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Summary Table

Category	Key Information
Risk Factors	Maternal diabetes, maternal alcohol or retinoid exposure, chromosomal abnormalities (DiGeorge, trisomy 18, 21), family history of congenital heart disease
Etiology	Failure of tricuspid valve development during embryogenesis, leading to no direct communication between right atrium and right ventricle. The right ventricle is hypoplastic.
Patient Presentation	Neonate with cyanosis (often within first 24 to 48 hours of life), tachypnea, poor feeding, failure to thrive, episodes worsening when ductus arteriosus closes
Classic Physical Exam	Central cyanosis, single S2, holosystolic murmur at left lower sternal border (VSD flow), continuous murmur if PDA present, normal or mild hepatomegaly
Key Diagnostic Results	ECG: left axis deviation, LVH, right atrial enlargement (tall peaked P waves). CXR: variable pulmonary vascular markings depending on flow. Echocardiography is confirmatory.
Management	Prostaglandin E1 infusion to maintain ductal patency, balloon atrial septostomy if restrictive atrial communication, staged surgical palliation ending in Fontan circulation
Keywords	"Cyanotic neonate with left axis deviation," "single ventricle physiology," "absent tricuspid valve," "obligate right-to-left atrial shunt," "Fontan candidate"

1. Pathophysiology

Tricuspid atresia is a cyanotic congenital heart disease characterized by complete absence of the tricuspid valve, so there is no direct flow from the right atrium to the right ventricle. The right ventricle is therefore underdeveloped (hypoplastic). For the neonate to survive, two communications must exist:

An interatrial communication (ASD or patent foramen ovale) AND
A source of pulmonary blood flow (usually a VSD, a patent ductus arteriosus, or both)

Systemic venous return enters the right atrium and, because there is no outlet to the right ventricle, must cross to the left atrium through the ASD or PFO. This is an obligate right-to-left atrial shunt. In the left atrium, deoxygenated blood mixes with pulmonary venous return, producing a single mixing chamber. The left ventricle then ejects this mixed blood to both the systemic circulation (through the aorta) and the pulmonary circulation (through the VSD into the hypoplastic right ventricle and then the pulmonary artery, or via the PDA).

The degree of cyanosis depends entirely on the amount of pulmonary blood flow. If the VSD is small or pulmonary stenosis is present, pulmonary flow is reduced and the neonate is profoundly cyanotic, especially when the ductus arteriosus closes. If there is no pulmonary stenosis and the VSD is large, pulmonary flow may be excessive, leading to congestive heart failure with only mild cyanosis.

The left ventricle becomes the systemic pumping chamber and bears the entire workload, which explains the unusual finding of LVH on ECG in a cyanotic newborn. The left axis deviation reflects this dominant left-sided physiology, a feature that distinguishes tricuspid atresia from most other cyanotic lesions in infancy, which usually show right axis deviation and RVH.

2. Classification and Clinical Manifestation

Classification follows the relationship of the great arteries (Tandon and Edwards) and is then subdivided by pulmonary outflow.

Type	Great Arteries	Subtype	Pulmonary Flow	Clinical Picture
Type I (about 70 percent)	Normally related	Ia	Pulmonary atresia	Severe cyanosis from birth, ductal-dependent pulmonary flow
		Ib	Pulmonary stenosis or small VSD	Moderate cyanosis, decreased pulmonary markings
		Ic	No pulmonary stenosis, large VSD	Mild cyanosis, may develop heart failure
Type II (about 30 percent)	D-transposition	IIa	Pulmonary atresia	Severe cyanosis, ductal-dependent
		IIb	Pulmonary stenosis	Cyanosis with reduced flow
		IIc	No pulmonary stenosis	Pulmonary overcirculation, heart failure, minimal cyanosis
Type III (rare)	L-transposition or other	III	Variable	Variable presentation

Clinically, patients fall into two practical groups:

Those with reduced pulmonary blood flow (the majority) who present with cyanosis and require prostaglandin to maintain ductal patency.
Those with unrestricted pulmonary flow who present later with heart failure, tachypnea, and failure to thrive.

