12.1 Norwood Stage I Reconstruction, Sano Shunts & Blalock-Taussig Shunts

12.1 Norwood Stage I Reconstruction, Sano Shunts & Blalock-Taussig Shunts

Clinical Core: Functionally univentricular hearts represent an anatomical and physiological spectrum where a solitary ventricle must support both the systemic and pulmonary circulations in parallel. In conditions such as Hypoplastic Left Heart Syndrome (HLHS), the morphologic right ventricle acts as the solitary pump. The neonatal Norwood Stage 1 reconstruction is an urgent surgical procedure performed in the first 1 to 2 weeks of life to convert an unstable, ductal-dependent circulation into a stable parallel circulation. The pediatric cardiac sonographer must thoroughly master the three pillars of the Norwood reconstruction—neoaorta formation, unrestrictive atrial septectomy, and calibrated pulmonary blood flow source—and execute a disciplined spectral Doppler surveillance protocol to detect life-threatening postoperative residual lesions.

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Anatomical Substrates & The Single Ventricle Paradigm

A functionally univentricular heart exists whenever complex congenital malformations prevent biventricular septation and repair. The common anatomical substrates requiring single ventricle staged palliation include:

1. Hypoplastic Left Heart Syndrome (HLHS): Marked hypoplasia or atresia of the aortic valve, mitral valve, left ventricle, and ascending aorta. The systemic circulation is ductal-dependent, relying entirely on right-to-left shunting across the patent ductus arteriosus (PDA).
2. Tricuspid Atresia: Complete agenesis of the tricuspid valve with right ventricular hypoplasia. Systemic venous return crosses an ASD/PFO into the left atrium, mixing with pulmonary venous return in a solitary morphologic left ventricle.
3. Double Inlet Left Ventricle (DILV): Both atrioventricular valves (or a common AV valve) enter a dominant morphologic left ventricle. An outlet chamber (rudimentary RV) frequently gives rise to one of the great arteries through a restrictive bulboventricular foramen (BVF).
4. Unbalanced Atrioventricular Septal Defect (AVSD): The common AV valve orifice is committed predominantly (>60%) to either the right ventricle (causing LV hypoplasia) or the left ventricle (causing RV hypoplasia).
5. Complex Heterotaxy Syndromes: Asplenia (right isomerism) or polysplenia (left isomerism) associated with common AV canal, anomalous pulmonary venous connections, and univentricular atrioventricular connections.

Normal In-Series Circulation:        Single Ventricle Parallel Circulation:
[Systemic Venous]                    [Systemic Venous]   [Pulmonary Venous]
       │                                    │                   │
       ▼                                    └───►[Common Pool]◄───┘
[Right Ventricle]                                      │
       │                                               ▼
       ▼                                       [Single Ventricle]
[Pulmonary Circulation]                                │
       │                                      ┌────────┴────────┐
       ▼                                      ▼                 ▼
[Left Ventricle]                         [Aorta (Qs)]     [PAs (Qp)]
       │                                (High Resistance) (Low Resistance)
       ▼
[Systemic Circulation]               Volume Overload: Total Output = Qp + Qs

Hemodynamics of Parallel Circulation & The Qp:Qs Balancing Act

In univentricular physiology, systemic venous return (deoxygenated) and pulmonary venous return (oxygenated) mix completely within the common atrium or single ventricle. From this common pool, the single ventricle ejects into both the systemic and pulmonary arterial systems in parallel:

