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Summary Table

Category	Key Points
Risk Factors	Maternal diabetes, 22q11.2 deletion (DiGeorge syndrome), maternal alcohol use, viral infection during pregnancy
Etiology	Failure of the truncoconal septum to divide the embryonic truncus arteriosus into the aorta and pulmonary artery
Patient Presentation	Neonate (within first 2 to 4 weeks of life) with cyanosis, tachypnea, poor feeding, diaphoresis with feeds, and failure to thrive
Classic Physical Exam	Single loud S2, harsh systolic ejection murmur at the left sternal border, wide pulse pressure, bounding peripheral pulses, signs of congestive heart failure
Key Diagnostic Results	Chest x-ray: cardiomegaly with increased pulmonary vascular markings, right-sided aortic arch in roughly 30 percent of cases. ECG: biventricular hypertrophy. Echocardiography is confirmatory
Management	Medical stabilization for heart failure(diuretics, digoxin, afterload reduction), then surgical repair (Rastelli-type procedure with VSD closure and right ventricle to pulmonary artery conduit) within the first weeks of life
Keywords	"Single great vessel overriding a VSD," "single S2," "neonatal cyanosis with CHF," "DiGeorge association," "common arterial trunk," "wide pulse pressure," "bounding pulses"

1. Pathophysiology

During normal cardiac embryogenesis, neural crest cells migrate into the truncoconal ridges and form a spiral septum that divides the primitive truncus arteriosus into two separate vessels: the aorta and the pulmonary trunk. In truncus arteriosus, this septation fails completely, leaving a single arterial trunk that arises from both ventricles and gives rise to the systemic, pulmonary, and coronary circulations.

Because neural crest cells are also responsible for thymic and parathyroid development, this defect is strongly tied to 22q11.2 deletion (DiGeorge syndrome). Roughly one-third of patients with truncus arteriosus carry this microdeletion, which is a heavily tested association.

A large ventricular septal defect (VSD) is always present because the truncus overrides the interventricular septum. The single truncal valve, which has variable cusp number (most commonly tricuspid, but can be bicuspid or quadricuspid), is frequently regurgitant or stenotic.

The hemodynamic consequence explains the entire clinical picture. Mixed deoxygenated and oxygenated blood enters the common trunk, producing mild cyanosis in the newborn. As pulmonary vascular resistance falls during the first weeks of life, blood preferentially flows into the low-resistance pulmonary circulation. This results in pulmonary overcirculation, volume overload of both ventricles, and rapid onset of congestive heart failure. The diastolic runoff from the aorta into the pulmonary arteries (similar to a large patent ductus arteriosus physiology) creates the wide pulse pressure and bounding pulses.

The single semilunar valve produces a single S2 because only one valve closes during ventricular relaxation. This is one of the most reliable physical exam findings on the test.

If left untreated, persistent pulmonary overcirculation leads to irreversible pulmonary vascular disease (Eisenmenger physiology) within the first year of life, after which surgical repair is no longer feasible.

2. Classification of Clinical Manifestation

Collett and Edwards Anatomic Classification

Type	Origin of Pulmonary Arteries
Type I	Main pulmonary artery arises from the truncus and then bifurcates into right and left pulmonary arteries (most common, around 50 to 70 percent)
Type II	Right and left pulmonary arteries arise separately but close to each other from the posterior aspect of the truncus
Type III	Right and left pulmonary arteries arise from the lateral sides of the truncus
Type IV	No true pulmonary arteries; pulmonary blood flow comes from collaterals off the descending aorta (now reclassified as pulmonary atresia with VSD, not true truncus)

Van Praagh Classification

Type	Description
A1	Partially separated pulmonary arteries with main pulmonary segment present (similar to Collett I)
A2	Pulmonary arteries arise separately, no main pulmonary segment (similar to Collett II and III)
A3	One pulmonary artery absent
A4	Truncus associated with interrupted aortic arch

Clinical Severity Spectrum

Stage	Findings
Early neonatal (first week)	Mild cyanosis, soft murmur, often missed at discharge
2 to 4 weeks of age	Tachypnea, feeding difficulty, diaphoresis, hepatomegaly, frank congestive heart failure
Untreated infant (over 6 months)	Pulmonary vascular obstructive disease, paradoxically diminishing CHF symptoms but worsening cyanosis (Eisenmenger physiology)

