Tetralogy of Fallot

Introduction and Etiology

• Tetralogy of Fallot (TOF) is the most common cyanotic congenital heart disease, occurring in 25 to 40 per 100,000 live births and accounting for up to 10% of all congenital cardiac malformations.
• The embryological and morphological hallmark of TOF is the anterior and superior deviation (malalignment) of the infundibular or conal septum.
• The etiology is multifactorial with a strong genetic component, famously associated with a 22q11.2 microdeletion (DiGeorge syndrome or velocardiofacial syndrome).
• Additional chromosomal aneuploidies associated with TOF include Trisomy 21 (Down syndrome), Trisomy 18, and Trisomy 13, which collectively account for 5% to 7% of TOF cases.
• Established environmental risk factors include untreated maternal phenylketonuria, maternal diabetes, and maternal exposure to retinoic acid, clomiphene, or organic solvents.
• The recurrence risk of nonsyndromic TOF within families is estimated at 2.5% to 3% if one sibling is affected, increasing to 8% if two siblings are affected.

Anatomical Components and Pathophysiology

• TOF is anatomically defined by four coexisting features: (1) Right ventricular outflow tract obstruction (RVOTO), (2) a malalignment type ventricular septal defect (VSD), (3) dextroposition or overriding of the aorta over the ventricular septum, and (4) right ventricular hypertrophy (RVH).
• The RVOTO is typically multilevel, predominantly involving the subpulmonary infundibulum (the squeeze between the deviated outlet septum and hypertrophied septoparietal trabeculations) alongside valvular pulmonary stenosis or annulus hypoplasia.
• Hemodynamically, the large, unrestrictive VSD ensures that systolic pressures in the right ventricle (RV) and left ventricle (LV) are identical and at systemic levels.
• The direction of blood shunting across the VSD is entirely dependent on the severity of the RVOTO relative to systemic vascular resistance (SVR).
• If RVOTO is mild to moderate, the shunt may be balanced or left-to-right, resulting in an acyanotic presentation known as "pink" TOF.
• If RVOTO is severe, deoxygenated blood from the RV shunts right-to-left across the VSD into the overriding aorta, leading to persistent systemic arterial desaturation and clinical cyanosis.
• A right-sided aortic arch is present in 20% to 30% of patients with TOF, which has surgical implications.
• Coronary artery anomalies occur in 5% to 10% of cases, the most critical being the left anterior descending (LAD) artery arising from the right coronary artery and crossing directly anterior to the RVOT, precluding a standard right ventriculotomy during surgical repair.

Clinical Manifestations

• Infants with extreme RVOTO or pulmonary atresia present with severe cyanosis within the first few hours or days of life as the ductus arteriosus closes.
• Infants with classical TOF present with increasing cyanosis over the first few months of life as infundibular hypertrophy progresses and pulmonary blood flow becomes further restricted.
• Older, unoperated children display severe effort intolerance, dyspnea on minimal exertion, and characteristically adopt a squatting position to recover; squatting acutely increases venous return and systemic vascular resistance, decreasing the right-to-left shunt and augmenting pulmonary blood flow.
• Chronic hypoxemia leads to the development of prominent digital clubbing and compensatory polycythemia.
• Patients may experience delayed growth, failure to thrive, and delayed puberty if the oxygen saturations are chronically below 70%.

Hypercyanotic (Tet) Spells

• Hypercyanotic spells, or "Tet spells," are acute, life-threatening paroxysms of severe cyanosis, hyperpnea, restlessness, and gasping respirations.
• The underlying pathophysiology involves an acute spasm of the right ventricular infundibulum (RVOT) coupled with a drop in systemic vascular resistance, causing a sudden, massive increase in right-to-left shunting and severe systemic hypoxia and metabolic acidosis.
• These spells are frequently provoked by physical exertion, feeding, crying, defecation, or dehydration, and occur most commonly in the morning upon waking.
• Without prompt intervention, severe spells progress rapidly to syncope, seizures, cerebrovascular insults, or death.

Physical Examination

• General inspection reveals varying degrees of central cyanosis and clubbing of the fingers and toes.
• The precordium often exhibits a left anterior hemithorax bulge due to long-standing RVH, with a palpable right ventricular impulse or parasternal heave.
• A prominent systolic thrill is frequently palpable along the left sternal border in the third and fourth parasternal spaces.
• The first heart sound (S1) is typically normal.
• The second heart sound (S2) is classically perceived as single because the pulmonic component (P2) is exceptionally soft, delayed, or inaudible due to the stenotic, immobile pulmonary valve.
• A loud, harsh, long crescendo-decrescendo ejection systolic murmur is best heard at the mid-to-upper left sternal border.
• The murmur is generated entirely by the turbulent forward flow across the stenotic RVOT; the flow across the large VSD is silent due to equalized biventricular pressures.
• A critical clinical hallmark is that the intensity and duration of the systolic murmur paradoxically decrease or vanish entirely during a hypercyanotic Tet spell due to the acute reduction of forward flow across the RVOT.
• An early systolic ejection click may occasionally be heard, originating from the dilated aorta.

Diagnostic Investigations

Electrocardiogram (ECG)

• The ECG frequently shows prominent right axis deviation (RAD), often greater than 120 degrees.
• Right ventricular hypertrophy (RVH) is the most prominent finding, characterized by tall, pure R waves or an rSR' pattern in the right precordial leads (V1, V2R, V3R) and deep S waves in the left precordial leads (V5, V6).
• The T waves in the right precordial leads (V1) are upright; an upright T wave in V1 beyond the first week of life is a reliable indicator of pathological RVH.
• Tall, peaked P waves (P pulmonale) may be observed, indicating right atrial enlargement secondary to right ventricular non-compliance.

