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).
Anatomical and physiological differences between adults and children
Anatomical differences in the airway between adults and children
Compared to the adult, in the child:
• the tongue is relatively large and the larynx is in a higher position
• the epiglottis is longer and U-shaped
• in the younger child, the narrowest part of the upper airway is the cricoid ring: a tube passing easily through the laryngeal inlet may be too tight at the cricoid ring. Should insertion of a tracheal tube be necessary, its fit is critical. Too small a tube leads to an increase in resistance, large leak, and possible fluid ingress around the tube. Too tight a fit creates a risk of mucosal ischaemia and oedema, leading to stridor at extubation
• the larynx is smaller, so the reduction in diameter imposed by a tracheal tube will have a significantly larger effect on airway resistance to flow. The significance of apparatus dead space in comparison to the child’s total dead space becomes greater the smaller the child (Fig. 12.2). The two together may accommodate a high proportion of the tidal volume with a significant effect on carbon dioxide elimination.
Physiological differences in breathing between adults and children
• The chest wall of the child is more compliant, and contributes little to ventilation. The diaphragm moves less efficiently and contains fewer fatigue-resistant muscle fibres. Alveolar ventilation is, therefore, dependent more on rate; hence the respiratory rate in infants and younger children is higher than in adults. The high rate is achieved at the expense of the end expiratory pause. The latter is important for the efficient action of anaesthetic partial rebreathing systems (see below and Chapter 5).
• The work of breathing is higher in children, consuming relatively more oxygen. Airway maintenance devices and breathing systems, by presenting resistance to flow, contribute further to this. Though infants can cope with increased work of breathing in the short term, diaphragmatic fatigue will set in earlier than in adults.
• Closing capacity in small children may exceed and overlap functional residual capacity. This can be worsened by anaesthesia and the supine position, with resultant hypoxia. The ability to deliver positive end expiratory pressure or continuous positive airway pressure may help to overcome this.
• Compared to adults and older children, infants produce approximately twice as much carbon dioxide and consume twice the amount of oxygen relative to body weight.
Equipment
The anaesthetic machine
From its humble origins as a spartan trolley bearing just cylinders, vaporizers and the odd coffee stain, the anaesthetic machine has evolved into the anaesthesia workstation. The principles of the continuous flow machine remain the same, and this machine is most suited to paediatric practice. Draw-over apparatus presents too great a resistance to breathing to be used by children weighing under 20 kg and, if used, there must be some means of providing continuous flow, such as a manual bellows.3 For certain congenital cardiac conditions, use of air without obligatory oxygen flow is needed to avoid the pulmonary vasodilatation seen with higher oxygen concentrations. This requires a specially adapted anaesthetic machine. Aside from this unusual application, any continuous flow machine will suffice.
Transmission of infection
In an effort to prevent transmission of infection between patients, single-use equipment has become almost ubiquitous for paediatric anaesthesia in the developed world. The quality of components varies, but their build standard is generally ratified for use on one patient on one occasion only. This does not imply inferior quality. Lightweight durable plastics are particularly valuable in paediatric breathing systems, providing flexible low-resistance systems with a reduced tendency to ‘drag’ on other components. For adult patients, single-use breathing systems may be reused, provided an effective airway filter is used to isolate the system and anaesthetic machine from transmissible disease. Evidence is accumulating that paediatric filters are as efficient as the adult versions,4 but as yet the reuse of breathing systems is not encouraged. For more information see the section on breathing system humidification and filtration later in this chapter.
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
Facemasks
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.
Figure 12.6 A. A device for fixing tracheal tubes, The Portex RSP B. The same device locked onto the tube, self-adhesive strips (arrowed) are used to fix the device to the face.
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).
Table 12.4 Recommended sizes and dimensions of laryngeal masks showing also the largest tracheal tube (TT) diameter that may be passed through each size
Figure 12.11 A size 2 (the smallest) reinforced laryngeal mask airway. The knot is for illustrative purposes only.
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.
Other supraglottic airway devices
A number of other supraglottic airways have followed in the wake of the laryngeal mask. Their acceptance has been gradual and unbiased evaluation scarce. The outcome of appeals for new initiatives in further independent assessment of clinical performance is awaited.31
Airway adjuncts
Scaled versions of oral and nasal airway adjuncts exist for paediatric use (Figs 12.12 and 12.13) and are discussed in Chapter 6.
Gaining access to the airway
The laryngoscope
The larynx is usually seen with the direct laryngoscope. A variety of laryngoscope blade profiles exist. The choice is usually dependent upon the age of the patient and the personal preference of the anaesthetist (Fig. 12.15). Many practitioners use a straight blade for infant laryngoscopy; this blade picks up the relatively large epiglottis, affording a better view. For older children, a curved (small Macintosh pattern) blade will suffice. The little finger of the hand holding the laryngoscope may be used to apply external laryngeal pressure to improve the view. Flexible tracheal tube introducers can be used to railroad a tube into a larynx when a direct view cannot be obtained. These are available for use with tubes as small as 2.5 mm internal diameter.
