For the purposes of this volume only those children of 20 kg in weight or greater will be considered. In practice, this will usually equate with an age of about five years.
Pre-operative assessment in children should be as rigorous as in adults and questions should be addressed to the child even though the parents may answer for them. Most children are healthy but chronic conditions such as asthma, multiple allergies, congenital heart disease and systemic conditions (such as muscular dystrophy) may also be encountered. The presence of one congenital abnormality should stimulate the search for others. Chromosomal abnormalities may be linked particularly with congenital heart disease. Except for true emergency surgery, children with colds or upper respiratory tract infections should have their surgery cancelled and rescheduled to a later date. The inflamed airway is exquisitely sensitive to any kind of manipulation, resulting in laryngeal spasm. Laryngeal spasm in children is particularly dangerous because of the rapid onset of severe desaturation, made more marked by their higher metabolic rate.
There is no universally good premedicant for children, trimeprazine makes many children irritable and uncontrollable in an unpredictable way and the injectable premedicants are probably better avoided because of the distress caused by the injection. Day case admission can result in insufficient time for anxiolytic pre medication to have effect (and the use of sedatives in day surgery may be undesirable). All children should have topical local anaesthetic cream or gel applied to the proposed venepuncture site at least 1 hour before anaesthesia. Drug doses in children should always be calculated on a weight-related basis, a calculation which will give an approximation of the required dose. If dilution of a drug is proposed then each syringe should be labelled with the drug name and concentration. Ambiguity must be avoided at all costs.
Anaesthetic equipment for patients under 20 kg body weight is quite specialised and there is no gradation to adult equipment. For all paediatric patients the breathing system dead space and resistance should be kept to a minimum by avoiding catheter mounts, angle pieces and valves. Controlled ventilation may be preferable because of the inevitable increase in dead space after induction of anaesthesia and the increased work of breathing. The Mapleson E or F system is preferable for patients less than 20 kg but can also be used for heavier patients if the volume of the expiratory limb is more than the calculated tidal volume and the fresh gas flow for spontaneous ventilation is more than 2.5 times minute volume. Tidal volume approximates to 8 ml/kg in the child and a respiratory rate of about 20 per minute is usual. If ventilation is to be controlled then a minute volume divider type of ventilator should only be used for tidal volume settings greater than 300 ml. The reason for this is that below this the ventilator becomes inaccurate in delivery due to the higher proportion of compressible volume related to total tidal volume. For required tidal volumes less than 300 ml either a "T piece occluder' type system should be used or a Mapleson D with a ventilator such as the Nuffield Anaesthesia Ventilator Series 200.
If tracheal intubation is proposed then account should be taken of the increased resistance to breathing that this introduces. The resistance to flow in a tube is inversely related to the fourth power of the radius and so halving the diameter will increase resistance by 16 times. Any tracheal tube will have a smaller internal diameter than the natural airway, particularly if the tube is cuffed. Unless there is a risk of tracheal soiling uncuffed tubes are preferable because of the larger internal diameter that this allows. The anatomy of the child larynx is different from the adult (Figure SC.6). In consequence, the use of cuffed tubes in the < 10 age group renders the subglottic region vulnerable to oedema particularly if an overly large tube is introduced with force. Even small amounts of secretion in a tracheal tube will significantly increase the resistance to gas flow.
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Figure SC.6 Anatomy of the infant larynx
Intravenous fluids in children should be given via a burette giving set or a volume controlled pump and should always be calculated on a weight related basis. A minimum of 2 ml/kg/H should be given to those not receiving oral fluids. Blood replacement can be very difficult to calculate and is based on replacing loss for loss when 10% blood volume has been spilt. Losses are assessed by swab weighing and accurate suction measurement. Blood volume may be estimated as 80 ml/kg in small children falling to 70 ml/kg in adults. Blood loss of 8 ml/kg will, therefore, need replacing during surgery.
Post operative analgesia should not be withheld because the dose calculations are inconvenient. Much of the surgery carried out on children lends itself very well to regional anaesthesia when used to supplement light general anaesthesia and this method provides very high quality early post operative analgesia. Although intramuscular opioids remain popular, there are many other routes of administration for analgesic drugs. Most children over six years of age can use PCA to advantage using skills learned at computer controls. Subcutaneous infusions of opioids may be useful and oral preparations (such as Oromorph®) should not be forgotten. Rectal administration of non steroidal agents (such as diclofenac) is rapidly gaining ground.
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