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10 Neonatal Physiology and Transport

possible to predict lung maturity antenatally by measuring amniotic
fluid phospholipid concentrations.
Air flow is proportional to the fourth power of the radius of the
airway, and a small reduction in calibre (for example, by mucosal
oedema) can have a major effect on resistance to air flow and therefore
on the work of breathing. Decreased ventilation will result in alveoli
being perfused but not aerated, creating an intrapulmonary shunt, with
a fall in peripheral oxygen saturation and an increase in the partial
pressure of carbon dioxide in the arterial blood (paCO ).
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Aspiration of vomitus is common in surgical babies at all phases
of their management and is a leading cause of airway oedema, lung
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contamination, and death. It can be prevented simply by never
allowing a surgically ill baby to be nursed supine. A neonate cannot turn
over to protect his airway, and vomiting in a supine position inevitably
leads to aspiration. Babies are perfectly happy on their sides or prone,
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and the culture of nursing babies supine has little merit. The canard
that it reduces the risk of sudden infant death syndrome (SIDS) is vastly
outweighed by the numbers lost each year to aspiration pneumonia.
Similarly, analgesia is important for postoperative respiratory care, as
a baby in pain will not breathe deeply, or cry, and will have diminished
respiratory excursion, leading to atelectasis, intrapulmonary shunting,
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and ultimately infection. After thoracic or upper abdominal surgery,
adequate analgesia may obviate the need for postoperative ventilation. 13
Immaturity of the respiratory centre is held to be the cause of
apnoea in prematurity. This usually responds to tactile stimulation
but may require treatment with theophylline. The risk of apnoea
following a general anaesthetic remains for up to a year postnatally in
formerly premature babies. All such babies undergoing an anaesthetic
Figure 2.4: Schematic diagram of foetal circulation, with red indicating arterial for whatever reason should be kept under observation, with apnoea
blood, blue indicating venous blood, and purple indicating admixed blood. monitoring, for 24 hours after surgery.
Clinical Evaluation
Because babies cannot vary their tidal volume, their initial response to
is unsustainable. Pulmonary vascular resistance can be increased, and inadequate ventilation is to increase the rate of breathing. Due to the
the foetal circulation reproduced, by hypoxia, acidosis, catecholamine flexible cartilaginous nature of the chest wall, any increase in the work
secretion, hypothermia, or hypoglycaemia, as well as conditions that of breathing is manifest by intercostal, sternal, and subcostal recession
primarily cause pulmonary hypertension. as well as alar flaring. As the neonate tries to increase positive end
The circulating blood volume of a term neonate is in the order of 80 expiratory pressure (PEEP) to maintain alveolar patency, grunting may
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ml/kg body weight. This small volume means that precision is essential occur. The increased work of breathing will eventually tire the baby,
in the prescription of intravenous fluids, as an apparently trivial error who will be unable to sustain these compensatory tactics and will go
of 1 ml/kg per hour in a 3-kg baby will result in an error of 10% of the into respiratory failure.
circulating blood volume by the end of the day. Similarly, all losses Babies with clinical signs of respiratory insufficiency should receive
should be carefully measured and replaced. supplementary oxygen pending investigation with a chest x-ray and
Respiratory Adaptation blood gas analysis, if available. Any increased work of breathing
associated with abdominal distention can often be ameliorated by
During normal delivery, the fluid that has filled the lungs during foetal the passage of an orogastric tube and maintenance of gastrointestinal
life is expelled and the lungs are expanded with air during the first breath. decompression. Viscid tracheal secretions can sometimes be suctioned
Along with lung expansion, there is a reduction in pulmonary vascular resis- following humidification, best effected by nebulisation with saline.
tance and a redirection of blood flow to allow gas exchange.
Neonates are obligatory nasal breathers and obligatory diaphragmatic Nutrition
breathers. Resistance to air flow is increased by nasogastric intubation, and The provision of energy as well as the substrate for growth and devel-
for this reason—as well as the danger of perforation of the cribriform plate opment is critical to the neonate, and the provision of adequate nutrition
during insertion —orogastric intubation is preferred in this group of patients. is particularly important for the developing brain. Perinatal deficiencies
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Abdominal distention of any cause will impair diaphragmatic may have lifelong consequences for the patient, particularly with regard
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mobility and therefore impede breathing. The number of alveoli in the to brain growth and development. Nutrition is also pivotal to wound
neonatal lung is less than 10% of the adult quota, but new alveoli are healing, temperature maintenance, and immune function.
continually added up to 8 years of age. Despite this paucity of alveoli, Babies who start life with the handicap of intrauterine growth
the resting neonate requires more oxygen per kilogram body weight than retardation, and those with surgical disorders that are not promptly
an adult, so the neonate is at risk if oxygen requirements are increased recognised, are at particular risk of neonatal malnutrition. Whereas
or if any pathology diminishes the surface available for gas exchange. normal babies can be fed through the alimentary tract, the surgically ill
Alveolar stability is maintained by surfactant, a phospholipid neonate is frequently unable to tolerate feeding.
wetting agent produced by the type II pneumocyte, which reduces In the developed world, this conundrum is resolved by using total
the surface tension in the fluid lining the alveoli. Adequate levels of parenteral nutrition (TPN), but in many developing countries this is
surfactant are achieved around 35 weeks of gestation. Babies born unavailable. The standard of care in the developed world has evolved
before this are at risk of developing hyaline membrane disease. It is on the back of the availability of TPN and is often inappropriate care