16.4 Transesophageal (TEE) & Epicardial Echocardiography in Congenital Heart Surgery

16.4 Transesophageal (TEE) & Epicardial Echocardiography in Congenital Heart Surgery

Clinical Core: Intraoperative echocardiography is an indispensable cornerstone of modern pediatric congenital heart surgery. Immediate real-time imaging guides surgical strategy, verifies anatomical repairs before chest closure, and prevents costly, morbid reoperations. Two primary modalities are utilized in the operating room: Pediatric Transesophageal Echocardiography (TEE) and Epicardial Echocardiography. While TEE provides continuous, hands-free monitoring from retrocardiac and transgastric acoustic windows, it requires strict weight-based probe selection to prevent airway obstruction or esophageal trauma in neonates. Conversely, epicardial echocardiography—performed by placing a sterile-sheathed high-frequency transducer directly on the exposed beating heart—provides unparalleled near-field spatial resolution and perfect Doppler alignment along the right ventricular outflow tract (RVOT) and branch pulmonary arteries, while serving as the primary modality of choice when TEE is anatomically contraindicated.

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Pediatric Transesophageal Echocardiography (TEE)

Transesophageal echocardiography places the multiplane ultrasound transducer in the esophagus and stomach directly posterior to the left atrium, interatrial septum, atrioventricular valves, and left ventricular inflow. By bypassing the sternum, ribs, lungs, and surgical dressings, TEE provides continuous, exceptionally clear acoustic visualization during open-heart surgery and interventional catheterization.

Pediatric TEE Transducer Selection Hierarchy by Patient Weight:

 1. Micro-Multiplane TEE Probe:          2. Mini-Multiplane TEE Probe:           3. Adult Multiplane TEE Probe:
    ┌───────────────────────────────┐       ┌───────────────────────────────┐       ┌───────────────────────────────┐
    │ Weight: <5 kg (down to 1.5 kg)│       │ Weight: 5 kg to 20 - 25 kg    │       │ Weight: >25 to 30 kg ONLY     │
    │ Shaft Diameter: 5.2 - 6.0 mm  │       │ Shaft Diameter: 7.0 - 8.0 mm  │       │ Shaft Diameter: 10.0 - 11.5 mm│
    │ Tip Diameter: ~7 mm           │       │ Full Multiplane (0° - 180°)   │       │ Adult Footprint & Rigidity    │
    │ Neonatal / Small Infant Probe │       │ Pediatric Multiplane Probe    │       │ Adolescent / Adult Probe      │
    └───────────────────────────────┘       └───────────────────────────────┘       └───────────────────────────────┘

Weight-Based TEE Probe Sizing & Anatomic Hazards

In pediatric patients, the esophagus is narrow, thin-walled, and compliant, running immediately posterior to the membranous, uncalcified posterior wall of the trachea. Inserting an improperly sized probe risks catastrophic airway compromise, vascular obstruction, or esophageal perforation. Strict adherence to weight-based probe sizing is mandatory:

1. Micro-Multiplane TEE Probes ($<5\ \text{kg}$):
   • Specifically engineered for neonates and infants weighing under 5 kg (validated down to $1.5-2.0\ \text{kg}$ in premature neonates).
   • Features an ultra-slim shaft diameter ($5.2$ to $6.0\ \text{mm}$) and miniature articulating tip.
   • Operates at high frequencies ($7$ to $10\ \text{MHz}$), optimizing near-field resolution for tiny cardiac structures.
2. Mini-Multiplane TEE Probes ($5\ \text{kg}$ to $20-25\ \text{kg}$):
   • Designed for older infants, toddlers, and young children.
   • Shaft diameter measures $7.0$ to $8.0\ \text{mm}$ with full $0^{\circ}$ to $180^{\circ}$ electronic multiplane rotation.
3. Standard Adult Multiplane TEE Probes ($>25-30\ \text{kg}$):
   • Shaft diameter measures $10.0$ to $11.5\ \text{mm}$.
   • Strictly restricted to adolescents and older children weighing $>25-30\ \text{kg}$. Never force an adult or mini probe into a neonate or small infant!