Chest Radiograph (CXR)

• The classic, pathognomonic appearance is the "boot-shaped heart" (Coeur en sabot).
• This silhouette is formed by a narrow mediastinal waist, a pronounced concavity at the left upper cardiac border (the "pulmonary bay" resulting from main pulmonary artery hypoplasia), and an upturned left ventricular apex lifted off the diaphragm by the hypertrophied right ventricle.
• The overall cardiothoracic ratio is typically normal.
• Pulmonary vascular markings are noticeably decreased, resulting in abnormally clear or oligemic lung fields reflecting reduced pulmonary blood flow.
• A right-sided aortic arch is visible in up to 30% of cases, identified by its indentation on the right side of the air-filled tracheobronchial shadow.

Transthoracic Echocardiography

• Echocardiography is the definitive non-invasive diagnostic modality.
• Parasternal long-axis and subcostal views clearly demonstrate the hallmark anterior malalignment of the conal septum, the resulting subpulmonary stenosis, the large VSD, and the aorta overriding the septal crest.
• The exact level(s) of RVOT obstruction—infundibular, valvular, or supravalvular—are localized using 2D imaging, while the pulmonary valve annulus and branch pulmonary arteries are measured and converted to Z-scores to guide surgical planning (e.g., the need for a transannular patch).
• Continuous wave Doppler across the RVOT classically demonstrates a "double envelope" or "dagger-shaped" profile; the early, lower velocity dagger shape represents dynamic infundibular obstruction, while the later, parabolic envelope reflects the total maximum gradient.
• Color and pulsed-wave Doppler confirm right-to-left or bidirectional shunting across the VSD.

Cardiac Catheterization and Angiography

• Diagnostic catheterization is largely reserved for complex cases where echocardiography cannot adequately define distal branch pulmonary artery anatomy or if there is suspected pulmonary atresia with Major Aortopulmonary Collateral Arteries (MAPCAs).
• Selective right ventriculography vividly outlines the heavily trabeculated right ventricle, the variable length and distensibility of the infundibular stenosis, and the thickened, narrowed pulmonary valve orifice.
• Aortography or selective coronary arteriography is performed to definitively outline the coronary anatomy, specifically ruling out an anomalous left anterior descending artery traversing the RVOT.

Management

Treatment of Hypercyanotic (Tet) Spells

• Place the infant in a knee-chest position immediately; this traps venous blood in the lower extremities and acutely raises systemic vascular resistance, reducing right-to-left shunting.
• Administer 100% oxygen via face mask or nasal cannula to maximize systemic oxygen content, though its effect on pulmonary vascular resistance is limited due to the fixed RVOT obstruction.
• Intramuscular or subcutaneous Morphine (0.2 mg/kg) or Ketamine is administered to provide sedation, abolish hyperpnea, and suppress central catecholamine release, thereby alleviating infundibular spasm.
• Rapidly correct metabolic acidosis with an intravenous bolus of Sodium Bicarbonate (1-2 mEq/kg).
• Administer intravenous fluid boluses (e.g., normal saline 10 mL/kg) to correct hypovolemia and optimize right ventricular preload.
• If spells persist, intravenous alpha-adrenergic agonists such as Phenylephrine or Methoxamine are given to sharply elevate systemic vascular resistance, forcing blood back through the pulmonary circuit.
• Intravenous Beta-blockers (e.g., Propranolol or Metoprolol) can be utilized to reduce dynamic infundibular contractility.
• Refractory spells require emergent elective intubation, paralysis, and mechanical ventilation to stabilize the patient for urgent palliative or corrective surgery.
• Oral Propranolol is widely utilized as chronic prophylactic therapy to prevent recurrent spells in infants awaiting definitive surgical repair.

Surgical and Transcatheter Interventions

Postoperative Complications and Long-Term Follow-up

• Survival after complete surgical repair is excellent, with long-term survival rates approaching 85% to 94% at 30 years post-surgery.
• Postoperatively, over 90% of patients exhibit a Right Bundle Branch Block (RBBB) on ECG due to inevitable surgical trauma to the conduction system during VSD patch closure and RVOT muscle resection.
• Prolongation of the QRS duration (specifically >180 ms) is a highly predictive marker for right ventricular dilation and significantly correlates with a risk of malignant ventricular tachycardia (VT) and sudden cardiac death.
• Severe pulmonary valve regurgitation (PR) is the most frequent and problematic long-term hemodynamic complication, particularly in patients who underwent transannular patch repair.
• Chronic volume overload from severe PR inevitably leads to progressive right ventricular dilation, RV systolic and diastolic dysfunction, biventricular dysfunction, and diminished exercise tolerance.
• Consequently, 25% to 45% of repaired TOF patients require re-intervention by adulthood, most commonly surgical or transcatheter pulmonary valve replacement (PVR).
• Additional long-term morbidities necessitating lifelong cardiology surveillance via echocardiography and Cardiac Magnetic Resonance Imaging (CMR) include residual VSD shunts, recurrent RVOT obstruction, peripheral branch pulmonary artery stenosis, and progressive aortic root dilation with subsequent aortic insufficiency.
• Patients displaying recurrent ventricular arrhythmias, unexplained syncope, or significant ventricular dysfunction combined with a wide QRS duration should be aggressively evaluated for the prophylactic placement of an Implantable Cardioverter-Defibrillator (ICD).