Paediatrics

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Chapter 9 Paediatrics

General Paediatrics

Definitions

• Neonate – first 28 days of life or <44 weeks post-conception
• Infant – 1 month to 1 year
• Child – >1 year to adolescence
• Low birth weight – 2500 g at birth
• Premature – less than 37 weeks.

Physiological changes at birth

• Tactile stimulus triggers respiratory centre. Inflation of lungs reduces pulmonary vascular resistance.
• Increased flow to lungs increases left atrial pressure, closing the foramen ovale.
• Cessation of placental flow increases SVR. Combined with decreased PVR, flow through ductus arteriosus reverses. High PaO2 causes ductal smooth muscle to constrict, and closure occurs (reversible for several days).

Fetal circulation is shown in Figure 9.1.

image

Figure 9.1 Fetal circulation.

Neonatal physiology

CVS

• Tendency to revert to fetal circulation for 2 weeks is triggered by acidosis and ↑ PaCO2.
• Relatively little contractile tissue in heart (30%), so increased cardiac output achieved by increased heart rate rather than stroke volume. Ventricular thickness equal by 6 months.
• Cardiac index is 2–3 times that of an adult.
• Heart rate at term is 120/min. Rises to 160/min by 1 month and decreases to adult rates by 15 years.
• PNS more developed than SNS. Therefore there is a tendency to bradycardia. Response to adrenergic drugs is diminished.
• Ductus arteriosus reopens following fluid overload.

Respiratory

• Obligatory nasal breathing until 5 months. Nasal passages account for 30–50% of airway resistance. May be unable to convert to mouth breathing if there is nasal obstruction.
• Large tongue obstructs the airway and makes laryngoscopy difficult. Epiglottis is longer, narrower and angled away from the axis of the trachea.
• Larynx is higher (C3–C4) and more anterior than in adults (C4–C5).
• Narrowest part of upper airway is cricoid cartilage. Even minimal oedema causes a large increase in airway resistance (Hagen–Poiseuille law).
• Short trachea at term: ≈︀4 cm.
• Tendency to apnoea if <50 weeks post-conceptual age, due to immature respiratory centre.
• Negative pressure of 40–80 cmH2O is required for initial lung expansion.
• Surfactant produced from 32 weeks by type II alveolar pneumocytes.
• No bucket handle rib movement, therefore increased minute volume is achieved by increasing respiratory rate.
• Diaphragmatic > intercostal respiration. Fewer type I muscle fibres used in prolonged work so earlier diaphragm fatigue.
• Rapid respiratory rate of 32 breaths/min.
• FRC <CC due to decreased outward recoil of chest wall.
• Small FRC, high O2 consumption; therefore rapid desaturation.
• Increased laryngeal irritability with susceptibility to laryngospasm.
• Increased susceptibility to respiratory infection.

GI

• Acute gastric dilatation is common, so consider a NG tube.
• Lower oesophageal sphincter incompetence predisposes to aspiration pneumonia.
• Immature hepatic function and reduced blood flow, increasing half-life of drugs excreted by hepatic metabolism. Phase I reactions attain adult levels within 1 week; phase II reactions within 3–4 months.

Renal

• Immature renal function with reduced GFR; therefore reduced ability to excrete drugs, dependent upon renal clearance.
• Reduced concentrating ability.
• 50% N2 forms new tissue; therefore less renal N2 load.

CNS

• Immature blood–brain barrier with increased permeability to lipid-soluble drugs.
• Motor nerve endings differentiate to form end plates at 26–28 weeks, but process still incomplete at term.
• Increased risk of intraventricular haemorrhage, particularly with abrupt fluctuations in cerebral blood flow and CVP, due to underdeveloped perivascular connective tissue.
• Increased proportion of low-affinity μ2 opiate receptors (mediate respiratory depression).
• Increased sensitivity to volatiles due to increased progesterone, increased β-endorphins, immature blood–brain barrier and decreased protein binding.

Blood

• 70% HbF at term; P50 = 2.7 kPa (3.5–4.0 kPa in adult) with leftward shift of O2 dissociation curve.
• Blood volume 80 mL.kg−1 at term.
• Physiological anaemia is greatest at 3–4 months, with Hb = 10 g.dL−1.

Change of haematocrit with age is shown in Figure 9.2.

image

Figure 9.2 Changes in haematocrit with age.

Temperature

• Increased surface area:body weight ratio. Therefore increased heat loss.
• Thermoneutral temperature at term = 33°C.
• Noradrenaline > adrenaline released with stress to trigger non-shivering thermogenesis in brown fat. Brown fat stores are limited. Shivering does not occur before 3 months.
• Immature thermoregulatory mechanisms, further inhibited by GA.
• Hypothermia causes acidosis, persistent fetal circulation, hypoxia, diaphragm fatigue, intraventricular haemorrhage and coagulopathy.

Glucose

• Minimal glycogen reserves for gluconeogenesis.
• Poorly developed glucose homeostasis.
• High glucose requirements at term. Tendency to hypoglycaemia in small-for-dates babies, infants of diabetic mothers and sick neonates.

Pharmacokinetic and pharmacodynamic differences

• Decreased lean body mass.
• Increased total body water (increased central volume by 50%).
• Decreased total body fat. Therefore, increased VD of water-soluble drugs, decreased VD of fat-soluble drugs.
• Decreased protein binding.