• Total Cardiac Output: The single ventricle pumps the combined sum of systemic blood flow ($Q_s$) and pulmonary blood flow ($Q_p$): $Q_{\text{total}} = Q_p + Q_s$. This imposes chronic, massive volume overload on the single ventricle.
• Vascular Resistance Governance: Blood divides between the two vascular beds strictly according to downstream resistance ($Q_p / Q_s = \text{SVR} / \text{PVR}$).
• Systemic Steal (Overcirculation): Because normal postnatal pulmonary vascular resistance (PVR) falls rapidly below systemic vascular resistance (SVR), blood preferentially floods the compliant pulmonary bed ($Q_p:Q_s > 3:1$). This produces systemic hypoperfusion, metabolic acidosis, oliguria, necrotizing enterocolitis (NEC), and myocardial ischemia.
• Excessive Cyanosis (Undercirculation): If the pulmonary source is overly restrictive or PVR is elevated, $Q_p:Q_s$ drops below $1:1$, resulting in profound hypoxemia ($SpO_2 < 70%$) and tissue hypoxia.
• Ideal Hemodynamic Target: Staged palliation aims to maintain a balanced $Q_p : Q_s \approx 1:1$, delivering systemic arterial oxygen saturation between 75% and 85% while preserving systemic organ perfusion.

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The Classical Norwood Stage 1 Reconstruction

Performed via median sternotomy on cardiopulmonary bypass with deep hypothermic circulatory arrest (DHCA) or selective antegrade cerebral perfusion, the Norwood operation accomplishes three mandatory objectives:

Norwood Stage 1 Reconstruction Pillars:

1. Neoaorta Reconstruction:       2. Wide Atrial Septectomy:        3. Pulmonary Blood Flow Source:
   [Native Ascending Ao (1-3 mm)]     [Restrictive Interatrial Septum]     [Calibrated Shunt]
                 │                                   │                              │
                 ▼                                   ▼                              ▼
   [Fused Side-by-Side with MPA]      [Widely Excised to Annulus]         [mBTT (3.0-3.5 mm Gore-Tex)]
                 │                                   │                              OR
                 ▼                                   ▼                    [Sano (5.0-6.0 mm RV-PA)]
   [Arch Augmentation Patch]          [Guarantees Unobstructed PVR]                 OR
   (Dacron / Homograft / Bovine)      (Mean LA-RA Gradient <2 mmHg)       [Bilateral PA Bands (Hybrid)]

1. Neoaorta Reconstruction

• The main pulmonary artery (MPA) trunk is transected just proximal to its bifurcation into the right and left branch pulmonary arteries.
• The diminutive native ascending aorta (often only 1 to 3 mm in diameter in aortic atresia) is incised longitudinally along its medial curvature down to the aortic root. The coarctation shelf and ductal tissue in the transverse and descending arch are completely resected.
• The transected MPA trunk is joined side-by-side to the hypoplastic ascending aorta. A large cryopreserved pulmonary homograft, aortic homograft, or bovine pericardial patch is then sutured to augment the neoaortic root, the transverse arch, and the proximal descending aorta past the ductal insertion.
• Coronary Perfusion: Coronary blood flow is driven retrograde down the native diminutive ascending aorta from the high-pressure neoaortic root. Any narrowing at this native aorta-neoaorta junction causes catastrophic coronary ischemia.

2. Wide Atrial Septectomy

• In HLHS, pulmonary venous blood returning to the left atrium must cross the interatrial septum to reach the systemic right ventricle.
• The surgeon completely excises the septum primum and septum secundum, creating a large, non-restrictive interatrial communication. This prevents left atrial hypertension, pulmonary venous congestion, and pulmonary edema.

3. Branch Pulmonary Artery Disconnection & Reconstruction

• The pulmonary artery bifurcation is detached from the ventricular outflow tract. The confluence defect is closed with an autologous pericardial or homograft patch, preparing the branch pulmonary arteries to receive a dedicated, calibrated systemic shunt.

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Pulmonary Blood Flow Supply: mBTT Shunt vs. Sano Conduit

Because the pulmonary trunk is incorporated into the neoaorta, the surgeon must construct a dedicated, limited source of pulmonary blood flow. The two primary techniques are the modified Blalock-Thomas-Taussig (mBTT) shunt and the Sano right ventricle-to-pulmonary artery (RV-PA) conduit.

