Prematurity
The WHO defines prematurity as childbirth occurring at <37 completed weeks of gestation, irrespective of birthweight. It is divided as: extremely preterm (<28 weeks), very preterm (28–32 weeks), and moderate to late preterm (32–37 weeks). Prematurity is generally associated with an increased rate of neonatal and infant morbidity. Risks increase from low birthweight (LBW) newborns (<2.5 kg) to very low birthweight (VLBW) newborns (<1.5 kg).
1 Outline the anatomical and physiological differences in the premature infant presenting for surgery
2 Be aware of the potential difficulties related to chronic lung disease
3 Discuss an analgesic plan for elective surgery in the ex-premature infant.
2D01; 2D02
Baby J was born at 28 weeks’ gestation, with a birthweight of 1.2 kg. He was admitted to the special care baby unit (SCBU) where he required CPAP and supplemental oxygen, nasogastric feeding, monitoring of blood sugars, and provision of glucose. He is now 37 weeks corrected gestational age (CGA), has been successfully weaned from his CPAP and oxygen therapy, and has been established on oral feeding. He is ready for discharge home but has previously been noted to have an inguinal hernia. Plans are made for him to attend his local paediatric hospital for repair of his hernia. He has an appointment to attend in 4 weeks’ time; by then, he will be 41 weeks CGA.
◆ Anatomical factors: neonates are obligate nasal breathers until 4–6 months of life; this is thought to be because of a relatively large tongue in a small oral cavity and an epiglottis that can reach the uvula. This leaves little in the way of an air space to breathe through. If there is nasal congestion, this can dramatically increase the work of breathing through the narrow nasal passages. The differences between the paediatric and adult airways are magnified due to the size of the premature neonate. The larynx is anterior, and the epiglottis is large and floppy; therefore, many anaesthetists would use a straight-bladed laryngoscope to facilitate endotracheal intubation. The cricoid cartilage is the narrowest part of the airway, and the trachea is short, around 4 cm at term; therefore, precision with tube placement is paramount
◆ Physiological factors: the premature neonate’s diaphragm fatigues easily due to a reduced level of type 1 muscle fibres. Their chest wall is highly compliant, and their accessory muscles of respiration are inefficient. Pathology, such as URTIs, can cause a significant increase in the work of breathing which can result in respiratory distress or apnoeic spells. Neonates have a higher metabolic rate and therefore consume a greater volume of oxygen per kilogram per minute. They also have a very small functional residual capacity (FRC). These two factors combined can lead to extremely rapid rates of oxygen desaturation during periods of apnoea, e.g. during induction of anaesthesia or secondary to illness and exhaustion. The immature CNS and the respiratory centre, in particular, do not respond to hypoxia and hypercarbia in a predictable manner. Hypoxia and hypercarbia can cause respiratory depression and apnoea, as opposed to causing a stimulus to respiratory activity in the older child and adult
◆ Ventilatory support: the premature neonate may require respiratory support by way of both invasive and non-invasive ventilation or supplemental oxygen. Neonates requiring prolonged oxygen support (>28 days) are said to be oxygen-dependent, and some may be discharged on home oxygen. Prolonged ventilation may result in pneumonia, barotrauma, hyaline membrane disease, and residual chronic lung damage.
In utero, the PVR is greater than the SVR due to the presence of fluid within the alveoli, causing hypoxic pulmonary vasoconstriction. As a result, the muscle of the LV is not well developed and is therefore unable to significantly increase the force of ejection brought about by the increased preload with a resultant fixed stroke volume. The major determinant of cardiac output is the HR. The sympathetic nervous system is relatively immature, compared with the more dominant parasympathetic nervous system. This is why hypoxia can result in bradycardia and lead to a significant reduction in cardiac output.
Premature infants have a higher risk of intraventricular haemorrhage with subsequent long-term neurological defects. The risk increases with increasing prematurity, LBW, respiratory distress syndrome, coagulopathy, hypoxia, and acidosis. There is poor autoregulatory control of cerebral blood flow, and fluctuations in the BP may increase the risk of intraventricular haemorrhage. Both overtransfusion and anaemia should be avoided.
