7.3 Truncus Arteriosus, Double Outlet Right Ventricle (DORV) & Aortopulmonary Window

7.3 Truncus Arteriosus, Double Outlet Right Ventricle (DORV) & Aortopulmonary Window

Clinical Core: Outflow tract and conotruncal malformations encompass a continuum of developmental arrest in the partitioning and ventriculoarterial alignment of the embryonic arterial pole. Truncus Arteriosus represents a complete absence of conotruncal septation, leaving a solitary great vessel supplying the systemic, pulmonary, and coronary circulations. In Double Outlet Right Ventricle (DORV), both great arteries arise predominantly (>50%) from the morphologic right ventricle, characterized by bilateral infundibula and variable VSD relationships. Aortopulmonary Window (AP Window) represents a localized defect between the ascending aorta and pulmonary trunk in the presence of two separate semilunar valves. Meticulous multiplane echocardiographic differentiation among these complex entities is paramount for surgical planning.

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Truncus Arteriosus (Common Arterial Trunk)

Embryology & Anatomic Architecture

Between the 5th and 7th weeks of human gestation, the aortopulmonary septal complex fails to fuse and spiral down to meet the conal septum. Consequently, a single, solitary great arterial vessel—the common arterial trunk—exits the heart base:

• Single Semilunar Valve (Truncal Valve): Overrides a massive, non-restrictive conoventricular malalignment ventricular septal defect.
• Universal Circulation Supply: The common trunk directly feeds the systemic ascending aorta, the pulmonary arterial tree, and both coronary arteries.
• Absence of Separate RVOT: There is complete absence of an anatomically separate subpulmonary infundibulum or native pulmonary valve apparatus.
• Fibrous Continuity: Strict fibrous continuity is maintained between the posterior cusp of the common truncal valve and the anterior leaflet of the mitral valve.

                         TRUNCUS ARTERIOSUS ARCHITECTURE
                             ┌────────────────────┐
                             │   Ascending Aorta  │
                             └─────────┬──────────┘
                                       │
                        Pulmonary ◄────┼────► Coronary Arteries
                        Arteries       │
                                ┌──────┴──────┐
                                │Truncal Valve│ (2-6 cusps, 30% quadricuspid)
                                └──────┬──────┘
                                       │
                        ┌──────────────┴──────────────┐
                        │   Large Conoventricular VSD │
                        │       (Aortic Override)     │
                        └──────┬──────────────┬───────┘
                               ▼              ▼
                           [Right Vent]   [Left Vent]

Truncal Valve Morphology & Hemodynamic Dysfunction

The truncal valve is inherently dysplastic, thickened, and nodular. Detailed multiplane 2D and 3D transesophageal or transthoracic interrogation reveals marked variability in cusp architecture:

• Cusp Number: Tricuspid in 60% to 65%, Quadricuspid in 30% to 35%, Bicuspid in 5%, and rarely pentacuspid or hexacuspid (<1%).
• Truncal Valve Regurgitation (TR): Clinically significant (moderate-to-severe) truncal regurgitation is present in 25% to 35% of patients. TR creates a devastating hemodynamic volume overload: during diastole, blood regurgitates from the common trunk back into both ventricles, while simultaneous runoff flows into the low-resistance pulmonary vascular bed. This produces marked myocardial ischemia, cardiogenic shock, and severe ventricular volume overload.
• Truncal Valve Stenosis: Present in 10% to 15% due to restricted leaflet excursion or thickened nodular raphes.

