Congenital Heart Defects

• Abnormal Septation and Chamber Development: Defects result from failure of the endocardial cushions (atrioventricular septation and truncus arteriosus division) or atrial/ventricular septum formation. The endocardial cushions form the lower atrial septum, upper ventricular septum, and atrioventricular valves; their hypoplasia produces atrioventricular septal defects (AVSD). The ostium secundum atrial septal defect (ASD) results from inadequate development of the septum secundum and/or excessive resorption of the septum primum. Ventricular septal defects (VSDs) arise from failure of muscular, membranous, or outlet septation. These defects permit abnormal left-to-right shunting (acyanotic defects) or result in admixture of systemic and pulmonary circulations when combined with right-to-left shunting (cyanotic defects).

• Abnormal Conotruncal Development: The truncus arteriosus normally divides into the aorta and pulmonary artery via neural crest cell migration and proliferation. Failure of this division produces transposition of the great arteries (TGA), where the aorta arises from the right ventricle and the pulmonary artery from the left ventricle, resulting in two separate, incompatible circulations (systemic and pulmonary). Tetralogy of Fallot results from anterior-superior displacement of the infundibular septum, creating a constellation of findings: VSD, right ventricular outflow tract obstruction, right ventricular hypertrophy, and an overriding aorta. Truncus arteriosus communis results from complete failure of truncal division, with a single arterial trunk supplying the systemic, pulmonary, and coronary circulations.

• Hemodynamic Consequences of Shunting: Left-to-right shunts (ASD, VSD, patent ductus arteriosus) initially bypass pulmonary vascular resistance because systemic vascular resistance exceeds pulmonary vascular resistance. This creates pulmonary overcirculation with increased pulmonary blood flow (Qp) relative to systemic flow (Qs), manifested as increased pulmonary artery pressure, volume overload of the left atrium and ventricle, and potential congestive heart failure. Over time, chronic exposure to high pulmonary blood flow and pressure induces pulmonary vascular remodeling—intimal thickening, medial smooth muscle proliferation, and adventitial fibrosis—progressively increasing pulmonary vascular resistance. This process, termed Eisenmenger syndrome, eventually reverses the shunt direction, creating right-to-left shunting with cyanosis and paradoxically worsening symptoms despite anatomic improvement. In contrast, right-to-left shunts (Tetralogy of Fallot, TGA) cause systemic desaturation and cyanosis by permitting deoxygenated blood to bypass the lungs, although they reduce pulmonary overcirculation and may delay pulmonary vascular disease development.

• Obstructive Lesions and Ventricular Pressure Overload: Stenotic lesions (aortic stenosis, pulmonary stenosis, coarctation of the aorta) increase afterload for the affected ventricle, inducing concentric left ventricular hypertrophy (LVH) or right ventricular hypertrophy (RVH). While initial hypertrophy maintains cardiac output via the Frank-Starling mechanism, progressive fibrosis and increased chamber stiffness eventually produce diastolic and systolic dysfunction. Severe stenosis may result in insufficient cardiac output, reduced coronary perfusion pressure (in aortic stenosis), or inadequate pulmonary perfusion (in critical pulmonary stenosis), manifesting as syncope, angina, or acute decompensation with low cardiac output.

• Ductal-Dependent Circulations: Certain lesions are ductus arteriosus-dependent, relying on patency of the fetal ductus arteriosus (a vessel connecting the pulmonary artery to the aorta) for survival. In ductus-dependent pulmonary blood flow (critical pulmonary stenosis, critical pulmonary atresia, severe Tetralogy of Fallot), the ductus maintains antegrade pulmonary blood flow, and its closure results in severe cyanosis and potential cardiovascular collapse. In ductus-dependent systemic blood flow (critical coarctation of the aorta, hypoplastic left heart syndrome, critical aortic stenosis), the ductus maintains lower body perfusion, and its closure precipitates shock with metabolic acidosis and end-organ dysfunction. These lesions require urgent prostaglandin E1 (PGE1) infusion to maintain ductal patency and preserve vital blood flow.