Retinopathy of prematurity is related to the gestational age and duration of oxygen therapy. PaCO2 and pH levels also have an affect. Lower levels of oxygen saturation (around 90%) are acceptable in this age group. In neonates born before 32 weeks’ gestation, early low oxygen saturation levels and late high oxygen saturation levels have been found in a meta-analysis to reduce the risk of retinopathy of prematurity.
Preterm infants have a large surface area to body weight ratio. This leads to an increased rate of heat loss. This is compounded with less ‘insulation’ in the form of subcutaneous fat. They are reliant on brown fat stores for non-shivering thermogenesis, but, under anaesthesia, especially with volatile agents, this process is inhibited. Before the age of 3 months, shivering does not occur.
In children <1 month old, there is a real risk of hypoglycaemia due to reduced glycogen storage. They require a source of glucose in their IV fluids if feeding is not established.
Total body water is expressed as a percentage of the body weight. It is higher for a term neonate than it is for an adult, with a term newborn being 75% water (40% extracellular fluid, ECF; 35% intracellular fluid, ICF) and an adult being 60% water (20% ECF; 40% ICF). During the first 6 months of life, there is a gradual decrease of body water as percentage of the body weight to 60%. Infants normally lose weight during the first week after birth. Term newborns usually lose 5–10% of their weight, almost all of which is water loss. This weight loss is greater in preterm than term infants and is associated with a diuresis. The post-natal diuresis is approximately 1–3 mL/kg/hour in term infants and is greater in preterm infants. Preterm neonates have proportionally more water (>80%), and they may lose 10–15% of their weight in the first week of life. Small-for-gestational-age preterm infants may also have a particularly high body water content. Estimated circulating blood volume is 90–100 mL/kg in a premature neonate, 85 mL/kg in a full-term neonate, 80 mL/kg in a child, and 70 mL/kg in an adult.
Neonates have a decreased capacity to concentrate or dilute urine in response to changes in the intravascular volume status and are at risk of dehydration or fluid overload. This is more pronounced in preterm infants. The normal maturation of renal function increases with increasing gestational age.
Poor red blood cell production from immature bone marrow results in anaemia. Thrombocytopenia may occur, and sepsis should be actively excluded. All newborns should receive vitamin K at delivery, as a very small number of newborns (1 in 10 000) may have vitamin K deficiency bleeding.
Reduced cellular and tissue immunity increases the risk of sepsis.
Hepatic enzyme function is immature. There may be difficulty establishing feeding as a consequence of a poor suck reflex. This reflex does not begin until about the 32nd week of pregnancy and is not fully developed until about 36 weeks. Premature infants with LBW are at risk of necrotizing enterocolitis. The exact cause of necrotizing enterocolitis is unknown, and it is not known whether some underlying pathology contributes to premature birth and LBW.
Patients with bronchopulmonary dysplasia require higher peak ventilation pressures and inspired oxygen to achieve an acceptable SpO2. Post-operative nasal CPAP or ventilation may be required.
The risks of post-operative apnoeic episodes increase after general anaesthesia, even in babies born at term. For those born with a gestational age of 32 weeks, the risk of apnoea is <1% after 56 weeks post-conceptual age. With a gestational age of 34 weeks, the risk of occurrence of apnoea is <1% at 54 weeks post-conceptual age.
Spinal anaesthesia is indicated in patients such as this case or where there are significant risks of administering general anaesthesia, e.g. a difficult airway.
Performing spinal anaesthesia in this patient group is technically very challenging, and a good understanding of the anatomical landmarks and their correlation to spinal cord levels is required. There are significant differences in where Tuffier’s line crosses the spinal axis in neonates, compared with older children and adults. The top of the iliac crests is thought to be at the L5–S1 level in neonates and infants, up to the age of 1 year. The spinal cord ends at L3 level at birth, compared to L1/L2 in older children and adults.
The procedure is performed with the patient awake, and it must be borne in mind that the duration of action of a subarachnoid block is much shorter than in an adult. Therefore, an experienced surgeon should be scrubbed and ready to commence the surgical procedure, whilst spinal anaesthesia is performed. When there are bilateral procedures required, prioritization of the most severely affected or highest risk side should take place first.
