Truncus Arteriosus

01Pathophysiology

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.

02Classification of Clinical Manifestation

Collett and Edwards Anatomic Classification

Van Praagh Classification

Clinical Severity Spectrum

03Diagnostic Workup

Diagnostic Test Summary

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.

04Management and Treatment

Treatment Summary Table

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.

05Differential Diagnosis and Distractors

06Traps 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.