12.2 Superior Cavopulmonary Connection (Bidirectional Glenn / Hemi-Fontan)

12.2 Superior Cavopulmonary Connection (Bidirectional Glenn / Hemi-Fontan)

Clinical Core: The Bidirectional Glenn (BDG) procedure—also termed the Superior Cavopulmonary Connection (SCPC)—marks the second milestone in staged single-ventricle palliation. Performed electively between 4 and 6 months of age, this operation eliminates the high-pressure, volume-wasting neonatal systemic-to-pulmonary shunt (mBTT or Sano) and diverts upper-body systemic venous return directly into the pulmonary arterial tree. By shifting upper-body venous return away from the common ventricular pool, the BDG dramatically unloads the single ventricle, halting progressive chamber dilation and preserving ventricular geometry. The pediatric echocardiographer must evaluate the patency and phasicity of the cavopulmonary anastomosis, measure branch pulmonary artery dimensions, and surveil for veno-venous collaterals and microscopic pulmonary arteriovenous malformations.

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Transition from Parallel to Partial In-Series Circulation

Following Stage 1 palliation, the single ventricle remains subjected to chronic, massive volume overload because it must pump both systemic blood flow ($Q_s$) and pulmonary blood flow ($Q_p$). As the infant grows, this parallel arrangement becomes unsustainable, leading to progressive ventricular dilation, spherical remodeling, and functional atrioventricular valve regurgitation.

Stage 1: Complete Parallel Circulation      Stage 2: Partial In-Series (Bidirectional Glenn)
[Upper Venous]    [Lower Venous]           [Upper Body Venous (SVC)]
      │                 │                                │
      └────────┬────────┘                                ▼
               ▼                               [Pulmonary Capillaries] (Passive Flow)
       [Single Ventricle]                                │
               │                                         ▼
       ┌───────┴───────┐                         [Pulmonary Veins]
       ▼               ▼                                 │
  [Aorta (Qs)]    [PAs (Qp)]                             ▼
(Total Output = Qp + Qs; Volume Overload)        [Single Ventricle] ◄── [IVC Venous]
                                                         │
                                                         ▼
                                                   [Aorta (Qs)] (Volume Unloaded)

Physiological Consequences of the Bidirectional Glenn

1. Ventricular Volume Unloading: Upper-body systemic venous drainage represents approximately 35% to 45% of total systemic venous return in an infant. Diverting superior vena caval flow directly into the pulmonary bed removes this volume from the ventricular preload pool. The single ventricle's total stroke volume decreases by roughly 40% to 50%, reducing ventricular wall stress, reversing eccentric hypertrophy, and improving myocardial efficiency.
2. Elimination of Diastolic Steal: Dividing the mBTT shunt or excising the Sano conduit eliminates aortic runoff. Systemic diastolic arterial blood pressure normalizes, substantially enhancing coronary artery perfusion pressure.
3. Stable Arterial Oxygenation: Because only the upper-body venous return traverses the lungs, the pulmonary-to-systemic blood flow ratio ($Q_p : Q_s$) is typically 0.6:1 to 0.8:1. Consequently, systemic arterial oxygen saturation ($SpO_2$) naturally stabilizes between 80% and 85% on room air.

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Anatomical Prerequisites & Surgical Timing

Because the Glenn connection relies entirely on passive venous drainage without a subpulmonary ventricle, successful execution requires a receptive, low-resistance pulmonary vascular bed:

• Surgical Timing (4 to 6 Months of Age): In the first weeks of life, neonatal pulmonary arteriolar muscular media is thick, maintaining high pulmonary vascular resistance (PVR). By 3 to 4 months of age, physiological involution of this vascular smooth muscle is complete, lowering PVR to mature levels.
• PVR Threshold: Documented PVR must be <= 2.0 Wood units $\cdot \text{m}^2$ (rarely exceeding 2.5 Wood units $\cdot \text{m}^2$). If PVR is elevated, passive cavopulmonary flow halts, causing acute superior vena cava syndrome and cerebral venous hypertension.
• Mean Pulmonary Artery Pressure (mPAP): Resting mean PAP must be <15 mmHg (ideally <= 12 mmHg).
• Branch Pulmonary Artery Caliber: Branch pulmonary arteries must be well-developed without focal hypoplasia or kinking:
  • McGoon Ratio: $(\text{Diameter of RPA} + \text{Diameter of LPA}) / \text{Diameter of Descending Aorta at Diaphragm} \ge 1.5 \text{ to } 1.8$.
  • Nakata Index: Total cross-sectional PA area indexed to body surface area $\ge 150 \text{ mm}^2/\text{m}^2$.
• Preserved Ventricular Function & AV Valve Competence: Single ventricle end-diastolic pressure (EDP) must be <= 10 to 12 mmHg, and atrioventricular valve regurgitation must be no more than mild.