Anatomical Classifications: Collett & Edwards vs. Van Praagh

Two established classification systems describe the morphological origin of the pulmonary arteries from the common trunk:

    COLLETT & EDWARDS I              COLLETT & EDWARDS II             COLLETT & EDWARDS III
   ┌──────────────────────┐         ┌──────────────────────┐         ┌──────────────────────┐
   │      [Aorta]         │         │      [Aorta]         │         │      [Aorta]         │
   │         │            │         │         │            │         │   LPA   │   RPA      │
   │   MPA ──┤            │         │  LPA ───┤            │         │    ▲    │    ▲       │
   │  ┌─┴─┐  │            │         │  RPA ───┘            │         │    │    │    │       │
   │ RPA LPA │            │         │ (Separate Adjacent)  │         │ (Distant Origins)    │
   └──────────────────────┘         └──────────────────────┘         └──────────────────────┘

1. Collett & Edwards Classification (1949)

• Type I (50% to 60%): A distinct, short main pulmonary artery (MPA) arises from the left posterolateral aspect of the common trunk and subsequently bifurcates into the right (RPA) and left (LPA) pulmonary arteries.
• Type II (30% to 35%): The RPA and LPA arise independently but from closely adjacent orifices on the dorsal (posterior) wall of the common arterial trunk, without an intervening main pulmonary segment.
• Type III (5% to 10%): The RPA and LPA arise independently from widely separated, distant lateral walls of the common arterial trunk.
• Type IV (Historical): Pulmonary blood flow derived entirely from descending aortic collaterals without native pulmonary branch origins from the trunk (now recognized as Tetralogy of Fallot with Pulmonary Atresia and MAPCAs, rather than true truncus).

2. Van Praagh Classification (1965)

• Type A1: Equivalent to Collett & Edwards Type I (discrete MPA trunk present).
• Type A2: Separate origins of RPA and LPA from the dorsal or lateral truncal wall (combining Collett & Edwards Types II and III).
• Type A3: Solitary pulmonary artery (usually RPA) originates from the common trunk, while the contralateral pulmonary artery (usually LPA) is supplied retrogradely via a PDA or collateral artery.
• Type A4: Truncus arteriosus accompanied by Interrupted Aortic Arch (IAA, typically Type B), severe aortic hypoplasia, or discrete coarctation. A massive patent ductus arteriosus continues directly into the descending thoracic aorta. This variant requires complex neonatal arch reconstruction simultaneously with truncal repair.

Syndromic Association: 22q11.2 Microdeletion (DiGeorge Syndrome)

Approximately 30% to 40% of infants with Truncus Arteriosus harbor the chromosome 22q11.2 microdeletion (DiGeorge / Velocardiofacial syndrome). The association is particularly high in patients with Van Praagh Type A4 (IAA) and those with a Right Aortic Arch (present in 30% to 35% of all truncus cases). Preoperative recognition is vital due to neonatal risks of hypocalcemic seizures (parathyroid hypoplasia) and impaired immunity (thymic hypoplasia).

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Double Outlet Right Ventricle (DORV)

Anatomical Definition & Infundibular Architecture

Double Outlet Right Ventricle is defined as a congenital ventriculoarterial connection anomaly where both great arteries arise entirely or predominantly (>50% of the circumference of both semilunar valves) from the morphologic right ventricle:

• The 50% Rule: If the aortic valve is >50% committed to the RV and the pulmonary valve is 100% committed to the RV, the heart is categorized as DORV.
• Bilateral Infundibulum (Bilateral Conus): The anatomical hallmark of classic DORV is the presence of both a subaortic muscular conus and a subpulmonary muscular conus. The subaortic conus physically displaces the aortic valve anteriorly and superiorly away from the ventricular septum. Consequently, fibrous continuity between the aortic valve and the mitral valve is completely lost (mitral-aortic discontinuity), providing a key echocardiographic diagnostic differentiator from Tetralogy of Fallot.