3. Diagnostic Workup

Diagnostic Test Summary

Test	Role	Findings
Pulse oximetry / newborn screening	Initial screening	Low oxygen saturation, often less than 95 percent, with possible pre-ductal and post-ductal differential
Chest x-ray	Initial imaging	Cardiomegaly, increased pulmonary vascular markings, right-sided aortic arch in about 30 percent
ECG	Adjunct	Biventricular hypertrophy, normal axis
Echocardiogram (transthoracic)	Best initial AND confirmatory test	Single great vessel overriding a VSD, single semilunar valve, defines anatomic type
Cardiac CT or MRI	Adjunct for surgical planning	Delineates pulmonary artery anatomy and aortic arch
Cardiac catheterization	Reserved for unclear anatomy or evaluation of pulmonary vascular resistance	Quantifies shunt and pulmonary pressures
FISH / chromosomal microarray for 22q11.2	Genetic workup	Confirms DiGeorge syndrome

Workup Walkthrough

In a vignette, a newborn presents with cyanosis and respiratory distress. The first step is pulse oximetry and a chest x-ray, which together suggest a congenital heart defect with increased pulmonary blood flow. Conditions causing cyanosis with increased pulmonary markings include the "Three T's plus one": Truncus arteriosus, Transposition of the great arteries, Total anomalous pulmonary venous return, and (single ventricle physiology with) unobstructed pulmonary flow. The chest x-ray pattern alone does not distinguish these.

The best initial test in any neonate with suspected congenital heart disease is transthoracic echocardiography, which also serves as the confirmatory or gold standard test for truncus arteriosus. Echocardiography demonstrates the pathognomonic finding: a single great vessel arising from the heart, overriding a large VSD, with a single semilunar (truncal) valve. It also evaluates truncal valve regurgitation, defines the anatomic subtype, and identifies any aortic arch anomalies.

ECG typically shows biventricular hypertrophy because both ventricles eject into a high-pressure systemic circuit.

Because of the strong link with DiGeorge syndrome, all patients require genetic testing for 22q11.2 deletion by FISH or chromosomal microarray. Calcium and immune function (T-cell subsets) should be checked because these patients can have hypocalcemia and T-cell immunodeficiency.

Cardiac catheterization is no longer routine for diagnosis and is reserved for cases where pulmonary vascular resistance must be measured (typically in older infants presenting late) or where echo cannot fully define the anatomy.

4. Management and Treatment

Treatment Summary Table

Phase	Intervention
Acute neonatal stabilization	Treat heart failure: furosemide 1 mg/kg IV every 6 to 12 hours, digoxin, afterload reduction with captopril or enalapril. Avoid supplemental oxygen above what is needed for adequate saturation, since oxygen lowers pulmonary vascular resistance and worsens overcirculation
Avoid prostaglandin E1	Unlike most ductal-dependent lesions, truncus is NOT ductal dependent. PGE1 worsens pulmonary overcirculation and is contraindicated in pure truncus (exception: when associated with interrupted aortic arch, where systemic flow is duct-dependent)
Definitive surgery	Complete repair within the first 2 to 6 weeks of life: VSD closure (committing the truncal valve to the left ventricle), separation of pulmonary arteries from the trunk, placement of a right ventricle to pulmonary artery valved conduit (Rastelli-type repair)
Long-term follow-up	Conduit replacement is required as the child grows (typically every 5 to 10 years), management of truncal valve regurgitation, endocarditis prophylaxis for high-risk procedures
Genetic and immunologic care	If 22q11.2 deletion confirmed: irradiated and CMV-negative blood products, monitor calcium, evaluate T-cell function, delay live vaccines if immunodeficient

Step-by-Step Management

The next best step when the diagnosis is suspected in a neonate is echocardiography and pediatric cardiology consultation. Once confirmed, the child is admitted to a cardiac intensive care unit.

Acute medical management focuses on treating congestive heart failure. Diuretics (furosemide) reduce pulmonary congestion. Digoxin and afterload-reducing agents (ACE inhibitors) improve cardiac output. A subtle but high-yield point is that excess supplemental oxygen should be avoided, because oxygen is a pulmonary vasodilator and lowering pulmonary vascular resistance further increases left-to-right shunting and worsens heart failure. Target oxygen saturation is typically around 75 to 85 percent until surgical repair.

Prostaglandin E1 is generally contraindicated in isolated truncus arteriosus because the lesion is not duct-dependent and prostaglandin would worsen pulmonary overcirculation. The important exception is truncus arteriosus with interrupted aortic arch (Van Praagh A4), in which systemic perfusion to the lower body depends on ductal patency, and PGE1 must be started.

