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PA26.{4,9} | Congenital Heart Disease & Cardiomyopathies — SDL Guide (Part 2)

Eisenmenger Syndrome — Shunt Reversal

Four-panel medical diagram showing how a large uncorrected left-to-right cardiac shunt causes pulmonary vascular remodeling, increased PVR, right-to-left shunt reversal, cyanosis, and paradoxical embolism. — **Panel A:** Large uncorrected left-to-right shunt through a ventricular septal defect, left ventricle, right ventricle, pulmonary artery, lungs, increased pulmonary blood flow.. **Panel B:** Sequential pathogenesis: large L→R shunt, chronic increased pulmonary blood flow, pulmonary arteriolar remodeling, progressive rise in PVR, PVR ≥ SVR causing shunt reversal.. **Panel C:** Pulmonary arteriole cross-section with medial hypertrophy, intimal fibrosis, narrowed lumen, Heath-Edwards grades 1-2 reversible, grades 3-4 irreversible, grades 5-6 end-stage.. **Panel D:** Established Eisenmenger syndrome with right-to-left shunt reversal, deoxygenated blood entering systemic circulation, central cyanosis, clubbing, polycythaemia, paradoxical embolism, and contraindication to surgical closure..

Eisenmenger syndrome is the final common pathway of large, uncorrected left-to-right shunts.

Mechanism:
1. Large L→R shunt → chronic increase in pulmonary blood flow
2. Pulmonary arterioles undergo medial hypertrophy and intimal fibrosis (Heath-Edwards grading of pulmonary vascular disease)
3. Pulmonary vascular resistance (PVR) rises progressively
4. When PVR ≥ systemic vascular resistance (SVR), shunt reverses → R→L
5. Deoxygenated blood enters systemic circulation → central cyanosis, clubbing, polycythaemia

Heath-Edwards changes (irreversible at Grade 4+):
• Grade 1-2: Medial hypertrophy, intimal proliferation — reversible with repair
• Grade 3-4: Plexiform lesions, fibrinoid necrosis — irreversible
• Grade 5-6: Angiomatoid lesions — end-stage

Clinical significance: Once Eisenmenger is established, surgical closure of the defect is contraindicated — it removes the 'pop-off valve' for the RV, causing acute RV failure. Only heart-lung transplantation offers cure.

CLINICAL PEARL

Paradoxical embolism is a life-threatening complication of any cardiac right-to-left communication. A deep vein thrombosis (DVT) travels to the right heart → crosses the defect → enters the systemic circulation → causes stroke or peripheral arterial occlusion. Even a small PFO (patent foramen ovale) — present in ~25% of adults — can transmit emboli during Valsalva manoeuvres that transiently raise right atrial pressure. Always think of paradoxical embolism in a young patient with cryptogenic stroke and no arterial risk factors.

Tetralogy of Fallot — The Archetypal Cyanotic CHD

Multi-panel diagram of Tetralogy of Fallot showing the four anatomical defects, right-to-left shunting pathophysiology, boot-shaped heart on chest X-ray, right ventricular hypertrophy histology, and Tet spell relief by squatting. — **Panel A:** A. Tetralogy of Fallot anatomy: large sub-aortic perimembranous VSD, overriding aorta, pulmonary stenosis, right ventricular hypertrophy, anterior-superior displacement of infundibular septum, right ventricle, left ventricle, pulmonary trunk, aorta, blue deoxygenated flow, purple mixed flow.. **Panel B:** B. Pathophysiology: RV outflow obstruction, increased RV pressure, right-to-left shunt across VSD, mixed blood entering aorta, early fixed cyanosis.. **Panel C:** C. Boot-shaped heart on CXR: coeur en sabot, upturned apex from RVH, concave pulmonary bay, small pulmonary artery trunk.. **Panel D:** D. Microscopy of RVH: increased myocyte diameter, enlarged nuclei, no myofiber disarray.. **Panel E:** E. Tet spells and squatting: crying or fever lowers SVR and worsens cyanosis; squatting raises SVR by compressing femoral arteries, reduces right-to-left shunt, increases pulmonary blood flow, improves oxygenation..

Tetralogy of Fallot (ToF) is the commonest cyanotic congenital heart defect, accounting for ~10% of all CHD. It arises from anterior-superior displacement of the infundibular (outlet) septum during development.

The 4 components (all follow from one developmental error):
1. Ventricular septal defect (large, sub-aortic, perimembranous)
2. Overriding aorta (aorta straddles the VSD, receiving blood from both ventricles)
3. Pulmonary stenosis (infundibular ± valvular; the obstruction that drives R→L shunting)
4. Right ventricular hypertrophy (consequence of obstruction, not a primary defect)

Pathophysiology: RV outflow obstruction → RV pressure rises ≥ LV pressure → blood preferentially exits via VSD → aorta receives deoxygenated blood → early, fixed cyanosis from birth.