     NORMAL OUTFLOW TRACT                     DOUBLE OUTLET RIGHT VENTRICLE (DORV)
┌─────────────────────────────┐             ┌─────────────────────────────────────┐
│      [Aortic Valve]         │             │   [Aortic Valve]   [Pulmonary Valve]│
│             │               │             │         │                 │         │
│ (Direct Fibrous Continuity) │             │ (Subaortic Conus) (Subpulm Conus)   │
│             │               │             │         └────────┬────────┘         │
│    [Mitral Valve Leaflet]   │             │       [Bilateral Infundibulum]      │
│                             │             │                  │                  │
│   (Normal Left Ventricle)   │             │   (Mitral-Aortic Discontinuity)     │
│                             │             │                  │                  │
│                             │             │      BOTH ARTERIES EMERGE FROM RV   │
└─────────────────────────────┘             └─────────────────────────────────────┘

Classification of DORV by VSD Relationship

Because virtually all blood leaving the left ventricle must transit the ventricular septal defect to exit the heart, the anatomical relationship of the VSD to the two semilunar valves dictates both the clinical pathophysiology and the surgical reconstructive strategy:

┌─────────────────────────────────────────────────────────────────────────────┐
│                     DORV CLASSIFICATION BY VSD LOCATION                     │
├──────────────────────────┬──────────────────────────┬───────────────────────┤
│       VSD Location       │  Physiology / Phenotype  │  Surgical Baffle Path │
├──────────────────────────┼──────────────────────────┼───────────────────────┤
│ 1. Subaortic (50-60%)    │ TOF-like (with PS) or    │ LV ──► VSD ──► Aorta  │
│                          │ Large VSD (without PS)   │ (Intraventricular)    │
├──────────────────────────┼──────────────────────────┼───────────────────────┤
│ 2. Subpulmonic (30-35%)  │ TGA-like (Parallel circ) │ Arterial Switch +     │
│    (Taussig-Bing)        │ Severe cyanosis; Coarct  │ VSD baffle to PA      │
├──────────────────────────┼──────────────────────────┼───────────────────────┤
│ 3. Doubly Committed (5%) │ High volume L-to-R shunt │ LV ──► VSD ──► Aorta  │
│                          │ Risk of cusp prolapse    │ (Intraventricular)    │
├──────────────────────────┼──────────────────────────┼───────────────────────┤
│ 4. Non-committed (10%)   │ Remote VSD (Inlet/Apex)  │ Single Ventricle      │
│                          │ Complex intraventricular │ (Fontan Pathway)      │
└──────────────────────────┴──────────────────────────┴───────────────────────┘

1. Subaortic VSD (50% to 60% of DORV Cases)

• Anatomy: The VSD is located in the conoventricular septum directly beneath the subaortic infundibulum.
• Pathophysiology: Left ventricular effluent streams preferentially across the defect into the aorta.
  • With Pulmonary Stenosis (Fallot-type DORV): Presents with dynamic cyanosis and clinical behavior identical to Tetralogy of Fallot.
  • Without Pulmonary Stenosis: Presents with massive left-to-right shunting, pulmonary overcirculation, and congestive heart failure.
• Surgical Repair: Straightforward biventricular repair via an intraventricular tunnel patch directing LV blood across the VSD into the subaortic conus, with RVOT enlargement or conduit if pulmonary stenosis coexists.

2. Subpulmonic VSD (30% to 35% - Taussig-Bing Anomaly)

• Anatomy: The VSD is positioned directly beneath the subpulmonary infundibulum, separated from the aortic valve by the infundibular septum.
• Pathophysiology: Left ventricular blood streams preferentially through the VSD into the pulmonary artery, while deoxygenated systemic venous blood from the RV enters the aorta. This produces parallel circulatory physiology identical to d-TGA, presenting with severe, profound cyanosis.
• Associated Arch Obstruction: Approximately 50% of Taussig-Bing neonates have aortic coarctation, tubular hypoplasia of the aortic arch, or interrupted aortic arch.
• Surgical Repair: Neonatal Arterial Switch Operation (ASO) combined with an intraventricular baffle tunneling the VSD into the neoaorta (former pulmonary valve) and arch reconstruction.

3. Doubly Committed VSD (3% to 5%)

• Anatomy: Absence of the muscular infundibular septum leaves a large subarterial defect committed to both the aortic and pulmonary semilunar valves.
• Surgical Repair: Intraventricular baffling to the aorta, typically with semilunar valve suspension.

