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Summary Table

Component	Detail
Risk Factors	Congenital heart disease; maternal diabetes, rubella, phenylketonuria; family history of CHD; associated with genetic syndromes (DiGeorge syndrome/22q11.2 deletion, especially PA/VSD)
Etiology	Failure of pulmonary valve development during embryogenesis, resulting in complete obstruction of right ventricular outflow tract
Patient Presentation	Cyanotic newborn within hours to days of life; cyanosis worsens as the ductus arteriosus closes; may present with tachypnea without respiratory distress
Classic Physical Exam	Central cyanosis unresponsive to supplemental oxygen; single S2 (absent pulmonary component); continuous murmur of PDA or collaterals; no ejection click
Key Diagnostic Results	Echocardiography shows absent pulmonary valve opening, determines RV size and coronary anatomy; CXR may show decreased pulmonary vascular markings with normal or small heart size (PA/IVS) or boot-shaped heart (PA/VSD); hyperoxia test fails to raise $PaO_2$ above 100 mmHg
Management	Prostaglandin $E_1$ (PGE1) infusion to maintain ductal patency is the immediate lifesaving step; definitive treatment is staged surgical repair
Keywords	"Cyanotic newborn," "cyanosis unresponsive to oxygen," "single S2," "duct-dependent pulmonary circulation," "absent pulmonary valve," "prostaglandin infusion"

1. Pathophysiology

Pulmonary atresia is a cyanotic congenital heart defect characterized by complete absence of anatomic continuity between the right ventricle and the pulmonary artery. The pulmonary valve fails to develop, creating a membranous or muscular plate where the valve should be. This means the right ventricle cannot eject blood into the pulmonary circulation through normal pathways.

Because the right ventricle has no forward outlet, systemic venous blood returning to the right heart has no direct route to reach the lungs. The infant's survival depends entirely on alternative pathways for pulmonary blood flow. The two principal shunts that sustain life are the patent ductus arteriosus (PDA), which allows aortic blood to flow retrograde into the pulmonary arteries, and the patent foramen ovale (PFO) or an atrial septal defect (ASD), which permits right-to-left shunting at the atrial level so that blood can reach the left heart and then be redirected to the lungs via the PDA. This is why pulmonary atresia is classified as a duct-dependent lesion: once the ductus arteriosus undergoes physiologic closure in the first hours to days of life, pulmonary blood flow drops precipitously and the infant develops profound, life-threatening cyanosis.

The severity of presentation depends heavily on the subtype. In PA with intact ventricular septum (PA/IVS), the right ventricle is often hypoplastic because it has been pressure-loaded against a blind outflow tract throughout fetal life. The high RV pressure may force the development of sinusoidal communications between the RV cavity and the coronary arteries, creating an RV-dependent coronary circulation. This is a critical concept: if the coronary blood supply depends on the high-pressure RV, surgical decompression of the RV can cause coronary steal and myocardial ischemia. In contrast, PA with VSD functions similarly to severe tetralogy of Fallot. The VSD allows right-to-left shunting at the ventricular level, and the pulmonary blood supply often comes through major aortopulmonary collateral arteries (MAPCAs) rather than through a well-formed ductus alone.

The net result in both subtypes is obligatory mixing of oxygenated and deoxygenated blood, producing systemic arterial desaturation. The degree of cyanosis correlates directly with how much blood ultimately reaches the pulmonary vascular bed.

2. Classification and Clinical Manifestation

Subtype	RV	Septum	Coronary Anatomy	Source of Pulmonary Blood Flow	Clinical Features
PA with Intact Ventricular Septum (PA/IVS)	Hypoplastic to severely hypoplastic RV; thick, hypertrophied wall	Intact ventricular septum	May have RV-dependent coronary circulation via sinusoidal connections	PDA (primary), PFO/ASD for atrial-level shunting	Severe cyanosis at birth; single S2; no VSD murmur; hepatomegaly from RV failure; tricuspid regurgitation murmur may be present
PA with VSD	Variable RV size (may be near-normal)	Large, unrestrictive VSD	Normal coronary anatomy (typically)	PDA and/or MAPCAs	Cyanosis severity varies with collateral supply; continuous murmur from MAPCAs; may resemble severe TOF clinically; boot-shaped heart on CXR

An important clinical distinction: neonates with PA/IVS tend to present earlier and more critically because the RV contributes almost nothing to forward flow, and ductal closure is catastrophic. Those with PA/VSD may have a more variable timeline depending on the extent of collateral vessels, though they remain critically ill.