• Genetic Factors and Chromosomal Abnormalities: Trisomy 21 (Down syndrome) represents the most common chromosomal association with CHD, occurring in 40-50% of individuals with Down syndrome; the most frequent lesions are atrioventricular septal defects (40-50% of trisomy 21 CHD cases), followed by secundum ASD and VSD. Trisomy 18 (Edwards syndrome) presents with complex CHD in >90% of cases, typically featuring VSD, patent foramen ovale (PFO), and polyvalvular disease. Trisomy 13 (Patau syndrome) associates with VSD, ASD, and patent ductus arteriosus. Turner syndrome (45,X) presents with left-sided obstructive lesions, particularly bicuspid aortic valve (30% prevalence), coarctation of the aorta (10-15%), hypoplastic left heart, and aortic stenosis, reflecting abnormal development of structures derived from the second pharyngeal arch. 22q11 deletion syndrome (DiGeorge syndrome) occurs in 5-8% of all CHD cases and presents with conotruncal abnormalities (Tetralogy of Fallot in 20-25% of deletions, truncus arteriosus communis in 30-35%, TGA, and interrupted aortic arch), along with thymic hypoplasia, cleft palate, hypocalcemia, and immune deficiency.

• Maternal Conditions and Teratogens: Maternal diabetes mellitus, particularly pregestational insulin-dependent diabetes, increases CHD risk 2-3 fold, with characteristic lesions including left ventricular outflow tract obstructions (aortic stenosis), cardiomyopathy, and sacral agenesis (caudal regression syndrome). Maternal systemic lupus erythematosus (SLE) and other autoimmune conditions increase neonatal lupus with congenital heart block (1-2% of SLE pregnancies), resulting from transplacental passage of maternal anti-Ro/SSA and anti-La/SSB antibodies that target fetal cardiac conduction tissue. Maternal drug exposure includes thalidomide (Ebstein anomaly), retinoic acid (Tetralogy of Fallot, TGA, truncus arteriosus), phenytoin (fetal hydantoin syndrome: aortic/pulmonary stenosis, PDA, ASD, VSD), alcohol (fetal alcohol syndrome: ASD, VSD, PDA, complex lesions), and lithium (Ebstein anomaly and tricuspid atresia). Maternal infections, particularly rubella in the first trimester, cause patent ductus arteriosus, peripheral pulmonary stenosis, and atrial septal defects.

• Familial and Genetic Syndrome Associations: Williams syndrome (microdeletion 7q11.23) produces supravalvular aortic stenosis and pulmonary artery stenosis due to elastin gene deletion. Noonan syndrome (PTPN11 and other gene mutations) causes pulmonary valve stenosis (60% of cases), hypertrophic cardiomyopathy, and ASD. Marfan syndrome (FBN1 mutation) results in aortic root dilatation with aortic regurgitation and dissection risk, along with mitral valve prolapse. Holt-Oram syndrome (TBX5 mutations) features atrial septal defects with skeletal abnormalities (thumb/radius dysplasia). Ellis-van Creveld syndrome (ciliary protein mutations) presents with ASD, single atrium, and common atrium. Heterotaxy syndromes (left/right axis determination gene mutations including ZIC3, NODAL, LEFTY2) produce complex CHD with abnormal organ situs, including asplenia (associated with cyanotic heart disease and functional asplenia complications) or polysplenia.

• Maternal Age and Recurrence Risk: Advanced maternal age (>35 years) increases risk of chromosomal abnormalities and associated CHD. Familial recurrence follows patterns dependent on lesion type: if one parent has CHD, recurrence risk is 2-4% for offspring; if one child has CHD, recurrence in subsequent siblings is 2-4% for isolated lesions but increases to 5-10% for familial clustering of the same lesion type. Monozygotic twin concordance is 40-50%, indicating incomplete genetic penetrance and environmental modulation.