4. Non-Committed / Remote VSD (10% to 15%)

• Anatomy: The VSD is situated distant from both semilunar valves—most commonly in the posterior-inferior inlet septum (AV canal type) or apical trabecular septum, separated from the outflow tracts by extensive muscular tissue or the tricuspid valve tensor apparatus.
• Surgical Dilemma: Constructing an intraventricular tunnel from a remote VSD to the subaortic valve would require an enormous prosthetic patch that would compromise right ventricular cavitary volume and obstruct the tricuspid inflow. Consequently, remote DORV frequently mandates conversion to single-ventricle staged palliation (Glenn and Fontan pathway).

Spatial Great Artery Relationships in DORV

In DORV, the great arteries do not exhibit a single fixed spatial relationship. On parasternal short-axis imaging, the sonographer identifies four distinct alignments:

1. Side-by-Side (Aorta to the right of PA): Most frequent in subaortic VSD (~60%).
2. D-Malposed (Aorta anterior and to the right of PA): Classic spatial relationship in the Taussig-Bing anomaly (~30%).
3. Normal Spiral Crossover: Aorta posterior and rightward (~10%).
4. L-Malposed (Aorta anterior and to the left of PA): Associated with L-looped ventricles or heterotaxy syndromes (<5%).

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Aortopulmonary Window (Aortopulmonary Septal Defect)

Embryology & Definition

An Aortopulmonary Window (AP Window) is a rare conotruncal anomaly (<0.5% of all congenital cardiac malformations) resulting from a localized failure of the embryonic aortopulmonary septum to form or fuse between the ascending aorta and the pulmonary trunk.

Crucial Diagnostic Hallmark: Two Separate Semilunar Valves

Unlike Truncus Arteriosus, which has only a single common truncal valve, an Aortopulmonary Window always features TWO separate, anatomically distinct semilunar valves (aortic and pulmonary valves), each supported by its own fibrous annulus and separated by normal sinus tissue. The interventricular septum is typically intact in isolated AP window (though associated VSD occurs in ~10%), whereas Truncus Arteriosus obligatorily features a large malalignment VSD.

     TRUNCUS ARTERIOSUS                         AORTOPULMONARY WINDOW
┌─────────────────────────────┐             ┌─────────────────────────────┐
│      [Common Trunk]         │             │   [Asc Aorta]  [Pulm Trunk] │
│             │               │             │         ▲           ▲       │
│             ▼               │             │         └──[Window]─┘       │
│    ONE TRUNCAL VALVE        │             │                             │
│       (2-6 cusps)           │             │   Aortic Valve  Pulm Valve  │
│             │               │             │     (3 cusps)    (3 cusps)  │
│             ▼               │             │   TWO DISTINCT SEPARATE     │
│     Large VSD Obligatory    │             │      SEMILUNAR VALVES       │
└─────────────────────────────┘             └─────────────────────────────┘

Richardson Anatomical Classification

The Richardson classification categorizes AP windows according to their position between the semilunar valves and the pulmonary artery bifurcation:

1. Type I (Proximal Defect, ~70%): Located in the proximal portion of the aortopulmonary septum immediately above the sinuses of Valsalva and semilunar valves, with a very short or absent inferior rim above the valve cusps.
2. Type II (Distal Defect, ~20%): Located in the upper portion of the aortopulmonary septum near the pulmonary artery bifurcation and origin of the right pulmonary artery (RPA), with a well-formed inferior rim above the semilunar valves.
3. Type III (Total / Confluent Defect, ~10%): Extensive defect involving virtually the entire length of the aortopulmonary septum from the semilunar valve annuli to the pulmonary bifurcation.
4. Type IV (Intermediate / Anomalous Origin): Defect associated with anomalous origin of the right pulmonary artery from the ascending aorta (AORPA).