3. Diagnostic Workup

Test	Role	Key Finding
Pulse oximetry / hyperoxia test	Screening	Failure of $PaO_2$ to rise above 150 mmHg on 100 percent oxygen
Chest radiograph	Initial assessment	Variable cardiac silhouette; pulmonary vasculature decreased (most common) or increased
ECG	Strongly suggestive	Left axis deviation, LVH, right atrial enlargement
Echocardiography	Gold standard / confirmatory	Absent tricuspid valve, hypoplastic right ventricle, ASD, VSD, great artery relationship
Cardiac catheterization	Pre-surgical hemodynamics	Pulmonary artery pressures, pulmonary vascular resistance, anatomy clarification
Cardiac MRI / CT	Adjunct	Detailed anatomy, especially for surgical planning

The workup begins at the bedside. A cyanotic neonate who fails to improve with supplemental oxygen (the hyperoxia test) has structural cyanotic heart disease until proven otherwise. The chest radiograph is ordered next to assess heart size and pulmonary vascular markings, which helps narrow the differential among cyanotic lesions.

The ECG is where tricuspid atresia announces itself. A neonate with cyanosis plus left axis deviation and LVH is the classic clue. Most cyanotic lesions in infancy (tetralogy of Fallot, transposition with VSD, pulmonary atresia with intact septum) produce right axis deviation. Left axis deviation in this setting is an important pointer.

Echocardiography is the confirmatory test (gold standard). It demonstrates the absent or atretic tricuspid valve as a dense band of echogenic tissue at the expected valve position, a hypoplastic right ventricle, the obligate atrial communication, the ventricular communication if present, and the relationship of the great arteries. Doppler interrogation quantifies pulmonary flow and identifies any pulmonary stenosis.

Cardiac catheterization is no longer required for diagnosis but is performed before each stage of surgical palliation to measure pulmonary artery pressures and pulmonary vascular resistance, both of which determine eligibility for the Glenn and Fontan operations.

4. Management and Treatment

Phase	Intervention	Timing	Goal
Acute neonatal	Prostaglandin E1 infusion, 0.05 to 0.1 mcg/kg/min IV	Immediately on suspicion	Maintain ductal patency for pulmonary flow
Acute neonatal	Balloon atrial septostomy (Rashkind procedure)	If restrictive atrial communication	Allow adequate right-to-left mixing
Stage 1 surgery	Modified Blalock-Taussig-Thomas shunt (if low pulmonary flow) or pulmonary artery banding (if high flow)	Neonatal period	Balance pulmonary and systemic circulation
Stage 2 surgery	Bidirectional Glenn (SVC to pulmonary artery anastomosis)	4 to 6 months of age	Reduce volume load on single ventricle
Stage 3 surgery	Fontan procedure (IVC routed to pulmonary artery)	2 to 4 years of age	Separate systemic and pulmonary circulations
Long-term	Anticoagulation, diuretics, afterload reduction, surveillance for arrhythmias and protein-losing enteropathy	Lifelong	Preserve Fontan circulation

The immediate priority is recognition and prostaglandin E1. Any neonate presenting with cyanosis that worsens in the first days of life, especially with a single S2 and an ECG showing left axis deviation, should be started on prostaglandin E1 at 0.05 to 0.1 mcg/kg/min before transfer to a tertiary center. Prostaglandin maintains patency of the ductus arteriosus, preserving the lifeline for pulmonary blood flow. Side effects include apnea (so intubation readiness is essential), fever, and hypotension.

If the atrial communication is restrictive, demonstrated by elevated right atrial pressure and a small atrial gap on echocardiography, balloon atrial septostomy is performed to enlarge the opening and allow adequate mixing.

Surgical management follows a three-stage palliation pathway because the right ventricle cannot be rehabilitated.

The first stage depends on physiology.
- Patients with reduced pulmonary blood flow receive a modified Blalock-Taussig-Thomas shunt, a synthetic graft from the subclavian artery to the pulmonary artery, providing controlled pulmonary perfusion.
- Patients with excessive flow receive pulmonary artery banding to limit flow and protect the pulmonary vasculature from irreversible hypertensive changes.
The second stage, the bidirectional Glenn, is performed around 4 to 6 months when pulmonary vascular resistance has fallen sufficiently. The superior vena cava is anastomosed directly to the pulmonary artery, allowing passive flow of upper body venous return into the lungs and unloading the single ventricle.

