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Respiratory Physiology And Support 15

however, the increased oxygen affinity of foetal haemoglobin may as inhaled nitric oxide or oxygen. However, if a significant shunt
limit the delivery of oxygen during periods of hypoxaemia. In the already exists, these interventions are less unlikely to be successful.
newborn infant, the oxygen-haemoglobin dissociation curve gradually Therefore, the optimal strategy is to recognise and treat alveolar
shifts to the right (toward decreased affinity), as adult haemoglobin hypoxia before pulmonary hypertension develops.
levels increase and 2,3-DPG levels rise. Thus, by the time the child Clinical Correlations
is 4 to 6 months of age, the oxygen affinity usually approximates that
of an adult. The following scenarios present typical situations and suggested treat-
Haemoglobin concentrations also change during the first weeks ments for paediatric respiratory problems.
after birth. The normal haemoglobin concentration of newborn Case Scenario #1
infants varies between 16.7 and 17.9 g/dl. Postnatally, haemoglobin Presentation
concentrations transiently increase initially, but then gradually fall You are seeing a newborn infant at 36 weeks gestational age. The
to reach minimum levels at 8 to 12 weeks of age. The primary child was born to a nulligravid 18-year-old mother who presented
explanation for the postnatal decrease in haemoglobin concentration with preterm labor. The child was born after a prolonged labor
is believed to be the decreased stimulus for haemoglobin synthesis and difficult assisted vaginal delivery. At the time of rupture of the
associated with markedly decreased erythropoietin levels in response amniotic membranes, meconium-stained amniotic fluid returned.
to the higher oxygen environment of the neonate. Examination of the placenta revealed evidence of a partial abruption.
During intrauterine life, the placenta functions as the organ for gas The infant’s Apgar scores were 7 at 1 minute and 8 at 5 minutes. Upon
exchange. In the foetus only about 12% of right ventricular output oropharyngeal suctioning in the delivery room, meconium-stained
circulates through the pulmonary circulation. The remaining right secretions were noted. The patient is tachypneic and has peripheral
ventricular output is shunted to the systemic circulation through the cyanosis. Auscultation reveals coarse bilateral breath sounds.
ductus arteriosus and foramen ovale. Blood is shunted away from
the lung due to the high resistance to flow in the foetal pulmonary 1. What is the likely cause of this patient’s respiratory distress?
vasculature, likely due the combined effects of hypoxic pulmonary 2. What are the options for supporting this patient?
vasoconstriction, the local release of vasoconstrictor leukotrienes
and anatomic compression of the pulmonary vasculature by the Treatment
surrounding liquid-filled lung. The newborn patient has respiratory distress syndrome. Given the dif-
At birth, the lung must immediately assume the role of gas ficult delivery and observed meconium staining of the amniotic fluid,
exchange. This remarkable transition depends upon the completion of it is likely this patient’s respiratory distress is due to meconium aspi-
several simultaneous events. First, the collapsed, fluid-filled alveoli ration syndrome. The management of patients with meconium aspira-
of the prenatal lung must be expanded with air, which begins with tion syndrome is primarily supportive. Pulmonary physical therapy
the first breath. At birth, more than 25 mm Hg of negative pressure and frequent suctioning should be instituted to assist with clearance
is required to overcome the surface tension and open the alveoli for of the airways. Oxygen therapy with continuous positive airway pres-
the first time. To accomplish this, the initial inspiratory efforts of the sures may improve the patient’s cyanosis and decrease the atelectasis
newborn infant are extremely powerful, generating negative pressures associated with disruption of surfactant function.
of up to 60 mm Hg. In addition, as the newborn’s lung fills with In many settings in Africa, the use of bubble continuous positive
oxygen, blood flow must be redirected such that poorly oxygenated airway pressure, known as “bubble CPAP”, may be an option in these
blood returning to the right atrium preferentially flows through types of cases. Bubble CPAP implies placing a short binasal pronged
the pulmonary circulation. The redistribution of blood flow in the nasal cannulae into the nasal passages of newborns who need some
newborn occurs as a result of the combined effects of an increase in extra airway pressure while their pathology improves and oxygenation
systemic vascular resistance and an eightfold decrease in pulmonary increases. The nasal cannulae is attached to the oxygen wall outlet
vascular resistance. The former is due to loss of the low-resistance at 8–14 l/min in an effort to maintain pharyngeal pressure with a
placental circulation. The latter is due to expansion of the pulmonary Y-connector so that an expiratory limb is produced. The expiratory
vasculature as the lung expands and the pulmonary vessels are no limb can then be placed in 8 cm water pressure; bubbling that stops
longer compressed by the fluid-filled lung, and due to vasorelaxation indicates an excessive loss of airway pressure and the need to confirm
in response to increased alveolar oxygen tension. As a result of these for leaks. The water pressure provides for continuous airway pressure,
adjustments, systemic pressure and left atrial pressure increase while which could allow for these types of patients to improve without the
right atrial and pulmonary artery pressure decrease, resulting first need for intubation.
in functional closure and then anatomic closure of the foetal shunts Steroid therapy may help to decrease airway inflammation,
(foramen ovale, ductus arteriosus). although corticosteroid therapy has not been shown to improve the
Newborn infants with significant impairments in lung function course or outcome associated with this disease. In addition, antibiotic
(e.g., RDS, pulmonary hypoplasia) that result in hypoxaemia are therapy should be instituted to prevent the frequent complication of
prone to persistent pulmonary hypertension of the newborn (PPHN) secondary bacterial pneumonia, especially if steroids are used.
due to hypoxic pulmonary vasoconstriction. In addition, abnormalities Case Scenario #2
of lung structure are frequently associated with abnormalities of the
pulmonary vasculature (i.e., pulmonary vascular hypoplasia), which Presentation
may also contribute to pulmonary hypertension. As pulmonary blood A 7-year-old boy is brought in after being rescued from a burning
pressure exceeds systemic blood pressure, blood flow will be shunted building. He was sleeping in his home when it caught fire. He was
again through the anatomic foetal shunts (foramen ovale and ductus rescued by a neighbor who heard the child screaming for help and
arteriosus), resulting in a condition referred to as persistent foetal coughing. On examination, the child has soot around the nares and in
circulation (PFC). The development of PFC further exacerbates the posterior pharynx, although there are no obvious facial burns. He
hypoxaemia as unoxygenated blood is shunted away from the is anxious and tachypneic, and his skin color is cherry-red.
pulmonary circulation to the systemic circulation (right-to-left shunt). 1. What is the likely cause of this patient’s anxiety and tachypnea?
Occasionally, PFC will respond to measures that increase systemic 2. What would be your initial treatment?
blood pressure or decrease pulmonary vascular resistance, such