Pathophysiology & Hemodynamics

Because the defect connects the high-pressure ascending aorta directly to the low-resistance pulmonary trunk:

• As pulmonary vascular resistance falls after birth, a massive left-to-right shunt floods the pulmonary circulation under systemic systolic driving pressure.
• This produces severe left atrial and left ventricular volume overload, tachypnea, diaphoresis, failure to thrive, and pulmonary edema.
• Physical examination reveals bounding peripheral pulses and a wide pulse pressure (due to diastolic runoff into the pulmonary bed).
• High Risk of Accelerated Eisenmenger Syndrome: Because the pulmonary vasculature is subjected directly to systemic pressure AND massive flow volume, irreversible pulmonary vascular obstructive disease develops very rapidly—frequently within the first 6 to 12 months of life if surgical patch closure is not performed.

Associated Congenital Cardiovascular Anomalies (~50% of Cases)

• Interrupted Aortic Arch (IAA): Present in 15% to 20% of cases, typically Type A (interruption distal to the left subclavian artery), in contrast to Truncus Arteriosus, which is predominantly associated with Type B.
• Coarctation of the Aorta & Arch Hypoplasia (15% to 20%).
• Berry Syndrome (The Lethal Conotruncal Tetrad): A rare, distinct congenital complex comprising:
  1. Distal aortopulmonary window (Type II)
  2. Anomalous origin of the Right Pulmonary Artery from the ascending aorta (AORPA)
  3. Interrupted Aortic Arch (Type A) or severe aortic coarctation
  4. Intact ventricular septum with a large Patent Ductus Arteriosus (PDA)
• Tetralogy of Fallot or VSD (~5% to 10%).

Echocardiographic Interrogation & Differential Diagnosis

• High Parasternal Short-Axis & Modified Outflow Views: Direct visualization of tissue dropout in the wall separating the ascending aorta from the main pulmonary artery.
• Confirming Two Semilunar Valves: Sweep from the ventricular base through the outflow tracts to definitively demonstrate two separate valve rings opening independently.
• Differentiating AP Window from PDA:
  • A PDA is located extracardiac and distal, connecting the proximal descending aorta (isthmus) to the roof of the main pulmonary artery near the left pulmonary artery origin.
  • An AP Window is located intrapericardial and proximal, connecting the ascending aorta to the main pulmonary artery trunk or origin of the RPA.
• Spectral Doppler: Continuous-wave Doppler demonstrates continuous, high-velocity left-to-right flow in small, restrictive windows. In large non-restrictive defects, flow velocities are low (<2.0 m/s), and flow is bidirectional during systole and diastole due to pressure equalization.

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Outflow Malformation Matrix: Truncus vs. DORV vs. AP Window

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Clinical Pearls & Sonographic Traps

[!WARNING] Differentiating Truncus Arteriosus from AP Window and TOF/PA: Always systematically count the semilunar valves and trace branch pulmonary artery origins. If there is only ONE semilunar valve overriding a large VSD, the diagnosis is Truncus Arteriosus (or TOF/PA if pulmonary arteries do not arise from the trunk). If there are TWO distinct semilunar valves with separate fibrous annuli, Truncus Arteriosus is ruled out; tissue dropout between the great arterial trunks represents an Aortopulmonary Window.

[!TIP] Evaluating the Post-Tricuspid Distance in DORV: When assessing a patient with DORV for biventricular intraventricular baffle repair versus single-ventricle palliation, the pediatric echocardiographer must measure the distance from the tricuspid valve annulus to the pulmonary and aortic valve annuli. If the distance between the VSD and the subaortic conus is obstructed by prominent tricuspid chordal attachments, straddling tricuspid valve, or an anomalous papillary muscle, tunneling the baffle through the RV cavity will cause acute tricuspid inflow or RV outflow tract obstruction, necessitating a Fontan pathway.

[!NOTE] Diastolic Steal in Truncus and AP Window: In both large AP windows and Truncus Arteriosus, the lack of an intact, resistive barrier between the high-pressure systemic circuit and the low-resistance pulmonary vascular bed produces torrential diastolic runoff. Interrogation of the abdominal descending aorta demonstrates prominent holodiastolic flow reversal (similar to a wide-open PDA or severe aortic regurgitation), stealing vital perfusion pressure from coronary, cerebral, and renal vascular beds.