The third and final stage is the Fontan procedure, in which the inferior vena cava is also routed to the pulmonary artery (typically via an extracardiac conduit). This completes the separation of systemic and pulmonary circulations. The single left ventricle now pumps only to the systemic circulation while venous return flows passively to the lungs.

Long-term issues include arrhythmias, thromboembolism (anticoagulation is standard after Fontan), protein-losing enteropathy, plastic bronchitis, hepatic dysfunction (Fontan-associated liver disease), and eventual ventricular failure that may require transplantation.

The "next best step" logic in a cyanotic neonate suspected of tricuspid atresia is: secure the airway and oxygenation, start prostaglandin E1, obtain confirmatory echocardiography, and arrange transfer for surgical evaluation. Do not wait for the echocardiogram to start prostaglandin if clinical suspicion is high.

5. Differential Diagnosis and Distractors

Differential Diagnosis	Why It Is Similar	Key Discriminator
Tetralogy of Fallot	Cyanotic neonate, decreased pulmonary markings, VSD on echo	ECG shows right axis deviation and RVH, not left. "Boot-shaped heart" on CXR. Tet spells with squatting in older infants.
Transposition of the great arteries (D-TGA)	Profound cyanosis at birth, ductal-dependent	ECG shows right axis deviation and RVH. CXR shows "egg on a string." Echo shows aorta arising from RV, PA from LV with intact tricuspid valve.
Pulmonary atresia with intact ventricular septum	Cyanosis, decreased pulmonary markings, ductal-dependent	Tricuspid valve is present on echo. RV may be hypoplastic but tricuspid inflow exists. ECG often shows RVH.
Hypoplastic left heart syndrome	Ductal-dependent cyanotic neonate, single ventricle physiology	Cyanosis with shock and weak pulses when ductus closes, not isolated cyanosis. ECG shows RVH, not LVH. Echo shows hypoplastic LV and aorta.
Ebstein anomaly	Cyanosis, tricuspid valve abnormality	Tricuspid valve is present but apically displaced. Severe tricuspid regurgitation murmur. CXR shows massive cardiomegaly ("box-shaped heart"). ECG with right bundle branch block, sometimes WPW.
Total anomalous pulmonary venous return	Cyanotic neonate, tachypnea	"Snowman" or figure-of-8 cardiac silhouette on CXR (supracardiac type). Severe pulmonary edema. Right axis deviation, RVH.
Truncus arteriosus	Cyanotic neonate with mixing	Single arterial trunk on echo. Bounding pulses, wide pulse pressure, often heart failure. ECG with biventricular hypertrophy.

6. Traps and High-Yield Pearls

The single most important clue, and the one item question writers reward, is the combination of cyanotic neonate plus left axis deviation plus LVH on ECG. This pattern stands out because almost every other cyanotic congenital heart disease presenting in infancy produces right axis deviation. If you see a vignette describing a blue baby whose ECG shows a "superior" or "northwest" axis with prominent left ventricular forces, tricuspid atresia should be at the top of the list.

A second classic trap is over-reliance on chest radiograph appearance. Pulmonary vascular markings can be decreased, normal, or increased depending on subtype, so the CXR alone will not make the diagnosis. Students who anchor on "decreased markings equals tetralogy of Fallot" miss tricuspid atresia type Ib.

Another frequent error is forgetting that prostaglandin E1 must be started before confirmatory imaging when a ductal-dependent lesion is suspected. The neonate who deteriorates at 24 to 48 hours of life, coinciding with physiologic ductal closure, is almost always ductal-dependent. Waiting for the echo before starting prostaglandin is a wrong-answer trap.

Students also confuse the Fontan procedure with a curative operation. It is palliative. The single left ventricle still bears the systemic load, and pulmonary flow is passive. Long-term complications (arrhythmias, protein-losing enteropathy, Fontan-associated liver disease) are testable.

Finally, do not be misled by the absence of a loud murmur. Because there is no flow across the atretic tricuspid valve, the auscultatory findings depend on associated lesions. A single S2 is often the only consistent abnormality, and its presence in a cyanotic neonate is itself a clue.

The core competency tested is recognizing a ductal-dependent cyanotic lesion early, identifying the unusual ECG signature of left axis deviation, initiating prostaglandin E1 promptly, and understanding the staged single-ventricle palliation pathway that ends with the Fontan circulation.

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