Modified Blalock-Thomas-Taussig (mBTT):        Sano (RV-to-PA Conduit):
       [Innominate Artery]                          [Single RV Wall]
               │                                           │
      [3.0-3.5 mm Gore-Tex]                       [5.0-6.0 mm Gore-Tex]
               │                                           │
               ▼                                           ▼
     [Right Pulmonary Artery]                   [Pulmonary Artery Confluence]
 continuous gradient (3-4 m/s)                  Systolic ejection jet (2-3 m/s)
 Marked diastolic runoff (coronary steal)       Preserved diastolic pressure

Detailed Comparison: Hemodynamics & Vulnerabilities

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The Hybrid Stage 1 Alternative

For high-risk neonates—such as those with extreme low birth weight (<2.0 kg), severe intracranial hemorrhage, end-organ shock, or necrotizing enterocolitis—conventional cardiopulmonary bypass and circulatory arrest carry prohibitive mortality. The Hybrid Stage 1 procedure provides temporary stabilization without open-heart surgery:

1. Bilateral Branch Pulmonary Artery Banding: Performed via median sternotomy without bypass. Bands (PTFE cuffs) are placed around the proximal RPA and LPA, constricting them to a circumference of ~6.0 to 7.0 mm (internal diameter ~2.0 mm). This creates controlled resistance, protecting the pulmonary vascular bed from high systemic pressures and balancing $Q_p : Q_s$.
   • Doppler Criteria: Continuous-wave Doppler across the bilateral bands must demonstrate high-velocity systolic flow acceleration (peak velocity >3.0 to 3.5 m/s, peak gradient >36 to 50 mmHg) with symmetric branch gradients.
2. Ductal Stenting / PGE1 Maintenance: Systemic perfusion to the descending aorta and coronary arteries is maintained by transcatheter implantation of a self-expanding metallic stent (7 to 8 mm diameter) across the ductus arteriosus, or by continuous intravenous prostaglandin $E_1$ ($PGE_1$) infusion.
3. Balloon Atrial Septostomy: Transcatheter balloon dilation or static blade septostomy creates an unrestrictive interatrial communication, relieving left atrial hypertension.

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The Damus-Kaye-Stansel (DKS) Anastomosis

In patients with a functionally single ventricle who have two well-developed great arteries but develop or are at risk for subaortic stenosis, the Damus-Kaye-Stansel (DKS) procedure is utilized.

Damus-Kaye-Stansel (DKS) Anastomosis Architecture:
       [Transected Main PA]         [Ascending Aorta]
                │                          │
                └────────────┬─────────────┘
                             ▼
                 [Side-to-Side Anastomosis]
                             │
                             ▼
                  [Common Double-Barrel]
                [Systemic Outflow Trunk]
                             │
                             ▼
           [Single Ventricle Ejection Protected]

• Anatomical Indication: Classically employed in single ventricle with transposed great arteries (e.g., Double Inlet Left Ventricle with l-TGA or tricuspid atresia with TGA) where the ventricular septal defect or bulboventricular foramen (BVF) acts as the only outlet to the subaortic chamber. As the myocardium hypertrophies, the BVF progressively constricts, causing life-threatening subaortic stenosis.
• Surgical Execution: The main pulmonary artery is transected proximal to its bifurcation. The ascending aorta and MPA trunk are joined side-to-side (or end-to-side), creating a double-barrel systemic outflow tract. The single ventricle ejects freely through both the native aortic valve and the pulmonary valve into the systemic aorta, bypassing the obstructed BVF.
• Pulmonary Blood Flow: Calibrated pulmonary blood flow is established simultaneously via an mBTT shunt or Sano RV-PA conduit.

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Comprehensive Postoperative Echocardiographic Surveillance Protocol

Following Norwood Stage 1 reconstruction, bedside echocardiography in the pediatric intensive care unit must methodically evaluate four critical anatomical targets:

1. Neoaortic Arch & Reconstruction Anastomosis (Re-coarctation Surveillance)

• Acoustic Window: Suprasternal long-axis and short-axis windows ("candy cane" and "crab" views) with extensive off-axis tilting.
• Pathology: Recurrent arch obstruction (re-coarctation) occurs in 10% to 20% of patients, most commonly at the distal anastomosis between the patch augmentation and the native descending thoracic aorta.
• Spectral Doppler Hallmarks:
  • High peak systolic velocity (>2.5 to 3.0 m/s).
  • Sawtooth Diastolic Runoff: Continuous, non-zero forward flow extending throughout all of diastole in the descending aorta (mean gradient >15 to 20 mmHg).
  • Damped, low-pulsatility (pulsus tardus et parvus) flow in the distal abdominal aorta from the subcostal window.