• Cyanosis (Hypoxemia): Central cyanosis, characterized by blue discoloration of the lips, tongue, and entire body (not just extremities), indicates arterial oxygen desaturation below 85 mmHg (normal >95%) and suggests right-to-left shunting or severe parenchymal lung disease. Cyanosis becomes clinically apparent when absolute deoxygenated hemoglobin exceeds 5 g/dL (in anemia, cyanosis appears later; in polycythemia, earlier). Cyanotic CHD pathophysiologically results from deoxygenated blood bypassing the lungs (right-to-left shunt) or mixing systemic and pulmonary circulations. The "Tet spell" or hypercyanotic spell in Tetralogy of Fallot represents acute increases in right-to-left shunting (due to increased RV outflow obstruction from dynamic infundibular narrowing or decreased systemic vascular resistance) manifesting as severe cyanosis, dyspnea, syncope, and potential cardiovascular collapse; spells are triggered by crying, defecation, or agitation. Infants with cyanotic CHD adopt a squatting position (flexed knees, hips), which increases systemic vascular resistance, decreases right-to-left shunting, and improves oxygenation.

• Dyspnea and Feeding Difficulties: Pulmonary overcirculation in left-to-right shunt lesions increases pulmonary venous return and left atrial pressure, causing pulmonary edema manifested as tachypnea, retractions, and crackles. Infants typically present with feeding difficulties and failure to thrive (poor weight gain) due to increased metabolic demands from elevated cardiac workload; a normal infant generates cardiac output matching 4-6 L/min/m², but acyanotic CHD with significant shunting may require 8-10 L/min/m² to maintain systemic perfusion. Dyspnea-on-exertion occurs earlier in life and with mild activity compared to older children without cardiac disease. Orthopnea and nocturnal dyspnea indicate significant pulmonary edema from congestive heart failure.

• Congestive Heart Failure (CHF): Left-to-right shunt lesions (ASD, VSD, PDA) predispose to CHF through pulmonary overcirculation and volume overload. Right-sided CHF manifests as hepatomegaly (which may be massive and extends below the costal margin), peripheral edema (uncommon in infants due to low systemic venous pressure), and ascites (late finding). Left-sided CHF presents with pulmonary edema, orthopnea, and crackles. The liver edge palpable >2-3 cm below the costal margin suggests hepatic congestion from elevated central venous pressure.

• Syncope and Exercise Intolerance: Outflow tract obstruction (aortic stenosis, pulmonary stenosis, hypertrophic cardiomyopathy) or cyanotic lesions with severe obstruction cause syncope due to inadequate cerebral perfusion during exertion when peripheral vasodilation increases oxygen demand while cardiac output plateaus. Sudden cardiac death during exertion is a catastrophic complication, particularly in aortic stenosis with severe obstruction and in arrhythmogenic conditions.

• Murmurs and Heart Sounds: Systolic ejection murmurs occur in left-to-right shunts (indicating increased flow across the pulmonary valve in ASD, or across the aortic valve in VSD) and obstructive lesions (aortic/pulmonary stenosis). The murmur in secundum ASD is classically a systolic ejection murmur at the left upper sternal border (pulmonary flow murmur) with wide, fixed splitting of S2 (the second heart sound doesn't narrow during inspiration because the right ventricle is already maximally filled). VSD murmurs are pansystolic (holosystolic), heard at the left sternal border, reflecting continuous flow from the higher-pressure left ventricle to right ventricle throughout systole. Aortic stenosis produces an early-to-mid systolic ejection murmur at the right upper sternal border, radiating to the neck (carotid radiation). Pulmonary stenosis generates a systolic ejection murmur at the left upper sternal border with delayed peak (late systolic click from right ventricular hypertrophy). Patent ductus arteriosus produces a continuous "machinery" murmur best heard at the left infraclavicular region, reflecting persistent flow during both systole and diastole due to the pressure gradient between the aorta and pulmonary artery.

• Cyanotic Newborn Presentation: Critical cyanotic lesions (TGA, critical pulmonary stenosis/atresia, severe Tetralogy of Fallot, total anomalous pulmonary venous return, truncus arteriosus, hypoplastic left heart) typically manifest in the first hours to days of life with profound cyanosis not responsive to supplemental oxygen (pulse oximetry often 60-75% despite 100% FiO2), severe dyspnea, metabolic acidosis, and cardiovascular collapse if ductal-dependent circulation closes. The "PaO2 gap" (A-a gradient) helps differentiate cardiac from pulmonary causes: in CHD with right-to-left shunting, the alveolar-arterial oxygen gradient is typically <15 mmHg (lungs function normally but blood bypasses them), whereas in pulmonary disease, A-a gradient exceeds 15 mmHg.

