11.5 Single-Ventricle Physiology & Staged Palliation

11.5 Single-Ventricle Physiology & Staged Palliation

Single-ventricle congenital cardiac anomalies—most notably Hypoplastic Left Heart Syndrome (HLHS)—present the most complex cardiopulmonary physiology encountered in neonatal and pediatric critical care. In these patients, a single morphological ventricle must simultaneously support both the systemic and pulmonary circulations operating in parallel. Blood distribution is strongly influenced by pulmonary and systemic resistance, but also by anatomic obstruction, shunt or conduit characteristics, ventricular function, valve function, preload, hemoglobin, and ventilation. Gas changes can be clinically important and should be interpreted in that larger hemodynamic picture. The Neonatal/Pediatric Specialist must master single-ventricle surgical staging, the hemodynamics of systemic steal, subambient gas blending, invasive pressure interpretation, and advanced noninvasive regional oximetry.

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Single-Ventricle Anatomy: Hypoplastic Left Heart Syndrome (HLHS)

Hypoplastic Left Heart Syndrome encompasses a spectrum of severe underdevelopment of left-sided cardiac structures:

• Pathological Components: Severe hypoplasia or atresia of the aortic valve, mitral valve, ascending aorta, and left ventricle. The ascending aorta is frequently a diminutive "hypoplastic string" ($1\text{ to }2\text{ mm}$ in diameter) that serves merely as a retrograde conduit to supply the coronary arteries.
• Obligate Parallel Circulation: Oxygenated blood returning to the left atrium cannot enter the hypoplastic left ventricle; it must traverse an obligate interatrial communication (PFO/ASD) to mix with deoxygenated systemic venous blood in the right atrium. The morphologic Right Ventricle (RV) serves as the single pumping chamber, ejecting the mixed stroke volume into the main pulmonary artery. From the pulmonary artery, blood travels either to the lungs via the pulmonary arteries or to the body via a widely patent ductus arteriosus (PDA) into the descending aorta.

                    PARALLEL SINGLE-VENTRICLE CIRCULATION
                                      │
                         [Morphologic Right Ventricle]
                               (Single Pump: Qtotal)
                                      │
                 ┌────────────────────┴────────────────────┐
                 ▼                                         ▼
     [Pulmonary Bed: Qp]                       [Systemic Organs: Qs]
     (Governed by PVR)                         (Governed by SVR)
                 │                                         │
                 └────────────────────┬────────────────────┘
                                      ▼
                      [Common Mixing: Right Atrium]

Because both circulations are supplied from a common ventricular output:

$Q_{\text{total}} = Q_p + Q_s$

The SVR-to-PVR relationship is a useful conceptual guide to flow distribution, but measured $Q_p/Q_s$ is not simply equal to $SVR/PVR$ because anatomy, shunt resistance, ventricular function, and pressure gradients also matter.

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The Three-Stage Surgical Palliation Pathway

HLHS cannot be converted into a normal two-ventricle circulation in most patients. Management pathways include staged single-ventricle palliation, transplantation in selected cases, and individualized goals-of-care decisions.

Stage 1: Norwood Procedure (with mBT Shunt or Sano Conduit)

• Timing: Performed during the first week of life (days 3–7).
• Three Surgical Goals:
  1. Neo-Aorta Reconstruction: The main pulmonary artery is transected and anastomosed to the diminutive hypoplastic ascending aorta and aortic arch, creating a large, unified systemic outflow tract from the right ventricle.
  2. Unrestricted Atrial Septectomy: Complete excision of the atrial septum to ensure completely unobstructed egress of oxygenated pulmonary venous blood from the left atrium into the right atrium.
  3. Regulated Pulmonary Blood Flow Source: Because the main pulmonary artery is now the neo-aorta, a new source of pulmonary blood flow is established using one of two techniques:
     • Modified Blalock-Taussig (mBT) Shunt: A synthetic PTFE (Gore-Tex) tube (typically $3.0\text{ to }3.5\text{ mm}$) connecting the right subclavian or innominate artery to the right pulmonary artery. Advantage: Simple to place. Disadvantage: Generates continuous diastolic runoff from the aorta into the pulmonary bed, lowering diastolic blood pressure and compromising coronary artery perfusion pressure.
     • Sano Conduit (RV-to-PA Conduit): A conduit from the right ventricle to the pulmonary arteries provides the pulmonary source. Compared with an mBT shunt it generally reduces aortopulmonary diastolic runoff, but it requires a ventriculotomy and does not guarantee superior coronary perfusion or outcome.