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Surgical Techniques: Classic BDG, Bilateral BDG & The Hemi-Fontan

Standard Bidirectional Glenn:               Bilateral Bidirectional Glenn (PLSVC):
           [SVC]                                      [RSVC]           [LSVC]
             │                                          │                │
             ▼                                          ▼                ▼
  [End-to-Side Anastomosis]                           [RPA]            [LPA]
             │                                          │                │
     ┌───────┴───────┐                                  └───────┬────────┘
     ▼               ▼                                          ▼
   [RPA]           [LPA]                              [Confluence Intact]
(Directs flow to both lungs)                (Requires two separate cavopulmonary hookups)

1. Standard Bidirectional Glenn (End-to-Side Anastomosis)

• Cardiopulmonary bypass is established. The previous mBTT shunt is doubly clipped and divided, or the Sano RV-PA conduit is completely excised and the ventriculotomy oversewn.
• The superior vena cava is transected clean from the right atrial appendage just above the cavoatrial junction. The cardiac end of the SVC is oversewn.
• The underside of the transected cephalic SVC is anastomosed end-to-side to the superior surface of the right pulmonary artery (RPA). Because the branch pulmonary arteries remain confluent, deoxygenated blood from the single SVC distributes passively into both the right and left pulmonary capillary beds (hence bidirectional).

2. Bilateral Bidirectional Glenn (Persistent Left SVC)

• In approximately 3% to 5% of single-ventricle patients, a Persistent Left Superior Vena Cava (PLSVC) coexists with a right SVC, frequently accompanied by absence or severe hypoplasia of the bridging innominate vein.
• Transecting only the right SVC would leave the left SVC draining deoxygenated blood into the coronary sinus or common atrium, causing progressive desaturation and volume overload.
• The surgical solution is a Bilateral Bidirectional Glenn: the right SVC is anastomosed to the RPA, and the left SVC is anastomosed independently end-to-side to the LPA.

3. The Hemi-Fontan Alternative

• Developed as an alternative Stage 2 technique to streamline subsequent Lateral Tunnel Fontan completion.
• Rather than transecting the SVC, the confluence between the SVC, right atrial appendage, and pulmonary artery is opened widely. A patch of cryopreserved homograft or PTFE is sewn internally across the cavoatrial junction to create an intra-atrial "dam" or baffle.
• This directs all SVC blood directly into the branch pulmonary arteries while excluding IVC blood from entering the lungs. At Stage 3 (Fontan completion), the surgeon simply excises this temporary intra-atrial baffle and completes the lateral tunnel.

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Normal Spectral Doppler Hemodynamics of the Glenn Circuit

Interrogation of the Bidirectional Glenn requires specialized transducer positioning and an understanding of passive venous physiology:

• Acoustic Windows: Suprasternal notch coronal view (profiling the vertical SVC entering the horizontal RPA), high right parasternal window, and subcostal sagittal/coronal sweeps.
• Laminar, Low-Velocity Profile: Continuous forward flow into the branch pulmonary arteries throughout both systole and diastole.
  • Normal Peak Velocity: 0.4 to 0.6 m/s (rarely exceeding 0.8 m/s).
  • Mean Pressure Gradient: Normal trans-anastomotic mean gradient is negligible (<1 to 2 mmHg).
• Respiratory Phasic Modulation: Normal flow exhibits striking respiratory modulation. During spontaneous inspiration, intrathoracic pressure drops, creating a "thoracic suction pump" effect that accelerates forward Glenn flow into the lungs. During expiration, forward velocity gently decreases. In mechanically ventilated patients with positive end-expiratory pressure (PEEP), this dynamic is reversed: positive-pressure inspiration impedes forward Glenn flow.

Normal Bidirectional Glenn Spectral Doppler Waveform:

Velocity
 (m/s)
  0.8 ┬
  0.6 ┼       ╭───╮             ╭───╮              (Inspiration: Velocity Rises)
  0.4 ┼  ╭───╯   ╰───╮     ╭───╯   ╰───╮
  0.2 ┼──╯           ╰─────╯           ╰────────   (Expiration: Velocity Dips)
  0.0 ┴───────────────────────────────────────── (Baseline: Continuous Forward Flow)
      ◄── Low-Velocity Biphasic Waveform ──►

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Postoperative Complications & Echocardiographic Surveillance

1. Glenn Anastomotic Stenosis & Branch PA Kinking

• Mechanism: Surgical tension, purse-string suture artifact, or intimal scarring can narrow the cavopulmonary anastomosis or kink the adjacent LPA/RPA.
• Echocardiographic Criteria:
  • Color Doppler reveals aliasing and localized flow acceleration at the anastomosis.
  • Pulsed-wave/Continuous-wave Doppler shows a turbulent, high-velocity jet (>1.2 to 1.5 m/s) with an elevated mean gradient (>3 to 4 mmHg).
  • Complete loss of normal respiratory phasicity (continuous monophasic jet).
  • Distal branch PA Z-score <-2.5 to -3.0.