Critical Airway & Vascular Hazards in Pediatric TEE

• Tracheal Compression: The posterior tracheal wall in infants lacks cartilaginous rings and consists of a compliant fibromuscular membrane. An oversized TEE probe or excessive anterior probe flexion compresses the trachea directly against the endotracheal tube (ETT). This causes immediate loss of tidal volume, dramatic spikes in peak inspiratory pressures, bronchospasm, and hypercarbia. The sonographer and anesthesiologist must verify uninterrupted ventilation upon probe insertion.
• Descending Aorta Compression: In small infants, an oversized probe can compress the adjacent descending thoracic aorta against the vertebral column, attenuating lower-body systemic perfusion.
• Thermal Injury: Ultrasound transducers generate heat during prolonged high-power scanning (especially color Doppler). Pediatric systems utilize continuous thermistor monitoring at the probe tip, automatically pausing transmission or throttling power if tip temperature reaches $42^{\circ}\text{C}$ to $43^{\circ}\text{C}$ to prevent esophageal mucosal burns.

Standard Pediatric TEE Acoustic Views

1. Upper Esophageal Views ($0^{\circ}$ to $90^{\circ}$): Profiles the aortic arch, ascending aorta, superior vena cava (SVC), pulmonary artery bifurcation, and anomalous upper pulmonary veins.
2. Mid-Esophageal Views ($0^{\circ}$ to $135^{\circ}$):
   • $0^{\circ}$ (4-Chamber): Interatrial septum, AV valves (mitral and tricuspid), ventricular septal alignment, and ventricular function.
   • $30^{\circ}$ to $60^{\circ}$ (Commissural / Short-Axis): En-face mitral valve, aortic valve cusps, and coronary ostia.
   • $60^{\circ}$ to $90^{\circ}$ (Bicaval / RV Inflow-Outflow): Superior and inferior vena cava entering RA, Eustachian valve, atrial septal defect rims, RVOT, and pulmonary valve.
   • $120^{\circ}$ to $140^{\circ}$ (Long-Axis): LVOT, aortic valve, subaortic region, and membranous ventricular septum.
3. Transgastric Views ($0^{\circ}$ to $90^{\circ}$): Probe advanced into the stomach and retroflexed against the gastric fundus. Yields cross-sectional basal, mid-papillary, and apical short-axis views of the ventricles for evaluating true cross-sectional ventricular contractility, biventricular sizing, and AV valve subvalvular chordal architecture.
4. Deep Transgastric Views ($0^{\circ}$ to $20^{\circ}$): Probe advanced deep into the stomach with steep retroflexion looking superiorly toward the heart base. Provides near-parallel ($0^{\circ}$) Doppler alignment across the LVOT, aortic valve, and subaortic conus.

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Epicardial Echocardiography in Congenital Heart Surgery

Epicardial echocardiography involves placing a high-frequency ultrasound transducer directly onto the exposed, beating myocardium, great arteries, and pericardial reflections within the sterile surgical field.

Epicardial Echocardiography Technique & Configuration:

 [ Surgeon's Sterile Field ]
        │
        ├─► Sterile Sheath containing Ultrasound Probe
        │   • Acoustic coupling gel inside sheath (Bubble-free!)
        │   • Rubber bands securing sheath snugly over transducer face
        │
        ├─► Fluid Well in Pericardial Cradle
        │   • Warm sterile normal saline fills pericardial well
        │   • Eliminates air interface between probe and epicardium
        │   • Acoustic standoff prevents near-field dead-zone artifact
        │
        └─► Direct Transducer Placement
            • High-Frequency Linear (10-15 MHz) or Micro-Curved Probe
            • Zero mechanical pressure applied to beating chambers