Surgical repair is definitive and should occur within the first 2 to 6 weeks of life, before pulmonary vascular disease develops. The operation involves closing the VSD (so the left ventricle ejects into the truncus, which becomes the neoaorta), detaching the pulmonary arteries from the trunk, and creating right ventricular outflow continuity using a valved conduit (RV to PA conduit). This is a form of Rastelli repair.

Long-term management centers on the inevitable need for conduit replacement as the child outgrows it, monitoring of the truncal (now neoaortic) valve for regurgitation, arrhythmia surveillance, and infective endocarditis prophylaxis for dental and surgical procedures involving infected tissue.

If 22q11.2 deletion is confirmed, irradiated and CMV-negative blood products must be used to prevent transfusion-associated graft-versus-host disease, calcium levels are monitored for hypoparathyroidism, and live vaccines are avoided until T-cell function is documented.

5. Differential Diagnosis and Distractors

Differential	Why It's Similar	Key Discriminator
Tetralogy of Fallot	Cyanotic neonate with a VSD and a great vessel overriding the septum	Tetralogy has TWO semilunar valves (normal split S2), pulmonary stenosis (decreased pulmonary markings on x-ray, "boot-shaped heart"), and right ventricular hypertrophy on ECG. Truncus has a SINGLE S2 and INCREASED pulmonary markings
Transposition of the great arteries (d-TGA)	Cyanotic neonate, often with cardiomegaly	TGA presents with severe cyanosis on day 1 of life, "egg on a string" cardiac silhouette, and is duct-dependent (PGE1 is lifesaving). Truncus presents with milder cyanosis worsening over weeks
Total anomalous pulmonary venous return (TAPVR)	Cyanosis, increased pulmonary markings, neonatal heart failure	TAPVR shows "snowman" or "figure of 8" cardiac silhouette (supracardiac type), and echo shows pulmonary veins draining into systemic circulation rather than left atrium. No single trunk
Large patent ductus arteriosus	Wide pulse pressure, bounding pulses, congestive heart failure	PDA produces a CONTINUOUS "machinery" murmur and is acyanotic. Truncus has a systolic ejection murmur and cyanosis
Aortopulmonary window	Single semilunar valve picture on rapid evaluation, CHF, increased pulmonary flow	In AP window, two SEPARATE semilunar valves are present (normal S2 split), the aorta and pulmonary artery are otherwise distinct, and there is an abnormal communication between them. Echo distinguishes
Pulmonary atresia with VSD	Was historically called "Type IV truncus"	True pulmonary arteries are absent; pulmonary flow comes from MAPCAs (major aortopulmonary collateral arteries). DECREASED pulmonary markings on x-ray, severe cyanosis
Single ventricle physiology	Cyanosis with mixing	Echo shows a single functional ventricle. Truncus has two ventricles with a VSD

6. Traps and High-Yield Pearls

The most common way students miss this question is by failing to register the single S2 in the physical exam description and instead anchoring on cyanosis to choose tetralogy of Fallot or transposition. The single S2 is the single most reliable bedside clue, and any cyanotic newborn vignette with a single S2, wide pulse pressure, bounding pulses, and increased pulmonary vascular markings is truncus arteriosus until proven otherwise.

A second trap is the prostaglandin question. Test writers love to give a cyanotic newborn vignette and ask for the next step in management. For most cyanotic congenital heart lesions, the answer is PGE1. For truncus arteriosus, it is the wrong answer because the lesion is not duct-dependent and PGE1 worsens pulmonary overcirculation. The exception is when the vignette mentions an interrupted aortic arch or differential pulses between upper and lower extremities, which signals duct-dependent systemic circulation and makes PGE1 correct.

A third trap is the DiGeorge association. When a vignette includes a neonate with cyanosis, hypocalcemic seizures, an absent thymic shadow on chest x-ray, or recurrent infections, the underlying lesion is almost always truncus arteriosus or interrupted aortic arch. Ordering a chromosomal microarray or FISH for 22q11.2 deletion is the next best step in workup beyond echocardiography.

A fourth trap involves oxygen administration. In most neonatal respiratory distress scenarios, supplemental oxygen is appropriate. In truncus and other left-to-right shunt lesions with pulmonary overcirculation, high inspired oxygen lowers pulmonary vascular resistance and paradoxically worsens heart failure. The correct management targets a saturation around 75 to 85 percent, not 100 percent.

The core competency tested here is recognizing the hemodynamic pattern of mixing plus pulmonary overcirculation in a neonate, identifying the single S2 as the discriminating physical sign, and sequencing the workup correctly with echocardiography first, then genetic testing for 22q11.2, while avoiding the reflex use of prostaglandin E1 and high-flow oxygen.

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