5.2 Patent Ductus Arteriosus (PDA) & Persistent Pulmonary Hypertension (PPHN)

Patent Ductus Arteriosus (PDA) & Persistent Pulmonary Hypertension (PPHN)

PDA and PPHN represent transitional circulation disorders where the pulmonary and systemic vascular beds fail to adapt correctly to extrauterine life. Although they can coexist, their pathophysiologies and therapies differ significantly.

Patent Ductus Arteriosus (PDA) in the Preterm Infant

In term infants, the ductus arteriosus closes within 24-72 hours of birth. In preterm infants, the ductus often remains patent due to immature smooth muscle, lower sensitivity to oxygen-induced constriction, and higher levels of circulating vasodilating prostaglandins (produced by the placenta and local tissues) and nitric oxide.

Pathophysiology

When PVR falls postnatally, a high-pressure gradient is established between the aorta and the pulmonary artery, driving a left-to-right shunt. This leads to:

1. Pulmonary Overcirculation: Excess blood flow to the lungs causes pulmonary edema, decreased lung compliance, and prolonged ventilator dependence.
2. Systemic "Steal": Blood is diverted away from the systemic circulation, causing hypoperfusion in target organs:
   • Renal: Decreased glomerular filtration rate (GFR) leading to oliguria and elevated creatinine.
   • Gastrointestinal: Ischemia of the mesenteric vessels, significantly increasing the risk of Necrotizing Enterocolitis (NEC) and spontaneous intestinal perforation.
   • Neurological: Fluctuations in cerebral blood flow, increasing the risk of Intraventricular Hemorrhage (IVH).

Clinical Indicators

• Widened pulse pressure: A low diastolic blood pressure (diastolic runoff into the pulmonary artery) resulting in a pulse pressure > 25-30 mmHg.
• Bounding peripheral pulses: Caused by the rapid runoff of blood from the aorta.
• Hyperactive precordium: Visible and palpable chest wall movement due to increased stroke volume.
• Murmur: A systolic or continuous "machinery-like" murmur heard best at the upper left sternal border.

Medical Closure Therapies

When a PDA is hemodynamically significant, pharmacologic closure is attempted using cyclooxygenase (COX) inhibitors to block prostaglandin synthesis.

1. Indomethacin (Indocin)

A potent, non-selective COX inhibitor.

• Regimen: Usually administered as a 3-dose course (0.2 mg/kg IV loading, followed by two doses of 0.1 to 0.25 mg/kg depending on postnatal age, given at 12-to-24-hour intervals).
• Adverse Effects: Transient renal impairment (oliguria, hyperkalemia, elevated creatinine), platelet dysfunction and impaired clotting, and mesenteric vasoconstriction (ischemia).
• Absolute Contraindications:
  • Active bleeding (e.g., active IVH, pulmonary hemorrhage, or GI bleeding).
  • Thrombocytopenia (platelet count < 50,000/mm³).
  • Documented or suspected Necrotizing Enterocolitis (NEC).
  • Renal failure (serum creatinine > 1.6-1.8 mg/dL or urine output < 0.6 mL/kg/hr).

2. Ibuprofen (NeoProfen)

An alternative non-selective COX inhibitor.

• Regimen: 3-dose course (10 mg/kg IV loading, followed by 5 mg/kg at 24 and 48 hours).
• Clinical Advantage: Ibuprofen has a significantly lower affinity for renal and mesenteric vascular beds compared to indomethacin. It achieves similar closure rates but with significantly less oliguria, lower creatinine elevations, and reduced mesenteric vasoconstriction.

Persistent Pulmonary Hypertension of the Newborn (PPHN)

PPHN is characterized by the failure of the normal pulmonary vascular transition at birth, leading to sustained elevation of pulmonary vascular resistance (PVR).

Pathophysiology

Because PVR remains equal to or greater than systemic vascular resistance (SVR), deoxygenated blood is shunted right-to-left through the patent foramen ovale (PFO) and/or patent ductus arteriosus (PDA) into the systemic circulation, causing refractory hypoxemia and acidosis.