Technique & Sterile Setup

1. Transducer Sheathing: A high-frequency linear vascular probe ($10$ to $15\ \text{MHz}$) or small-footprint micro-convex probe ($8$ to $12\ \text{MHz}$) is inserted into a long, sterile acoustic sheath. Acoustic coupling gel is placed inside the tip, and all air bubbles are meticulously smoothed out. Sterile rubber bands or elastic ties seal the sheath snugly over the acoustic lens.
2. The Saline Acoustic Standoff: The surgeon fills the pericardial well with warm, sterile normal saline. The fluid acts as an acoustic standoff, eliminating air interface artifacts and placing superficial structures (such as coronary buttons, right ventricular outflow patches, and conduit anastomoses) outside the near-field dead zone of the transducer.
3. Gentle Application: The probe is rested lightly on the fluid pool or epicardial surface without exerting downward compressive force. Excessive pressure will compress thin-walled right heart chambers, induce transient hypotension, or trigger malignant ventricular arrhythmias.

Clinical Indications for Epicardial Echo

1. Absolute Contraindications to TEE:
   • Esophageal atresia or tracheoesophageal fistula (TEF).
   • Recent esophageal surgery, esophageal perforation, or severe corrosive strictures.
   • Vascular rings (e.g., double aortic arch, right arch with aberrant left subclavian) causing severe extrinsic esophageal compression where probe advancement risks vascular or esophageal rupture.
   • Unrepaired pharyngeal or esophageal trauma.
2. Extremely Low Birth Weight Neonates ($<1.5-2.0\ \text{kg}$): Premature neonates whose small esophageal caliber cannot safely accommodate even a micro-multiplane TEE probe.
3. Superior Doppler Alignment for Outflow Tracts & Conduits:
   • On TEE, the ultrasound beam from the esophagus strikes the Right Ventricular Outflow Tract (RVOT), pulmonary valve, and branch pulmonary arteries at an angle approaching $70^{\circ}$ to $90^{\circ}$ (perpendicular). This severe angle of insonation severely underestimates flow velocities ($V_{\text{measured}} = V_{\text{true}} \times \cos\theta$).
   • Epicardial Advantage: Placing the probe directly on the anterior RVOT or main pulmonary trunk aligns the ultrasound beam completely parallel ($0^{\circ}$) to blood flow, providing accurate continuous-wave Doppler measurement of peak and mean systolic gradients across pulmonary valves, RV-to-PA conduits, and reconstructed branch pulmonary arteries.
4. High-Resolution Coronary Button & Microvascular Imaging:
   • Ultra-high-frequency linear transducers ($10-15\ \text{MHz}$) placed directly over the root of the neoaorta provide microscopic resolution of reimplanted coronary artery buttons following the Arterial Switch Operation (Jatene) or Ross Procedure. Epicardial echo immediately identifies ostial kinking, vessel torsion, flap dissection, or acute intraluminal thrombosis.
5. Resolving Ambiguous or Inconclusive TEE Findings: Clarifying complex residual VSD geometry, baffling pathways, or unroofed venous connections.

Standard Epicardial Imaging Planes

• Anterior Basal Short-Axis View: Transducer placed across the base of the heart, profiling the aortic valve, coronary ostia and buttons, pulmonary valve, and RVOT.
• Longitudinal RVOT / Main Pulmonary Artery View: Aligned along the long axis of the RVOT, profiling the infundibulum, pulmonary annulus, RV-PA conduit, and distal bifurcation into right and left pulmonary arteries.
• Apical / Subcostal Epicardial Views: Placed over the ventricular apex or diaphragmatic surface, providing unforeshortened 4-chamber and 2-chamber projections for ventricular septal defect interrogation and global biventricular contractility.

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Pre- and Post-Cardiopulmonary Bypass (CPB) Evaluation Protocol

Intraoperative evaluation follows a rigorous, sequential protocol divided into pre-bypass baseline assessment, de-airing surveillance, and post-bypass definitive verification.