2. Shunt Assessment: mBTT vs. Sano

• mBTT Shunt Evaluation:
  • Acoustic Window: High right parasternal or suprasternal notch coronal view.
  • Normal Flow: Continuous, high-velocity turbulent flow throughout both systole (3.0 to 4.0 m/s) and diastole (2.0 to 3.0 m/s).
  • Stenosis / Occlusion Alert: A peak systolic velocity <2.5 m/s with absent diastolic flow does not signify a normal low velocity; it indicates critical low cardiac output, impending shunt thrombosis, or severe anastomotic obstruction.
• Sano Conduit Evaluation:
  • Acoustic Window: Parasternal short-axis and RV outflow tract views.
  • Normal Flow: High-velocity systolic ejection jet (2.0 to 3.0 m/s); minimal or absent diastolic forward flow.
  • Obstruction Alert: Peak systolic velocity >3.5 m/s or progressive narrowing at the proximal ventricular insertion site or distal PA bifurcation.

3. Interatrial Communication (ASD / Septectomy Integrity)

• Acoustic Window: Subcostal coronal and sagittal imaging with low color velocity scales (30 to 40 cm/s).
• Normal Flow: Broad, low-velocity, non-turbulent left-to-right laminar flow across the atrial septum.
• Restrictive ASD Alert: A narrow, aliasing jet with a peak velocity >1.5 m/s or a mean gradient >4 to 5 mmHg indicates restriction. This causes acute pulmonary venous congestion, elevated left atrial pressure, and pulmonary edema, requiring urgent transcatheter balloon septostomy.

4. Systemic Ventricle & Atrioventricular Valve Competence

• Acoustic Window: Apical 4-chamber and subcostal coronal views.
• Surveillance Targets: The morphologic right ventricle (in HLHS) is prone to acute systolic failure under high afterload. The systemic atrioventricular (tricuspid) valve must be scrutinized for progressive regurgitation. Moderate-to-severe tricuspid regurgitation (TR) elevates atrial filling pressures, worsens volume overload, and is an independent predictor of interstage and post-Fontan mortality.

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Norwood Stage 1 Hemodynamic & Doppler Reference Table

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Clinical Pearls & Sonographic Traps

[!WARNING] The mBTT Shunt Angle of Insonation Trap: The modified Blalock-Thomas-Taussig shunt is an obliquely oriented prosthetic conduit coursing from the innominate/subclavian artery downward to the RPA. Interrogating the shunt strictly from the suprasternal notch frequently yields an insonation angle exceeding 50 to 60 degrees, severely underestimating peak velocities. The sonographer must utilize the high infraclavicular window or high right parasternal view to achieve a coaxial interrogation angle (theta <= 20 degrees).

[!TIP] Differentiating Neoaortic Re-coarctation from Shunt Runoff: Both an mBTT shunt and severe aortic coarctation produce diastolic forward flow in the descending thoracic aorta. To differentiate:

• In Isolated mBTT Shunt Runoff, the descending aortic systolic velocity is normal (1.0 to 1.5 m/s), and diastolic forward flow is smooth and continuous without an obstructive systolic step-up.
• In Neoaortic Re-coarctation, there is a dramatic systolic velocity step-up (>2.5 to 3.5 m/s) with high turbulence at the isthmus, followed by the classic dense, prolonged "sawtooth" holodiastolic tail.

[!NOTE] Coronary Perfusion via Retrograde Native Aorta: In neonates with HLHS and aortic atresia, the native ascending aorta is an extraordinarily slender "string" (often 1.5 to 2.5 mm in diameter) that functions solely as a conduit for retrograde coronary perfusion. During neoaorta reconstruction, any distortion, twisting, or compression of this native aortic stump halts coronary blood flow, triggering immediate, fatal myocardial infarction.