Chapter 30: Pulmonary Hypertension of the Preterm Infant

I. Intensive care

1. Definition

   1. The adult definition of primary pulmonary hypertension (PH) (mean pulmonary arterial pressure greater than 25 mm Hg at rest) is often applied to preterm infants with PH secondary to bronchopulmonary dysplasia (BPD), which may not necessarily be applicable.

   2. Clinically, pulmonary pressures in preterm infants are often reported as a percentage of systemic pressure with pulmonary pressures greater than 50% of systemic pressures being diagnostic for PH, and suprasystemic (>100%) pressures being considered a marker for severe disease.

2. Incidence

   1. As a result of growing awareness among clinicians, PH is more commonly being diagnosed in preterm infants with BPD.

   2. The Tracking Outcomes and Practice in Pediatric Pulmonary Hypertension (TOPP) registry reported that 12% of pediatric PH cases were secondary to chronic respiratory disease in their registry, with BPD being the most common respiratory disease associated with pediatric PH.

   3. Retrospective studies demonstrate that 17% to 43% of preterm infants with BPD may have PH. The only prospective study to date documented that 18% of ELBW infants were diagnosed with PH while in the intensive care setting.

3. Pathophysiology

   1. The interplay of factors leading to the development of PH in the preterm infant is poorly understood, but the development of PH is likely multifactorial. The underlying deficit may be a poorly developed vascular bed in the preterm lung due to alveolar hypoplasia accompanied by dysynaptic airway growth, resulting in a “fixed” component responsive only to lung growth and development.

   2. In vitro work suggests that imbalances in the nitric oxide pathway may also play a role, which may represent a “responsive” component amenable to pulmonary vasodilator therapies.

4. Risk factors

   1. Birthweight and BPD severity play a substantial role in the development of PH, but other factors are involved as not every ELBW infant with severe BPD develops PH.

   2. Infants born small for gestational age (SGA) also appear to be at higher risk for developing PH, but it is unclear whether this is due to a smaller pulmonary vascular bed or an SGA-associated in utero exposure.

   3. Other risk factors identified through retrospective studies include maternal preeclampsia and oligohydramnios. Preeclampsia may act through interference with VEGF pathways.

   4. Theoretical risk factors may include persistent ventilation-perfusion mismatch, intermittent hypoxia or hypercarbia, aspiration from dysphagia or gastroesophageal reflux, infections, suboptimal nutrition for lung growth, shunts that increase pulmonary blood flow (eg, intracardiac shunts, PDAs), and/or airway anomalies that impede gas exchange (eg, subglottic stenosis, airway malacia).

5. Clinical presentation

   1. Signs and symptoms

      1. Physical exam findings and basic clinical test results can be subtle or nonspecific for preterm infants with PH.

      2. Physical findings may include tachypnea, respiratory distress, hypoxemia, and signs of right heart overload or failure, such as parasternal lift or the quality of the second heart sound.

      3. Chest radiography may demonstrate cardiomegaly, right atrial/ventricular dilatation, and/or enlarged central arteries.

      4. EKG findings may include right axis deviation or right ventricular hypertrophy.

      5. Premature infants with PH are also subject to pulmonary hypertensive crises, which represent an acute worsening of PH, often associated ...