Stage 2: Bidirectional Glenn / Hemi-Fontan Procedure

• Timing: Commonly performed around 3 to 6 months, after PVR has fallen sufficiently and when anatomy, growth, oxygenation, ventricular function, and center strategy support proceeding.
• Surgical Execution: The prior systemic-to-pulmonary shunt (BT shunt or Sano conduit) is taken down. The Superior Vena Cava (SVC) is transected and anastomosed directly to the right pulmonary artery (end-to-side).
• Physiological Impact: Venous blood from the head and upper body reaches the pulmonary arteries without a subpulmonary ventricle, reducing single-ventricle volume load. Expected saturation and pressure are patient- and pathway-specific; obstruction, elevated PVR, collateral flow, lung disease, and ventricular dysfunction can impair the result.

Stage 3: Fontan Completion Procedure

• Timing: Performed at $2\text{ to }4\text{ years of age}$.
• Surgical Execution: The Inferior Vena Cava (IVC) is routed directly to the pulmonary artery via an extracardiac synthetic conduit (Gore-Tex) or an intra-atrial lateral tunnel.
• Fenestration when used: A small surgically created communication between the Fontan pathway and atrium. This acts as a safety pop-off: if PVR spikes post-operatively, blood shunts right-to-left into the atrium, preserving left-sided filling and cardiac output at the cost of mild desaturation.
• Result: Total cavopulmonary connection. All systemic venous blood flows passively through the pulmonary capillary bed driven entirely by central venous pressure (transpulmonary gradient: $\text{CVP} - \text{LAP}$). The single ventricle pumps exclusively oxygenated arterial blood to the systemic circulation.

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The Trap of "Excessive Oxygenation" & Systemic Steal

In unpalliated single-ventricle neonates and Stage 1 Norwood patients, the balance between pulmonary blood flow ($Q_p$) and systemic blood flow ($Q_s$) is fragile. Excessive pulmonary flow can reduce effective systemic flow and requires prompt recognition.

                      PULMONARY OVERCIRCULATION AND SYSTEMIC FLOW
                                       │
            [Trigger: Oxygen above need or unintended hyperventilation in susceptible physiology]
                                       │
                                       ▼
                    [Pulmonary vasodilation may lower PVR]
                                       │
                                       ▼
          [Pulmonary flow may rise relative to systemic flow]
                                       │
                 ┌─────────────────────┴─────────────────────┐
                 ▼                                           ▼
       [PULMONARY VASCULAR BED]                     [SYSTEMIC VASCULAR BED]
     • Pulmonary congestion may worsen           • Effective systemic flow may fall
     • Work of breathing may rise                 • Pulses/perfusion may deteriorate
     • Lung compliance may fall                   • Urine output may decline
     • Saturation can rise despite poorer DO2     • Lactate/metabolic acidosis may rise

Clinical Presentation of Systemic Steal

• The Oximetry Paradox: A rising saturation may accompany excessive pulmonary flow while systemic perfusion deteriorates. Saturation cannot by itself calculate Qp/Qs; interpret it with arterial and venous oxygen content, hemoglobin, lactate, pressure, urine output, NIRS trends, and echocardiography.
• Systemic Collapse: The infant develops mottled, cool extremities, delayed capillary refill ($>3\text{ to }4\text{ seconds}$), diminished or absent femoral pulses, profound oliguria ($<0.5\text{ mL/kg/hr}$), acute mesenteric ischemia, necrotizing enterocolitis, and severe lactic metabolic acidosis.

The Target Saturation Rule

To maintain an optimal balance where $Q_p/Q_s \approx 1.0$:

• Typical monitored range: Many Stage 1 pathways accept SpO2 around 75%–85%, but the prescribed range is patient- and center-specific. Interpret it with lactate, blood pressure, urine output, near-infrared spectroscopy, venous saturation, and echocardiography.
• PaO2 and SpO2: Follow the congenital-cardiac team's target for that anatomy and stage. A saturation above the prescribed range warrants prompt perfusion assessment, but it does not by itself diagnose systemic steal or define an emergency.

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