General anaesthesia

NCEPOD RECOMMENDATIONS FOR PAEDIATRIC ANAESTHESIA

• Anaesthetist must have sufficient paediatric experience
• One anaesthetist in each hospital must be responsible for paediatric anaesthesia
• Most problems occur in children <3 years
• Neonates (<28 days) must be anaesthetized in specialist units.

Weight (kg)

• 1–8 years = (age + 4) × 2
• >9 years = age × 3

Preoperative fasting

Half-life of gastric fluid (saline) is 11 min, prolonged by fat and glucose. Recent studies show that prolonged fasting may actually decrease gastric pH and increase volume. Clear fluids administered 2–3 h preoperatively do not alter gastric residual volume and cause less distress. See Association of Paediatric Anaesthetists Guidelines 2007 (below) for preoperative fasting guidelines.

CONSENSUS GUIDELINE ON PERIOPERATIVE FLUID MANAGEMENT IN CHILDREN, V1.1

Association of Paediatric Anaesthetists of Great Britain and Ireland 2007

Executive summary

1. Children can safely be allowed clear fluids 2 h before surgery without increasing the risk of aspiration.
2. Food should normally be withheld for 6 h prior to surgery in children aged 6 months or older.
3. In children under 6 months of age it is probably safe to allow a breast milk feed up to 4 h before surgery.
4. Dehydration without signs of hypovolaemia should be corrected slowly.
5. Hypovolaemia should be corrected rapidly to maintain cardiac output and organ perfusion.
6. In the child, a fall in blood pressure is a late sign of hypovolaemia.
7. Maintenance fluid requirements should be calculated using the formula of Holliday and Segar

Body weight Daily fluid requirement
0–10 kg 4 mL/kg per h
10–20 kg 40 mL/h + 2 mL/kg per h above 10 kg
>20 kg 60 mL/h + 1 mL/kg per h above 20 kg

8. A fluid management plan for any child should address three key issues:

i. any fluid deficit which is present
ii. maintenance fluid requirements
iii. any losses due to surgery, e.g. blood loss, 3rd space losses.

9. During surgery, all of these requirements should be managed by giving isotonic fluid in all children over 1 month of age.
10. The majority of children over 1 month of age will maintain a normal blood sugar if given non-dextrose containing fluid during surgery.
11. Children at risk of hypoglycaemia if non-dextrose containing fluid is given are those on parenteral nutrition or a dextrose containing solution prior to theatre, children of low body weight (<3rd centile) or having surgery of more than 3 h duration and children having extensive regional anaesthesia. These children at risk should be given dextrose containing solutions or have their blood glucose monitored during surgery.
12. Blood loss during surgery should be replaced initially with crystalloid or colloid, and then with blood once the haematocrit has fallen to 25%. Children with cyanotic congenital heart disease and neonates may need a higher haematocrit to maintain oxygenation.
13. Fluid therapy should be monitored by daily electrolyte estimation, use of a fluid input/output chart and daily weighing if feasible.
14. Acute dilutional hyponatraemia is a medical emergency and should be managed in paediatric intensive care.

Preoperative assessment

Examine for congenital defects, cardiorespiratory pathology due to prematurity, hypoglycaemia and hypocalcaemia.

Premedication

Bradycardia secondary to cholinergic effects of drugs (halothane, suxamethonium) and upper airway stimulation is reduced with anticholinergic premedication.

Intubation

• Tracheal tube size (>1 year) = age/4 + 4 mm
• Tracheal tube length = age/2 + 12 cm.

Volatiles

MAC is lower in the neonate, but increases to a peak at 6 months before declining. Age-related change in MAC of sevoflurane is less pronounced. Gas induction is fast because alveolar ventilation is large compared with FRC, high cardiac output and lower blood:gas solubility of volatiles in neonates. Sevoflurane produces less laryngeal irritation than desflurane and is more suitable for gas induction. Baroreflex response is maintained with isoflurane and sevoflurane but not with halothane or desflurane.

Volatiles depress respiration, depress intercostal muscles and ventilation becomes diaphragmatic. Therefore, intubate if <5 kg or <44 weeks post-conceptual age. Volatiles reduce SVR, worsening a right-to-left shunt if persistent fetal circulation is present.

MAC of nitrous oxide is not related to age. N2O may cause marked cardiovascular depression in neonates and diffusion into bowel may impair ventilation.

Variation in MAC of isoflurane with age is shown in Figure 9.3.

image

Figure 9.3 Variation in the MAC of isoflurane with age.

Barbiturates

Reduced VD of lipid-soluble drugs reduces redistribution of thiopentone thus prolonging recovery. Induction doses: 2–4 mg/kg in neonates; 7–8 mg/kg in infants; 5–6 mg/kg in older children.

Propofol

Pain on induction, more excitatory phenomenon and less postoperative nausea and vomiting than with thiopentone. Greater depression of laryngopharyngeal reflexes than with thiopentone so more suitable for use with laryngeal mask. Larger VD, faster metabolism and clearance. Therefore, a larger induction dose of 2.5–3.5 mg/kg is required.

Only licensed as an induction agent in children >3 years because of case reports of a syndrome of lactic acidaemia, bradyarrhythmias, hypotension, lipaemia and oliguria.

Benzodiazepines

Diazepam has similar VD to adult values but greatly prolonged t1/2 (20–50 h in neonates; 8–14 h in children). Midazolam is more suitable in children, with a t1/2 of 1.5 h.

Opioids

Increased sensitivity due to immature blood–brain barrier, reduced protein binding and decreased glucuronidation. Reduced clearance of all opioids. High doses reduce stress response and prevent pulmonary hypertension associated with airway instrumentation.

Muscle relaxants

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