• Acyanotic Newborn Presentation: Lesions causing pulmonary overcirculation (large VSD

Screening and initial evaluation

• Pulse oximetry screening for critical CHD: universal newborn screening endorsed by the AAP is performed at or after 24 hours of life, with saturations measured in the right hand (preductal) and either foot (postductal). A screen fails if saturation is below 90% in either site, or if saturations are 90-94% in both sites or differ by more than 3% on repeated measurements an hour apart. A failed screen mandates echocardiography, not simple repeat observation.
• Hyperoxia test: 100% FiO2 for ~10 minutes. Failure of PaO2 to rise above roughly 100 mmHg indicates fixed right-to-left intracardiac shunting rather than parenchymal lung disease, in which PaO2 rises substantially.
• Four-extremity blood pressures and pulses: an upper-extremity to lower-extremity systolic gradient with diminished femoral pulses suggests coarctation of the aorta.

Confirmatory testing

• Transthoracic echocardiography with color Doppler: the gold standard. It defines anatomy, shunt direction, Qp:Qs ratio, gradients, and ventricular function, and is the test that establishes the diagnosis in essentially every lesion.
• Chest radiograph: boot-shaped heart with decreased pulmonary vascularity in tetralogy of Fallot; egg on a string with narrow mediastinum in TGA; snowman/figure-of-8 in supracardiac total anomalous pulmonary venous return; 3 sign and inferior rib notching in coarctation; cardiomegaly with increased pulmonary vascular markings in large left-to-right shunts.
• ECG: right axis deviation and RVH in tetralogy and pulmonary stenosis; left axis deviation with RVH is characteristic of AV septal defect (superior QRS axis); RSR' in V1 with right atrial enlargement in secundum ASD.
• Cardiac catheterization: reserved for hemodynamics rather than anatomy — quantifying pulmonary vascular resistance and testing vasoreactivity when shunt reversal is suspected, and for interventional procedures. Cardiac MRI quantifies ventricular volumes and pulmonic regurgitation in repaired tetralogy, per AHA/ACC adult congenital heart disease guidance.
• Genetic testing: FISH or microarray for 22q11.2 deletion in conotruncal lesions; karyotype for suspected trisomies and Turner syndrome.

Immediate stabilization of the critical neonate

• Prostaglandin E1 (alprostadil) infusion: give to any cyanotic or shocked neonate with suspected ductal-dependent lesion, typically started at 0.05-0.1 mcg/kg/min, before echocardiography confirms the diagnosis. It maintains ductal patency and therefore pulmonary or systemic flow. Anticipate apnea, fever, and hypotension — have airway equipment at the bedside.
• Avoid indiscriminate high FiO2 in single-ventricle or ductal-dependent systemic circulations: oxygen lowers pulmonary vascular resistance and steals flow from the systemic circuit, worsening shock and acidosis.
• **Hypercyanotic (tet) spell**: knee-chest position first, then oxygen, morphine or intravenous fluid bolus, and a vasoconstrictor such as phenylephrine to raise systemic vascular resistance; a beta blocker (esmolol or propranolol) relaxes the dynamic infundibulum. Inotropes and anything that drops afterload are contraindicated because they increase right-to-left shunting.

Medical therapy for pulmonary overcirculation

• Loop diuretics (furosemide) for pulmonary edema, with afterload reduction using an ACE inhibitor (enalapril) and high-calorie feeds for failure to thrive in large VSD or AVSD.
• PDA in preterm infants: cyclooxygenase inhibition with indomethacin or ibuprofen, or acetaminophen as an alternative; contraindicated with active bleeding, significant renal dysfunction, or necrotizing enterocolitis — and absolutely contraindicated when the ductus is the lifeline in a ductal-dependent lesion.

Definitive management

• Catheter-based: device closure of secundum ASD and many PDAs; balloon valvuloplasty for valvar pulmonary stenosis; balloon atrial septostomy (Rashkind) to improve mixing in TGA.
• Surgical: complete tetralogy repair in infancy (commonly around 3-6 months), arterial switch for d-TGA within the first weeks of life, VSD patch closure, coarctation repair, and staged Norwood-Glenn-Fontan palliation for hypoplastic left heart.
• Endocarditis prophylaxis: per AHA/ACC, amoxicillin before dental procedures only for unrepaired cyanotic CHD, prosthetic repair material within 6 months, or residual defect adjacent to prosthetic material.
• Contraindicated: shunt closure once Eisenmenger physiology is established, and pregnancy in Eisenmenger syndrome.

