11.3 Right Ventricle to Pulmonary Artery (RV-PA) Conduits & Ross Procedure

11.3 Right Ventricle to Pulmonary Artery (RV-PA) Conduits & Ross Procedure

Clinical Core: Surgical management of complex pediatric left and right heart lesions frequently necessitates replacing semilunar valves or establishing continuity between the right ventricle and pulmonary arteries using biological autografts, allografts, or heterografts. The Ross Procedure replaces a diseased aortic valve with the patient's living pulmonary autograft, offering somatic growth potential at the expense of creating two valve positions requiring lifelong surveillance. Concurrently, Right Ventricle to Pulmonary Artery (RV-PA) Conduits are required across a broad spectrum of congenital cardiac repairs (Truncus Arteriosus, TOF with Pulmonary Atresia, Rastelli, and Ross). Because all extracardiac conduits have a finite lifespan, mastering the Doppler quantification of conduit stenosis, conduit regurgitation, and Transcatheter Pulmonary Valve Replacement (TPVR) is a cornerstone of pediatric echocardiography.

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Part 1: Extracardiac RV-to-PA Conduits

Extracardiac conduits are essential in pediatric cardiac surgery to bridge anatomical discontinuities between the right ventricle and the pulmonary arterial tree.

Types of Conduits

• Cryopreserved Homografts (Allografts): Cryopreserved human cadaveric donor valves. Pulmonary homografts are preferred over aortic homografts because they exhibit lower rates of dystrophic calcification, less fibroelastic thickening, and longer structural durability. While they accommodate somatic growth in small infants better than rigid prosthetic rings, they lack living cellular repair mechanisms and do not grow with the patient.
• Bovine Jugular Vein Conduits (Contegra): A natural bovine jugular vein segment containing an intact, competent trileaflet venous valve. Highly pliable and easily trimmed, with no rigid stent ring, making it ideal for infant RVOT reconstruction. However, it is uniquely susceptible to distal anastomotic intimal hyperplasia, aneurysmal dilation of the venous wall, and a pronounced predisposition to infective endocarditis.
• Porcine Valved Dacron Conduits (Hancock): A glutaraldehyde-preserved porcine aortic valve mounted inside a woven Dacron prosthetic tube. Prone to severe calcific cusp immobilization, leaflet tearing, and the development of an inner neointimal peel that severely narrows the functional conduit lumen.
• Non-Valved Polytetrafluoroethylene (PTFE / Gore-Tex) Conduits: Commonly utilized in neonates and small infants where small conduit caliber (<10 to 12 mm) precludes a functional valve. The resulting free pulmonary regurgitation is tolerated well in the short term, but mandates late valved conduit placement.

Mechanisms of Conduit Failure

1. Somatic Outgrowth: A conduit sized for an infant or small child (e.g., 10 to 12 mm) becomes relatively stenotic as the child grows into a larger body surface area, even in the complete absence of intrinsic tissue degeneration.
2. Dystrophic Calcification: Chronic mechanical stress, low-grade cell-mediated immune rejection, and calcium phosphate deposition induce progressive calcification of the conduit wall and valve cusps, causing rigid leaflet immobility and luminal narrowing.
3. Neointimal Peel & Fibroelastosis: Proliferation of a thick, fibrous, collagenous "peel" lining the inner lumen of synthetic or biological conduits creates circumferential subvalvar or supravalvar narrowing.
4. Valvular Regurgitation: Leaflet tear, prolapse, retraction, or calcific fixation results in free pulmonary regurgitation (PR), causing progressive right ventricular volume overload, RV dilation, and secondary tricuspid annular stretching.

Comprehensive Spectral Doppler Interrogation

To quantify conduit hemodynamics accurately, the sonographer must employ continuous-wave (CW) Doppler from multiple imaging windows (parasternal short-axis, high left parasternal, subcostal sagittal):

• Peak Instantaneous Gradient: Derived using the modified Bernoulli equation:

$\Delta P_{peak} = 4 \times (V_{max})^2$

• Mean Pressure Gradient: Obtained by tracing the continuous-wave Doppler spectral envelope across systole:

$\Delta P_{mean} = \text{Integrated Mean Systolic Gradient}$

• Severity Thresholds for Conduit Stenosis:
  • Mild Stenosis: Peak velocity <3.0 m/s (peak gradient <36 mmHg).
  • Moderate Stenosis: Peak velocity 3.0 to 4.0 m/s (peak gradient 36 to 64 mmHg).
  • Severe Stenosis: Peak velocity >4.0 m/s (peak gradient >64 mmHg) or an estimated right ventricular systolic pressure (RVSP via TR jet) exceeding two-thirds of systemic pressure.
• Differentiating Levels of Obstruction (PW Doppler Mapping): Sweep pulsed-wave Doppler along the conduit to distinguish:
  1. Proximal obstruction at the RV-conduit ventriculotomy anastomosis,
  2. Valvar stenosis at the conduit valve leaflets,
  3. Distal stenosis at the pulmonary artery bifurcation anastomosis, or
  4. Branch PA stenosis.

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Catheter Interventions & Transcatheter Pulmonary Valve Replacement (TPVR)

To avoid repeated open-heart sternotomies, failing RV-PA conduits are frequently managed in the cardiac catheterization laboratory using balloon angioplasty, bare-metal stenting, and Transcatheter Pulmonary Valve Replacement (TPVR).