Intraoperative Echocardiographic Surgical Workflow:

 1. Pre-CPB Baseline Interrogation:
    • Confirm primary anatomic diagnosis
    • Interrogate for secondary occult lesions (muscular VSDs, LSVC, PAPVC)
    • Establish baseline ventricular function & AV valve regurgitation
    │
    ▼
 [ Surgical Repair on Cardiopulmonary Bypass ]
    │
    ▼
 2. De-Airing Phase (Cross-Clamp Removal):
    • Continuous 2D monitoring of LA, LV apex, and aortic root
    • Guide venting to prevent coronary air embolism (RCA territory!)
    │
    ▼
 3. Post-CPB Hemodynamic & Structural Verification (Off Bypass):
    • Interrogate for residual shunts (VSD jet width, Qp:Qs)
    • Evaluate AV and semilunar valve competence / stenosis
    • Rule out RVOT / LVOT residual obstruction
    • Scrutinize ventricular contractility & coronary button patency
    • Inspect systemic / pulmonary venous pathways
    │
    ├─► Acceptable Result ────► Proceed to Sternal Closure
    └─► Major Residual Defect ─► Immediate Return to CPB for Revision

1. Pre-Cardiopulmonary Bypass Assessment

• Confirming the Surgical Blueprint: Verify the primary anatomical repair plan (e.g., exact VSD margins, degree of pulmonary annular hypoplasia in TOF, or AV canal bridging leaflet architecture).
• Screening for Unsuspected Associated Anomalies:
  • Search for additional "swiss-cheese" muscular VSDs that may be masked by a large membranous defect.
  • Confirm systemic venous drainage: rule out an unrecognized Persistent Left Superior Vena Cava (PLSVC), which requires separate cannulation to prevent flooding the operative field.
  • Verify all four pulmonary veins connect normally to the left atrium.
• Baseline Functional Quantification: Document pre-repair RV and LV systolic function, regional wall motion, and baseline degree of atrioventricular valve regurgitation.

2. De-Airing Surveillance

As the aortic cross-clamp is released and the heart fills with blood, air trapped in the pulmonary veins, left atrium, and trabeculated left ventricular apex can embolize into the systemic arterial circulation.

• Acoustic Surveillance: The echocardiographer continuously monitors the ascending aorta, left atrial appendage, and LV apex in the mid-esophageal long-axis view ($120^{\circ}$).
• Coronary Air Embolism Hazard: Air bubbles preferentially enter the anteriorly located Right Coronary Artery (RCA) ostium. Acute air embolism into the RCA produces sudden, severe right ventricular hypokinesis, acute RV dilation, and profound ST-segment elevation on lead II/III/aVF. Guiding surgical venting needles in the aortic root and LV apex until all microbubbles clear prevents acute graft failure.

3. Immediate Post-CPB Structural & Hemodynamic Verification

Once the patient is successfully weaned from cardiopulmonary bypass and stable hemodynamics are established, a systematic post-repair survey is performed:

A. Residual Intracardiac Shunts

• Residual Ventricular Septal Defects (VSD):
  • Interrogate the patch perimeter using color Doppler with optimized Nyquist limits ($50-60\ \text{cm/s}$). Small muscular pinhole leaks ($<1.5\text{ mm}$) with high-velocity left-to-right jets ($>3.5-4.0\ \text{m/s}$, reflecting low RV pressure) are hemodynamically insignificant and typically close spontaneously.
  • Critical Threshold for Re-Bypass: A residual jet width $>2$ to $3\text{ mm}$, low-velocity bidirectional shunting (reflecting elevated RV pressures), or a calculated pulmonary-to-systemic flow ratio $Q_p:Q_s > 1.5$ represents a major residual shunt that mandates an immediate return to cardiopulmonary bypass for patch revision before chest closure.
• Residual Atrial Shunts: Confirm complete patch closure of ASDs; assess intentional fenestrations in Fontan conduits or high-risk ASD patches (verifying right-to-left decompression flow).