2. Veno-Venous Collaterals

• Mechanism: If SVC pressure rises even slightly above baseline (e.g., due to mild PA narrowing or elevated PVR), tiny embryonic systemic veins dilate to decompress the high-pressure upper body into the lower-pressure inferior vena cava or azygos/hemiazygos system.
• Pathophysiology: These collaterals divert deoxygenated blood around the pulmonary capillary bed directly into the common atrium or IVC, establishing a right-to-left systemic venous shunt.
• Clinical Manifestation: Unexplained, progressive arterial cyanosis ($SpO_2 < 75%$) in an otherwise well-appearing infant.
• Echocardiographic Detection: Color Doppler from the suprasternal or high parasternal window shows anomalous tortuous channels decompressing caudally from the SVC or innominate vein toward the spine (azygos vein flow reversal). Confirmed via transcatheter angiography and treated with coil embolization.

3. Pulmonary Arteriovenous Malformations (PAVMs)

• The "Hepatic Factor" Concept: Normal lung microvasculature requires continuous exposure to an unidentified vasoactive biochemical substance synthesized by the liver ("hepatic factor"). Because the Glenn circuit directs only upper-body venous blood through the lungs, the pulmonary bed is deprived of hepatic venous blood.
• Microscopic Shunts: In the absence of hepatic factor, microscopic pulmonary capillary beds dilate into direct arteriovenous communications (PAVMs), allowing desaturated pulmonary arterial blood to bypass the alveoli without gas exchange.
• Contrast Echocardiography (Bubble Study) Diagnosis:
  • Agitated saline is injected into an upper extremity peripheral vein.
  • Positive Diagnostic Finding: Microbubbles transit through the pulmonary capillaries and appear in the systemic atrium and ventricle after 3 to 5 cardiac cycles.
  • Distinction from Intracardiac Shunts: Microbubbles from an intracardiac right-to-left shunt (e.g., residual ASD) appear immediately within 1 to 2 cardiac cycles. PAVM bubbles show delayed, dense, uniform appearance following capillary transit.

4. Superior Vena Cava Syndrome & Thrombus Formation

• Severe elevation of SVC pressure causes facial edema, periorbital swelling, chemosis, irritability, and upper-extremity venous distension. Sluggish flow in the SVC predisposes to in-situ thrombus formation, which can acutely occlude the Glenn pathway.

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Bidirectional Glenn Hemodynamic & Diagnostic Reference Table

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Clinical Pearls & Sonographic Traps

[!WARNING] The Upper vs. Lower Extremity Bubble Study Trap: When evaluating a post-Glenn patient with unexplained desaturation for pulmonary arteriovenous malformations:

• Injecting agitated saline into an upper-extremity vein sends bubbles directly into the SVC, through the Glenn, and across the pulmonary capillary bed, correctly identifying PAVMs via delayed systemic transit (3 to 5 beats).
• If agitated saline is mistakenly injected into a lower-extremity vein, the bubbles enter the IVC, flow directly into the right atrium, and cross unimpeded into the systemic ventricle within 1 beat, completely failing to test the pulmonary capillary bed!

[!TIP] Optimizing Color Scale for Low-Velocity Glenn Flow: Because normal forward velocity in the Glenn pathway is only 0.4 to 0.6 m/s, using a standard arterial color velocity scale (60 to 80 cm/s) will result in poor color filling and false-negative diagnoses of thrombosis. The sonographer must lower the color Doppler Nyquist limit to 20 to 30 cm/s to adequately visualize laminar flow and detect subtle non-obstructive mural thrombi.

[!NOTE] Inadvertent Systemic AV Valve Deterioration After Volume Unloading: Following the Bidirectional Glenn, the dramatic reduction in ventricular preload usually improves functional tricuspid/mitral regurgitation. If systemic AV valve regurgitation paradoxically worsens post-Glenn, suspect structural leaflet damage, papillary muscle ischemia, or severe geometric distortion of the ventricular annulus induced by surgical manipulation.