TPVR Devices

• Melody Valve (Medtronic): A bovine jugular vein valve sutured within a balloon-expandable platinum-iridium Cheatham-Platinum (CP) stent. Designed for conduit diameters between 16 and 22 mm.
• SAPIEN Valve (Edwards Lifesciences): Bovine pericardial tissue mounted inside a balloon-expandable cobalt-chromium stent frame (SAPIEN XT or SAPIEN 3), capable of expanding into larger conduits and native RVOTs up to 29 mm.

Post-TPVR Echocardiographic Surveillance Protocol

1. Stent Seating and Position: Evaluate in parasternal short-axis, high parasternal, and subcostal views. The radiopaque stent appears as two bright, parallel echogenic lines in the RVOT. Confirm stable anchoring without proximal migration into the RV cavity or distal embolization into the pulmonary bifurcation.
2. Stent Fracture Detection: High-mechanical-stress RVOT environments can cause fatigue fractures of stent struts. On 2D and zoom imaging, identify any strut disruption, discontinuity, or inward strut protrusion that could cause leaflet damage.
3. Residual Transvalvular Gradient: Baseline CW Doppler should show marked gradient reduction (typical post-implant peak velocity 1.5 to 2.2 m/s, peak gradient <20 to 25 mmHg). A progressive rise in velocity on serial exams suggests stent compression, leaflet thrombosis, or endocarditis.
4. Paravalvular Leak (PVL) vs. Valvar Regurgitation: Color Doppler sweeps around the circumference of the stent frame distinguish a paravalvular leak (color jet originating outside the stent struts) from intravalvular regurgitation (central jet through dysfunctional leaflets).
5. Branch Pulmonary Artery Patency: Confirm that the distal stent rim does not protrude across or jail the origin of the left or right pulmonary artery.

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Part 2: The Ross Procedure (Pulmonary Autograft)

Pioneered by Donald Ross in 1967, the Ross procedure is primarily indicated for severe congenital or acquired aortic stenosis and regurgitation in children and young adults where mechanical valve replacement is undesirable (to avoid lifelong systemic anticoagulation and permit somatic root growth):

Ross Procedure Surgical Anatomy:

1. Native Diseased Aortic Root/Valve  ──► Excised and Discarded
2. Native Pulmonary Root/Valve (Living Autograft) ──► Translocated to Systemic Aortic Position
3. Coronary Artery Buttons           ──► Re-implanted into Neoaortic Autograft
4. Reconstructed RVOT                 ──► Cryopreserved Pulmonary or Aortic Homograft

Echocardiographic Surveillance Targets After the Ross Procedure

1. Autograft Neoaortic Root Dilation: Like the arterial switch operation, the native pulmonary root is exposed to systemic vascular pressures. Over time, progressive dilatation of the neo-annulus, neo-sinuses of Valsalva, and neo-sinotubular junction develops. Serial 2D measurements in the parasternal long-axis view must be indexed to BSA and tracked as Z-scores. Neoaortic root Z-scores exceeding +3.0 to +4.0 carry risk of aneurysm formation and leaflet coaptation failure.
2. Neoaortic Valve Regurgitation (AR): Progressive autograft regurgitation results from root dilation stretching the commissures, geometric leaflet distortion, or intrinsic cusp prolapse. Quantify AR via color jet width/LVOT diameter ratio, pressure half-time, and holodiastolic flow reversal in the descending abdominal aorta.
3. Coronary Button Patency: Interrogate the reimplanted coronary ostia and surveil the LV myocardium for regional wall motion abnormalities or reduced global longitudinal strain.
4. RV-PA Homograft Degeneration: The pulmonary or aortic allograft in the RVOT position is subject to chronic immunological reaction and dystrophic calcification, culminating in progressive stenosis, regurgitation, or combined disease.

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Ross, Rastelli & Conduit Comparison Protocol

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Clinical Pearls & Sonographic Traps

[!WARNING] The Overestimated Conduit Gradient (Angle & Series Stenosis): When interrogating a curved extracardiac conduit, the velocity vector often changes direction across the ventriculotomy, valve, and distal anastomosis. Using a single transducer angle can severely misalign the Doppler beam. Furthermore, if a distal branch PA stenosis coexists with conduit valve stenosis, continuous-wave Doppler will superimpose both velocities, reporting an aggregate gradient. Always map with pulsed-wave Doppler to separate the conduit gradient from distal branch stenosis.

[!TIP] High Parasternal / Off-Axis Windows for RV-PA Conduits: Conduits are located immediately beneath the anterior chest wall and course superiorly and leftward. Standard apical and parasternal views often fail to align parallel to the conduit axis. Move the transducer one to two intercostal spaces higher on the left parasternal border and angle superiorly to achieve a pristine parallel intercept angle.

[!NOTE] Infective Endocarditis Vulnerability: Valved biological conduits (particularly bovine jugular vein Contegra conduits and Melody transcatheter valves) carry an extraordinarily high risk of infective endocarditis compared to native valves. Any new fever, unexplained rise in transconduit gradient, or worsening regurgitation mandates immediate, meticulous multiplane interrogation for mobile vegetations, stent sleeve abscesses, and leaflet destruction.