◆ Raise the ambient room temperature
◆ Minimize heat losses by limiting exposure for procedures, e.g. IV cannulation
◆ Cover the head with a bonnet
◆ Active warming measures, e.g. heating mattress and forced air warmers
◆ Warm and humidify anaesthetic gases.
◆ Simple analgesia, e.g. paracetamol
◆ Local anaesthetic by subcutaneous infiltration, nerve block (ilioinguinal), or neuraxial blockade (caudal)
◆ Avoid NSAIDs in <44 weeks’ gestation
◆ Aim to minimize opioid use.
Before performing any nerve block or central neuraxial procedure, the parent/guardian should be given sufficient information in a way they can understand to give informed consent. This should include both the benefits and reasonable risks. A patient information leaflet could be used to support this.
A caudal is a procedure allowing access to the epidural space to deliver local anaesthesia. It is most commonly used alongside general anaesthesia to provide post-operative analgesia for procedures performed below the umbilicus.
A good understanding of the anatomy of the sacrum is required, and an awareness of the levels of the spinal cord and dural sac termination is essential. In children, the procedure is most commonly performed under general anaesthesia, in the lateral position with hips and knees flexed. There should be a trained assistant present, a working IV cannula sited, and resuscitation equipment easily accessible.
This is an aseptic technique, performed with similar sterile precautions as when performing an epidural. The sacral hiatus should be identified with the sacral cornuae laterally. A 22G needle/cannula should be inserted at around 45°, and, when the sacrococcygeal ligament is punctured, the angle should be flattened off, and the needle/cannula advanced by a few millimetres. It is important to take care not to advance too far, as this can cause a dural puncture.
Time should be taken to allow for the CSF or blood to flow freely from the inserted needle/cannula if inadvertent placement has occurred. The needle/cannula should be aspirated, prior to the administration of local anaesthetic.
The dose of local anaesthetic administered should be predetermined and should not breach recommendations on the maximal safe dosing of the chosen local anaesthetic.
This is a simple technique with a low complication rate when appropriate care is taken. Therefore, a competent anaesthetist could perform this procedure, with supervision if required.
◆ Provides good post-operative analgesia
◆ Minimizes opioid requirement.
◆ Failure/technical difficulty
◆ Risks of intravascular injection
◆ Risks of dural puncture
◆ Rectal perforation
◆ Subcutaneous injection
◆ Urinary retention
◆ Leg weakness
◆ Nerve damage.
No. He should be admitted for apnoea monitoring post-operatively. General anaesthesia can increase apnoea risk for 12–24 hours post-operatively. Apnoeic episodes are common in preterm infants, with risk factors for apnoeas, including:
◆ Gestation age <45 weeks
◆ Preterm <34 weeks
◆ History of apnoeas
◆ Anaemia
◆ Chronic lung disease.
◆ Post-operative analgesia
◆ Glucose-containing IV fluids and blood glucose monitoring until feeding re-established
◆ Oxygen therapy, if required, to achieve an acceptable SpO2 level.
Summary
Delivering safe and effective anaesthesia and analgesia to an ex-premature infant requires meticuluous attention to detail of the basic principles of anaesthesia. Airway and ventilatory management must take account of the fact that many ex-premature infants may have significant chronic lung disease, causing difficult oxygenation throughout the perioperative period, and may be prone to apnoeas in the post-operative period. Fluid management should account for their basal maintenance requirements of fluid, electrolytes, and glucose. Hypothermia is common and may delay recovery, so efforts to minimize heat losses and maintain normothermia should be made at all times. Employing effective regional analgesic techniques, ideally avoiding systemic opioids where possible, should optimize the post-operative recovery.
Chen ML, Guo L, Smith LE, Dammann CE, and Dammann O (2010). High or low oxygen saturation and severe retinopathy of prematurity: a meta-analysis. Pediatrics, 125, e1483–92.
Doyle E and McCormack J (2012). Sacral epidural (caudal) block. In: McLeod G, McCartney C, and Wildsmith T, eds. Principles and practice of regional anaesthesia, pp. 153–7. Oxford University Press, Oxford.