Gross: Boot-shaped heart (coeur en sabot) on CXR — small pulmonary artery trunk, upturned apex from RVH, concave pulmonary bay.

Micro: RVH — increased myocyte diameter, enlarged nuclei, no disarray (unlike HOCM).

Tet spells (hypercyanotic episodes):
Sudden decrease in pulmonary blood flow → acute worsening cyanosis, loss of consciousness. Precipitated by crying, feeding, fever (↓SVR). Squatting raises SVR (compresses femoral arteries) → increases resistance to L→R flow → forces more blood through the pulmonary valve → temporarily improves oxygenation.

Complications: Polycythaemia (compensatory), paradoxical embolism, brain abscess (bypasses pulmonary filter), infective endocarditis, growth retardation.

Annotated Tetralogy of Fallot diagram showing VSD, overriding aorta, infundibular pulmonary stenosis, right ventricular hypertrophy, boot-shaped heart on CXR, and squatting physiology. — **Panel A:** Main coronal heart cutaway showing large ventricular septal defect (VSD), overriding aorta straddling the defect, narrowed right ventricular outflow tract, pulmonary artery, right ventricle, left ventricle, thickened right ventricular free wall, and right-to-left shunt arrows.. **Panel B:** Magnified right ventricular outflow tract showing subpulmonary or infundibular pulmonary stenosis, narrowed pulmonary outflow, pulmonary valve region, and turbulent reduced pulmonary blood flow.. **Panel C:** Clinical correlation showing boot-shaped heart on chest X-ray silhouette with upturned apex and concave pulmonary bay, plus mini-inset of squatting increasing systemic vascular resistance and promoting pulmonary blood flow..

SELF-CHECK

In Tetralogy of Fallot, why does squatting temporarily relieve the cyanotic spell?

A. Squatting increases venous return and dilates the pulmonary outflow tract

B. Squatting compresses femoral arteries, raising systemic vascular resistance and forcing more blood through the pulmonary valve

C. Squatting reduces heart rate and decreases myocardial oxygen demand

D. Squatting closes the overriding aorta by mechanical compression

Reveal Answer

Answer: B. Squatting compresses femoral arteries, raising systemic vascular resistance and forcing more blood through the pulmonary valve

Squatting kinks and compresses the large femoral arteries in the inguinal crease, acutely raising systemic vascular resistance (SVR). With SVR now higher than pulmonary resistance, the haemodynamic gradient favours pushing more RV blood through the narrow pulmonary valve into the lungs rather than across the VSD into the aorta. This transiently improves pulmonary blood flow and oxygenation.

Other Right-to-Left Shunts (The 5 Ts — Brief Overview)

A four-panel medical education diagram compares D-TGA, truncus arteriosus, TAPVC, and tricuspid atresia as cyanotic right-to-left shunts beyond Tetralogy of Fallot. — **Left strip:** Cyanotic congenital heart disease, right-to-left shunt, red oxygenated blood, blue deoxygenated blood, purple mixed blood.. **Panel A:** D-TGA with aorta arising from RV, pulmonary artery from LV, parallel systemic and pulmonary circulations, ASD/VSD/PDA mixing defect, PDA maintained by PGE1, egg-on-a-string CXR inset, arterial switch operation.. **Panel B:** Truncus arteriosus with single great arterial trunk arising from both ventricles, VSD, pulmonary arteries branching from common trunk, failed aortopulmonary septation, 22q11.2 deletion association.. **Panel C:** TAPVC with all four pulmonary veins draining to the right side via SVC, coronary sinus, or portal vein, ASD mixing defect, pulmonary venous obstruction causing severe pulmonary oedema at birth.. **Panel D:** Tricuspid atresia with absent tricuspid valve, no right ventricular inflow, obligatory ASD, usually VSD, hypoplastic RV, reduced pulmonary outflow..

Beyond Tetralogy of Fallot, four other cyanotic defects complete the '5 Ts':

Transposition of great arteries (TGA): Aorta arises from RV; pulmonary artery from LV. Two parallel circulations incompatible with life unless a mixing defect (ASD, VSD, or PDA) coexists. D-TGA is the commonest cyanotic lesion presenting in the neonatal period (first hours of life). Egg-on-a-string CXR appearance. Emergency: maintain PDA with prostaglandin E1; definitive: arterial switch operation.

Truncus arteriosus: Single great vessel arises from both ventricles (failure of aortopulmonary septation). Always associated with VSD. Associated with 22q11.2 deletion.

Total anomalous pulmonary venous connection (TAPVC): All four pulmonary veins drain into the right side (portal vein, SVC, coronary sinus) instead of the LA. Requires mixing defect (ASD) to survive. May be obstructed → severe pulmonary oedema at birth.

Tricuspid atresia: Complete absence of tricuspid valve → no RV inflow → obligatory ASD + usually VSD for survival. Hypoplastic RV.