B. Valvular Competence & Stenosis

• Atrioventricular Valve Repairs: In repaired AV canal defects or mitral cleft repairs, assess the degree of residual regurgitation. Document jet origin (central vs. commissural vs. zone of apposition). Measure the transmitral or transtricuspid mean diastolic pressure gradient via continuous-wave Doppler; an elevated mean gradient ($>5-7\ \text{mmHg}$) indicates surgically induced valve stenosis that may require chordal or annular release.
• Semilunar Valves: In reconstructed outflow tracts (Ross procedure, valve-sparing root, aortic valvuloplasty), assess for aortic or pulmonary regurgitation and transvalvular gradients.

C. Outflow Tract Obstruction

• Interrogate the Left and Right Ventricular Outflow Tracts using pulsed and continuous-wave Doppler.
• Fixed vs. Dynamic Obstruction: Differentiate between fixed anatomical obstruction (e.g., residual subaortic fibrous shelf, restrictive RVOT muscle bundles) and dynamic obstruction caused by hypovolemia and high circulating catecholamines in a hyperdynamic, underfilled ventricle.

D. Ventricular Performance & Coronary Patency

• Global & Regional Wall Motion: Systematically inspect all ventricular walls. A new localized wall motion abnormality (e.g., anterior wall akinesis following an Arterial Switch Operation) is a catastrophic red flag indicating coronary artery kink, twist, tension, or ostial spasm of the reimplanted coronary button. Immediate communication with the surgeon enables prompt coronary revision before irreversible myocardial infarction ensues.

E. Vascular Anastomoses & Conduits

• Verify unobstructed laminar flow across systemic-to-pulmonary shunts (BT shunts), bidirectional Glenn cavopulmonary anastomoses, extracardiac Fontan conduits, and aortic arch coarctation repair sites.

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Pediatric TEE vs. Epicardial Echocardiography Matrix

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Clinical Pearls & Sonographic Traps

[!WARNING] The Perpendicular RVOT Gradient Trap on TEE: When evaluating a newly placed RV-to-PA conduit or repaired TOF on post-CPB TEE, the continuous-wave Doppler jet across the RVOT may measure only 2.0 m/s (peak gradient 16 mmHg). Do not declare the outflow tract unobstructed! From the mid-esophageal TEE window, the beam strikes the pulmonary outflow tract nearly perpendicular ($80^{\circ}$), severely underestimating the true gradient. Place a sterile-sheathed high-frequency probe directly on the anterior RVOT for epicardial interrogation; perfect $0^{\circ}$ alignment may reveal a true velocity of 4.2 m/s (peak gradient 71 mmHg), diagnosing severe residual conduit stenosis.

[!TIP] Eliminating Air in Epicardial Sheaths: Microscopic air bubbles trapped between the transducer face and the plastic sterile sheath produce dense acoustic reverberation that completely obscures near-field cardiac structures. When preparing the epicardial probe, place an ample pool of sterile acoustic gel in the sheath tip, insert the transducer firmly, and smooth out all air bubbles with a sterile gauze pad before applying the securing rubber bands. Always ensure the surgeon fills the pericardium with warm sterile saline to serve as an acoustic standoff.

[!NOTE] Recognizing Coronary Button Kinking Off Bypass: Following an Arterial Switch Operation, weaning from bypass may appear stable initially, but within 10 minutes the left ventricular anterior wall becomes severely hypokinetic and the ECG demonstrates ST-segment elevation. Do not attribute this to general cardioplegic stunning! Immediately image the reimplanted left coronary button using high-frequency epicardial echo and color Doppler. Coronary kinking or stretch obstruction requires immediate re-institution of bypass and surgical revision to save the patient's life.