Complications of unrepaired disease

• Eisenmenger syndrome: chronic pulmonary overcirculation drives irreversible pulmonary vascular remodeling until pulmonary resistance exceeds systemic, reversing the shunt. Signals: new cyanosis, clubbing, a loud/palpable P2, and disappearance of the previously prominent shunt murmur.
• Paradoxical embolism and brain abscess: right-to-left shunting bypasses the pulmonary capillary filter, so venous thrombi and bacteria reach the systemic circulation. New focal deficit or fever with headache in a cyanotic child is a neurologic emergency requiring imaging.
• Infective endocarditis: turbulent high-velocity jets injure endothelium; suspect with persistent fever, new regurgitant murmur, and embolic phenomena.
• Secondary erythrocytosis and hyperviscosity: chronic hypoxemia drives erythropoietin; headache, visual change, and myalgia signal it. Concurrent iron deficiency worsens symptoms because microcytes are less deformable.
• Congestive heart failure and failure to thrive in large left-to-right shunts; necrotizing enterocolitis in preterm infants with ductal diastolic steal.
• Ductal closure in a ductal-dependent lesion: an emergency presenting as sudden cyanosis or shock with severe metabolic acidosis and absent femoral pulses.

Complications of treatment

• Complete heart block after VSD or AVSD patch closure, from injury to conduction tissue near the membranous septum — watch the post-operative ECG.
• Late arrhythmia and sudden death after tetralogy repair: chronic pulmonic regurgitation dilates the right ventricle and the ventriculotomy scar supports reentry; progressive QRS prolongation and ventricular tachycardia are warning signs. Emergency if sustained.
• Atrial arrhythmias and thromboembolism after Fontan, plus protein-losing enteropathy, plastic bronchitis, and Fontan-associated liver disease from chronically elevated systemic venous pressure.
• Recoarctation, aneurysm, and persistent systemic hypertension after coarctation repair; lifelong blood pressure surveillance is recommended by AHA/ACC.
• Prostaglandin E1 toxicity: apnea, fever, hypotension, and cortical hyperostosis with prolonged use.

• Cyanotic newborn, next best step: start PGE1 while arranging urgent echocardiography — do not wait for a definitive diagnosis, and do not attribute failure to improve on 100% oxygen to lung disease.
• Radiographic buzzwords: boot-shaped heart = tetralogy of Fallot; egg on a string with narrow mediastinum = d-TGA; snowman = supracardiac TAPVR; 3 sign with rib notching = coarctation.
• Auscultation: wide fixed splitting of S2 = secundum ASD (the murmur itself is a pulmonary flow murmur, not the shunt); machinery continuous murmur = PDA. A louder, harsher holosystolic murmur in VSD means a smaller defect — a large VSD equalizes pressures and may be relatively quiet. This inverse relationship is the classic distractor.
• Tet spell management: knee-chest position and phenylephrine raise systemic vascular resistance and reduce right-to-left shunting. Choosing an inotrope or a vasodilator is the trap answer.
• Differential cyanosis (pink upper body, blue lower body) points to coarctation or interrupted arch with a right-to-left PDA; reversed differential cyanosis (blue upper, pink lower) is nearly pathognomonic for d-TGA with coarctation or pulmonary hypertension.
• Syndrome associations examiners repeat: trisomy 21 → AV septal defect with left axis deviation on ECG; 22q11.2 deletion → truncus arteriosus and interrupted aortic arch; Turner syndrome → bicuspid aortic valve and coarctation; lithium → Ebstein anomaly; congenital rubella → PDA; maternal diabetes → TGA and left-sided obstruction.
• Eisenmenger physiology contraindicates defect closure and makes pregnancy extremely high risk; closing the defect removes the pop-off valve for the hypertensive right ventricle.
• Endocarditis prophylaxis is narrow: per AHA, unrepaired cyanotic CHD, prosthetic material within 6 months, or a residual defect at a prosthetic patch — not every child with a murmur.