3. Diagnostic Workup

Test	Purpose	Expected Findings
Hyperoxia test	Differentiate cardiac from pulmonary cyanosis	$PaO_2$ remains below 100 mmHg despite 100% $FiO_2$; confirms right-to-left shunting
Chest X-ray	Initial imaging	PA/IVS: decreased pulmonary markings, normal or small heart; PA/VSD: decreased markings, boot-shaped heart (similar to TOF), possible asymmetric lung vascularity from MAPCAs
Echocardiography	Best initial AND confirmatory test in neonates	Demonstrates atretic pulmonary valve, RV size/morphology, presence or absence of VSD, ASD/PFO, PDA patency, tricuspid valve size, and coronary sinusoidal connections
Cardiac catheterization	Preoperative planning	Defines coronary anatomy (critical in PA/IVS to detect RV-dependent coronary circulation); delineates MAPCA anatomy in PA/VSD; measures RV pressure
CT angiography / MRI	Anatomic delineation	Maps pulmonary artery confluence and MAPCA origins in PA/VSD; useful for surgical planning
Pulse oximetry screening	Neonatal screening	Pre- and post-ductal saturation difference or low saturations (<90%) trigger further evaluation

The diagnostic sequence in practice begins with pulse oximetry in the newborn nursery. A cyanotic neonate who does not improve with supplemental oxygen should undergo a hyperoxia test: this is the bedside test that separates cardiac from pulmonary causes of cyanosis. If $PaO_2$ fails to rise above 100 mmHg on 100% oxygen, the cause is presumed cardiac until proven otherwise.

Echocardiography is then the single most important diagnostic study. It is both the best initial test and the most accurate noninvasive test. On echo, you will see a blind-ending right ventricular outflow tract with no valve leaflet excursion, and the examiner can assess the size of the RV cavity, the tricuspid valve annulus diameter (which correlates with RV development), and whether coronary sinusoidal communications exist.

Cardiac catheterization is reserved for preoperative planning. In PA/IVS, catheterization is essential to determine whether the coronary circulation is RV-dependent, because this finding changes the entire surgical strategy. In PA/VSD, catheterization maps the MAPCA anatomy to guide unifocalization procedures. This is a high-yield testing point: the question stem may ask what additional study is needed before surgical intervention.

4. Management and Treatment

Phase	Intervention	Details
Immediate stabilization	Prostaglandin $E_1$ (PGE1) infusion	Starting dose 0.05 to 0.1 mcg/kg/min IV; maintains ductal patency; begin immediately upon clinical suspicion
Supportive care	Airway management, monitoring	Intubation if apnea develops (PGE1 side effect); avoid high $FiO_2$ (does not help and may accelerate ductal closure reflex); IV access, thermoregulation
PA/IVS: surgical	Depends on RV and coronary anatomy	If RV adequate and no RV-dependent coronaries: RV decompression (surgical valvotomy or catheter-based perforation + balloon valvuloplasty) with or without BT shunt. If RV hypoplastic: systemic-to-pulmonary shunt (modified Blalock-Taussig shunt, 3.5 mm Gore-Tex graft) as initial palliation, followed by eventual single-ventricle palliation (Glenn at 4 to 6 months, Fontan at 2 to 4 years). If RV-dependent coronary circulation: avoid RV decompression (risk of coronary ischemia); proceed directly to single-ventricle pathway or consider cardiac transplantation
PA/VSD: surgical	Staged complete repair	Modified BT shunt or PDA stenting in neonatal period for palliation; unifocalization of MAPCAs (connecting collaterals to native pulmonary arteries); definitive repair includes VSD closure + RV-to-PA conduit placement when pulmonary artery anatomy is adequate
Long-term	Follow-up	Lifelong cardiology surveillance; conduit replacements as child grows; endocarditis prophylaxis; monitoring for arrhythmias; exercise restriction counseling

The single most important management concept for the exam is this: PGE1 is the bridge to survival. The moment you suspect a duct-dependent cardiac lesion in a cyanotic neonate, start prostaglandin infusion. Do not wait for echocardiographic confirmation. The key side effect of PGE1 is apnea, so the infant should be monitored in an ICU setting with intubation capability readily available.

