance: connectors, tube length, shape of tube and tendency to collect secretions.

Table 12.1 Dimensions of some non-cuffed infant endotracheal tubes

MANUFACTURER INT. DIAMETER (mm) EXT. DIAMETER (mm)
Portex (silicone) 2.5 3.4
Sheridan (Ped-soft) 2.5 3.6
Portex (ivory) 2.5 3.6
Mallinkrodt (PVC) 2.5 3.6
Rusch (clearway) 2.5 4.0
Mallinkrodt (reinforced) 2.5 4.0
Portex (reinforced) 2.5 4.0
Rusch (rubber) 2.5 4.0
Portex (silicone) 2.5 4.2
Sheridan (Ped-soft) 3.0 4.2
Portex (ivory) 3.0 4.4
Mallinkrodt (PVC) 3.0 4.3
Rusch (clearway) 3.0 4.7
Mallinkrodt (reinforced) 3.0 4.7
Portex (reinforced) 3.0 4.7
Rusch (rubber) 3.0 4.7
Portex (silicone) 3.5 4.8
Sheridan (Ped-soft) 3.5 4.9
Portex (ivory) 3.5 5.0
Mallinkrodt (PVC) 3.5 4.9
Rusch (clearway) 3.5 5.3
Mallinkrodt (reinforced) 3.5 5.3
Portex (reinforced) 3.5 5.3
Rusch (rubber) 3.5 5.3

The decision to intubate, and which tracheal tube to use, is of great significance. Previous attempts to circumvent the problem of tube resistance included the use of shouldered and tapered tubes, but these designs have been shown to confer no real advantage (Fig. 12.5). Below the age of 10 years, uncuffed tracheal tubes were the norm and were believed to minimize the chance of mucosal damage and post extubation stridor. Despite this perceived advantage, the lack of an airway seal with uncuffed tubes can permit fluid to enter the tracheobronchial tree, contribute to atmospheric pollution, lead to inadequate ventilation and induce anaesthesia in surgeons working around the upper airway.

Widespread use of uncuffed tubes has been questioned.14 Cuffed tubes offer advantages (Table 12.2) and are safe when used appropriately.15 New and better designs of cuffed paediatric sized tubes are emerging but more work still remains to be done, particularly on cuff position in preformed (shaped, e.g. RAE type) tubes and on the relationships between tube length and diameter.15 A change in practice will take time, and selecting an uncuffed tube with a leak at an inflation pressure of 25 cm H2O will remain common practice, despite evidence that in short procedures at least, it confers no benefit.16

Table 12.2 A comparison of cuffed and uncuffed tubes

  Uncuffed tracheal tube Cuffed tracheal tube
Seal attempts a seal in the cricoid ring, but seal may not be effective used appropriately forms a good seal below cricoid in larger diameter of trachea
Effect on airway mucosa may be less prone to causing damage less evidence of long term safety currently available
Available lumen maximizes available lumen of artificial airway may reduce the available lumen
Other   – allows use of smaller diameter tubes which may impact post-intubation laryngeal morbidity
– choice of tube diameter less critical
– lower tube exchange rates
– new designs still needed with tube dimensions revised for new paradigm of cuffed tubes.

Tube size selection is critical particularly for uncuffed tubes; formulae provide only a guide to the correct tube size (Table 12.3). Coexisting medical conditions may influence tube size, for example: children with Down syndrome often require a tube 1–2 mm smaller than expected for their age.17 Likewise, the required length of tube can only be estimated. Some tubes incorporate marks intended to guide how far to advance the tube into the larynx under direct vision. Preformed tubes may have a mark indicating the position for fixation over the lip. The placing of such marks is inconsistent across tube sizes and manufacturers, and they should not be relied upon.

Fixation of the tube should aim to prevent displacement, maintain the tube position with head movement, and still be relatively easy to secure and adjust (Fig. 12.6). Simple tape fixation fulfills many of these criteria (Fig. 12.7). Nasal intubation in children is more secure and preferred in the intensive care setting, as the tube tends to move less, reducing trauma to the tracheal mucosa.

Flow at the interface of breathing system and tube is disturbed by changes in diameter and direction. Connectors aim to minimize this by smooth internal surfaces, gradual reductions in diameter and gentle direction changes. The commonest tube connector is the ISO 15 mm; another ISO standard system based on 8.5 mm connectors (Fig. 12.8) appears rarely used. Connectors do not reduce the available lumen as they dilate the tube at the point of insertion. Problems can arise when assembling small thin-walled parts, with buckling of the walls and possible occlusion of the lumen.18 Some older connectors remain in use as they are compact and may offer less resistance to gas flow (Fig. 12.9). Endotracheal tubes allow suction to be applied to the lower airway. To size a suction catheter for use, doubling the tube diameter in mm, gives the appropriate French gauge catheter size.19

Laryngeal Masks

Since its introduction, the Laryngeal Mask Airway (LMA) has been credited with a revolution in anaesthetic technique, and is widely used for spontaneous and controlled ventilation.20

Chapter 6, page 151, ‘Other laryngeal masks’, details the distinction in terminology used to describe these devices. The LMA has also proved valuable in managing the difficult airway, such as that encountered with Pierre Robin sequence.21 It can be used to guide fibreoptic intubation, or even blind passage of a tracheal tube,22 although caution is advised with the latter technique.23 A range of sizes, from neonatal to older child, is available (Fig. 12.10); size selection is based on patient weight (Table 12.4). Where more flexibility of the tube is needed, a reinforced version is available down to size 2 (Fig. 12.11).

The laryngeal mask has lower resistance to gas flow compared with a tracheal tube, causes minimal stimulation of the airway and offers some protection against pulmonary aspiration of fluid from above. The classic design does not protect against aspiration of regurgitated fluid. The ProSeal LMA is designed to overcome this limitation, and early experience of this device with children is encouraging.24 It is recommended the laryngeal mask be inserted in exactly the same way as for the adult patient.8 Positioning and securing the laryngeal mask is generally easily accomplished in children, though some difficulties may be encountered with the smaller sizes, 1 and 1.5.25 In any event, infants usually need intubation and controlled ventilation for anaesthesia of longer duration.

When used for controlled ventilation, a small leak often occurs around the laryngeal mask. Attempts to silence this leak by sealing the patient’s mouth around the LMA stem or placing oropharyngeal packs may result in gastric insuflation and are to be discouraged. Loss of gas is minimal with normal inflation pressure; successful use with controlled ventilation and a circle absorber has been described.26 Pressure controlled ventilation is advantageous, as it allows effective ventilation with lower airway pressures than volume controlled ventilation.27 Oesophageal pH studies during controlled ventilation reveal no greater incidence of gastro-oesophageal reflux with the laryngeal mask compared to tracheal tube or facemask.28

Examination of the airway under anaesthesia and some surgery around the airway can be accomplished with a laryngeal mask in place.29 This may allow better maintenance of anaesthesia and oxygenation during the procedure, compared with jet entrainment or apnoeic techniques. Care is needed with laryngeal mask cuff pressures, particularly if nitrous oxide is employed; unchecked pressures are usually higher than expected and may injure structures in and around the upper airway.30 Overall, the complication rate is low and the laryngeal mask and its variants have a place in anaesthesia for an increasingly wide range of paediatric patients.

Tracheostomy tubes

A full range of uncuffed tracheostomy tubes exists for use in children (Fig. 12.14). To avoid endobronchial intubation, the intratracheal length is kept short; hence accidental decannulation is easily achieved.