FRCA Notes

The Effects of Prematurity

This topic covers the curriculum item: 'describes the special problems of the premature and ex-premature neonate'.

Resources

  • In the UK, infants are resuscitated from the 23rd week of gestation
  • A term baby is one born >37 weeks gestation
  • Those <37 weeks post-conceptional age are 'pre-term' infants
  • Those up to 44 weeks post-conceptional age are 'neonates'

  • Babies can be classified by birth weight:
    • <2.5kg = low birth weight
    • <1.5kg = very low birth weight
    • <1kg = extremely low birth weight

  • 50% of those born >25 weeks and weighing >600g will survive without major morbidity
  • Those born >25 weeks and weighing <1000g will have some neurodevelopmental impairment in 75% of cases

Surfactant

  • Preterm infants are predisposed to respiratory distress;
    • Surfactant only actively secreted from 24 - 26th week
    • Alveolar development increases from 32nd week onwards

  • The lack of surfactant in preterm infants causes:
    • Increased alveolar surface tension
    • Higher alveolar instability, leading to collapse
    • Impaired gas exchange

  • Management involves synthetic surfactant, ventilation (non-invasive, invasive or oscillatory) and prolonged oxygen supplementation
  • These infants always need positive pressure ventilation under anaesthesia

Apnoeas

  • Preterm neonates are more prone to respiratory apnoeas (pause ≥20s or pause associated with hypoxia/bradycardia)
  • The cause is not fully understood; possibly related to neurological maturity
  • The clinical response to hypoxia is a period of hyperventilation, followed by apnoea and bradycardia

    • Risk factors increasing likelihood of apnoea
    Hypoglycaemia
    Hypo- or hyper-thermia
    Hypocalcaemia
    Anaemia of prematurity
    Lower gestational age

  • A term neonate has a minimal risk of post-operative apnoea after 4 weeks post-natal age and is typically suitable for day surgery
  • A preterm neonate should be monitored postoperatively until the 60th post-conceptional week i.e. not suitable for day surgery

  • Management of apnoeas
    • Avoid GA if possible
    • Stimulate the child and open their airway
    • Use face-mask oxygen or nasal CPAP
    • Close monitoring
    • Consider caffeine, either as a prophylaxis or for rescue
    • May require intubation

Ventilation

  • Ventilation in the premature neonate may be complicated by air leaks, necessitating chest drain insertion
  • Prolonged intubation and consequent airway oedema increases the risk of developing subglottic stenosis
  • Fewer Type 1 (slow twitch) muscle fibres in the diaphragm, so pre-term infants are more prone to respiratory fatigue

  • Congenital heart disease is 2x as common in preterm infants

Circulation

  • In the term neonate, the normal transitional circulation from foetal to adult is completed in the first few days of life
  • Preterm infants are at higher risk of a PDA (50%), leading to:
    • Excessive pulmonary blood flow and cardiac failure
    • Low systemic blood pressures and its complications e.g. necrotising enterocolitis (NEC)
    • Failure to wean from ventilation

Persistent Pulmonary Hypertension of the Newborn

  • Persistent pulmonary hypertension of the new-born is more common in the preterm infant
  • It is compounded by factors increasing pulmonary vascular resistance
  • Management includes increasing FiO2 or inhaled NO

  • Neurological injury is extremely common in preterm infants

  • Intraventricular haemorrhage (50% of those <1.5kg)
  • Retinopathy of prematurity, particularly if <30 weeks gestation
    • Exacerbated by high inspired oxygen concentrations in the neonatal period
  • Increased sensitivity to non-depolarising NMBA
  • Increased sensitivity to opioids (due to altered MOP and KOP)
  • Higher risk of hypothermia, with consequent alterations to drug metabolism, oxygen demand and work of breathing

  • Nephrogenesis only complete in the 36th gestational week
  • In the immature kidney there is inefficient handling of water, sodium and other electrolytes
  • Prone to hypocalcaemia
  • There is a relatively low GFR until the 2nd year of life

  • Increased risk of hypoglycaemia and aspiration in very low birth weight infants due to an inability to coordinate sucking/swallowing
  • Premature bowel may not tolerate enteral feeding
  • Risk of NEC
  • Increased unconjugated bilirubin levels

Hepatic

  • Hepatocyte number is only 20% of adult levels in the term infant, and even less so in the pre-term one
  • Proportionally larger free water compartment so may require higher doses of drugs than adults despite metabolic immaturity
  • Little gluconeogenetic ability in the immature liver + high metabolism predisposes to hypoglycaemia; may require 10% dextrose infusion

  • Relatively higher blood volume in the premature infant; 90 - 100ml/kg
  • Haemoglobin levels are equivalent to the term infant but fall rapidly in the face of physiological insult
  • Relative absence of vitamin K-dependent clotting factors required IM vitamin K supplementation

  • Impaired cellular and humoral immunity; only limited protection from maternal immunoglobulins