The surgical decision tree hinges on two anatomic questions. First, is the RV large enough to support a biventricular circulation? The tricuspid valve z-score is the primary metric; a z-score greater than -2 to -2.5 generally supports biventricular repair. Second, is the coronary circulation RV-dependent? If sinusoidal connections supply a substantial portion of the coronary bed, decompressing the RV would drop RV pressure and cause myocardial infarction. This is a classic exam trap: the test may describe a PA/IVS patient with coronary sinusoids and ask about the next step, where the wrong answer would be RV outflow tract surgery.

For PA/VSD, the concept of unifocalization is testable. MAPCAs arise from the aorta and supply lung segments in a disorganized fashion. The surgeon must connect these collaterals into a unified pulmonary arterial tree before definitive repair can be performed.

5. Differential Diagnosis and Distractors

Differential Diagnosis	Why It Is Similar	Key Discriminator
Tetralogy of Fallot (TOF)	Cyanotic newborn, boot-shaped heart on CXR, RV outflow obstruction, right-to-left shunt	TOF has a stenotic but patent pulmonary valve with infundibular narrowing; patients have tet spells (hypercyanotic episodes) that improve with squatting; pulmonary atresia has complete obstruction with no antegrade pulmonary flow
Tricuspid atresia	Cyanotic newborn, single S2, duct-dependent physiology	Tricuspid atresia shows absent tricuspid valve with obligatory left-to-right atrial shunt (ASD); ECG classically shows left axis deviation and LV hypertrophy (unusual in a cyanotic CHD); pulmonary atresia shows RV hypertrophy or diminutive RV
Transposition of the Great Arteries (TGA)	Cyanotic newborn unresponsive to oxygen, duct-dependent	TGA shows an "egg-on-a-string" heart on CXR with increased pulmonary vascular markings (parallel circulations with pulmonary overcirculation); pulmonary atresia shows decreased pulmonary markings
Ebstein anomaly	Cyanotic newborn with tricuspid valve abnormality, right atrial enlargement	Ebstein shows apical displacement of the tricuspid valve with "atrialized" RV; CXR shows a massively enlarged, "box-shaped" or "wall-to-wall" heart; pulmonary atresia has a normal to small heart
Critical pulmonary stenosis	Cyanotic neonate, duct-dependent, RV outflow obstruction	Critical PS has a severely stenotic but patent valve with some residual antegrade flow; a systolic ejection murmur is typically audible; in pulmonary atresia, there is no antegrade flow and the ejection murmur is absent
Persistent pulmonary hypertension of the newborn (PPHN)	Cyanotic newborn unresponsive to oxygen	PPHN often follows perinatal stress (meconium aspiration, sepsis); shows pre- and post-ductal saturation gradient (higher pre-ductal); echocardiography reveals structurally normal heart with elevated pulmonary pressures

6. Traps and High-Yield Pearls

The most common way students lose points on pulmonary atresia questions is by failing to initiate PGE1 immediately. The vignette will describe a cyanotic newborn who does not improve with oxygen, and students may select "obtain echocardiography" or "obtain CXR" as the next best step. While those are correct diagnostically, the next best step in management is PGE1 infusion. Never delay prostaglandin for confirmatory imaging when duct-dependent physiology is suspected.

A second trap involves the distinction between PA/IVS and PA/VSD. The vignette may describe coronary sinusoidal communications and ask about surgical planning. Students who do not understand RV-dependent coronary circulation will incorrectly choose RV decompression, which would cause fatal myocardial ischemia. The correct pathway for these patients is single-ventricle palliation or transplant listing.

A third testing point is differentiating pulmonary atresia from critical pulmonary stenosis. Both are duct-dependent and cyanotic, but critical PS retains a harsh systolic ejection murmur and some antegrade flow on echo, while pulmonary atresia has no murmur from the RVOT and no forward flow. This distinction matters because critical PS may be treated with catheter-based balloon valvuloplasty alone, whereas pulmonary atresia requires a more complex surgical approach.

The core competency being tested is the ability to recognize duct-dependent cyanotic heart disease, initiate prostaglandin without delay, and understand how anatomy (RV size, coronary dependence, MAPCA distribution) dictates the surgical strategy.

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