Classification and Etiology

• Primary PPHN: Idiopathic; characterized by abnormal muscularization of the pulmonary arterioles.
• Secondary PPHN: Triggered by lung diseases that cause hypoxia and acidemia, such as Meconium Aspiration Syndrome (MAS), Respiratory Distress Syndrome (RDS), neonatal sepsis, or pulmonary hypoplasia associated with Congenital Diaphragmatic Hernia (CDH).

Clinical Assessment and Diagnostic Shunt Testing

When evaluating an infant for PPHN vs. structural cyanotic CHD, specific bedside tests are utilized.

1. Pre- and Post-Ductal Oxygenation Monitoring

• Method: Place one pulse oximeter probe on the right hand (pre-ductal; receives blood before the ductus arteriosus via the innominate artery) and a second probe on either foot or the left hand (post-ductal; receives blood mixed with right-to-left ductal flow).
• Interpretation: A difference of > 10% in oxygen saturation (SpO2) or > 15-20 mmHg in arterial partial pressure of oxygen (PaO2) confirms a significant right-to-left shunt at the ductal level, indicating PPHN. If no difference is seen, shunting may still be occurring solely at the atrial level (PFO).

2. Hyperoxia Test

• Method: Place the infant in 100% fraction of inspired oxygen (FiO2) for 10-15 minutes and obtain an arterial blood gas.
• Interpretation:
  • Cyanotic CHD: PaO2 typically remains < 100 mmHg due to fixed anatomical right-to-left shunting.
  • PPHN / Parenchymal Lung Disease: PaO2 typically rises > 150 mmHg (though in severe PPHN with fixed pulmonary vasospasm, the response may be blunted).

3. Echocardiography

The gold standard diagnostic tool. It rules out structural CHD (such as TGA or HLHS), confirms right-to-left shunting at the PFO/PDA, and estimates pulmonary artery pressures by measuring tricuspid regurgitant jet velocity or assessing flattening/bowing of the interventricular septum.

Therapeutic Management of PPHN

The goal of therapy is to reduce PVR, maintain SVR, and optimize oxygenation to break the cycle of hypoxia, acidosis, and pulmonary vasoconstriction.

1. Inhaled Nitric Oxide (iNO) Therapy

iNO is a selective pulmonary vasodilator. It is inhaled into ventilated alveoli where it diffuses into pulmonary vascular smooth muscle, activating guanylate cyclase to increase cyclic guanosine monophosphate (cGMP), causing smooth muscle relaxation. It is rapidly bound to hemoglobin in the bloodstream, preventing systemic vasodilation.

• Dosing: Initiated at 20 parts per million (ppm).
• Toxicity Monitoring:
  • Methemoglobinemia: Nitric oxide binds with hemoglobin to form methemoglobin, which cannot transport oxygen. Monitor methemoglobin levels (maintain < 2-3%).
  • Nitrogen Dioxide (NO2): NO reacts with oxygen to form toxic NO2 gas. Continuous monitoring of circuit NO2 levels must be maintained (< 0.5-1.0 ppm).
  • Rebound Pulmonary Hypertension: Avoid sudden withdrawal of iNO. Wean incrementally (e.g., to 10 ppm, 5 ppm, then by 1 ppm decrements) to prevent rebound bronchoconstriction and pulmonary hypertension.

2. Ventilation and Oxygenation Support

• Maintain normal functional residual capacity (FRC) to minimize PVR (both atelectasis and alveolar overdistension increase PVR).
• Target arterial pH of 7.35-7.45 and PaO2 of 50-80 mmHg. Avoid aggressive hyperventilation (hypocapnia causes cerebral vasoconstriction, increasing the risk of neurodevelopmental deficits).

3. Sedation and Vasoactive Support

• Minimizing handling and administering sedation (e.g., fentanyl) prevents agitation-induced pulmonary hypertensive crises.
• Maintain systemic blood pressures using volume or inotropes to ensure SVR exceeds PVR, minimizing right-to-left shunting.