Coronary Artery Abnormalities in Transposition of the Great Arteries (d-TGA)

Overview

The Leiden Classification (most widely used)

• Sinus 1 = leftward/anterior facing sinus
• Sinus 2 = rightward/posterior facing sinus
• Coronary branches denoted as: LAD, LCx (Cx), and R (RCA)

Coronary Patterns in d-TGA

High-Risk Anatomical Variants

• One coronary runs within the aortic wall (intramural/intramural-interarterial course) between the aorta and pulmonary artery
• Variants: Intramural LCA, Intramural LAD, or Intramural RCA
• Carries significant risk of early sudden cardiac death (SCD)
• Substantially increases surgical mortality of the ASO because the button technique must be modified (unroofing or trap-door technique)

• The entire coronary circulation arises from one ostium
• Technically challenging to transfer during ASO but not a contraindication
• Requires meticulous surgical planning to avoid kinking or tension

• The Cx arises as a branch of the RCA from sinus 2
• Important to recognize because both ostia arise from the same sinus or from the RCA; missing the Cx button is a well-described surgical error

Surgical Implications (Arterial Switch Operation)

1. Both coronary ostia are excised as "buttons" with a rim of aortic wall
2. They are reimplanted into the neoaorta (native pulmonary artery root) after the great vessels are transected and switched
3. The Lecompte maneuver (anterior translocation of the pulmonary bifurcation) protects the translocated coronary arteries from compression
4. Precise anatomic knowledge preoperatively is required for unusual patterns, though ASO is offered to all patients regardless of coronary pattern since techniques exist for every variant

Key Teaching Points

• Coronaries always originate from the facing sinuses (adjacent to the PA) in TGA - this is the rule
• The Leiden classification is the standard nomenclature
• The usual pattern (1LCx; 2R) accounts for ~68% of cases
• Intramural course (~3%) is the highest-risk variant due to SCD risk and surgical difficulty
• No coronary pattern is a contraindication to the ASO; surgical technique is adapted accordingly

Sources: Fuster and Hurst's The Heart, 15th ed.; Sabiston Textbook of Surgery, 21st ed.; Braunwald's Heart Disease, 12th ed.

Direction of Shunting in d-TGA at Each Level

The Core Concept: Parallel Circulations

Key Principle: Bidirectional (Obligatory) Shunting

• For every milliliter of deoxygenated blood that crosses from the systemic circuit to the pulmonary circuit (left-to-right shunt at that site), an equal volume of oxygenated blood must cross in the opposite direction (right-to-left) to maintain net zero blood balance
• This bidirectional obligatory mixing is what sustains systemic oxygenation
• The net shunt at any site depends on the local pressure gradient

Shunt Direction at Each Level

1. Atrial Level (ASD / Patent Foramen Ovale)

• Net direction: Left-to-Right (LA → RA)
• The left atrium in d-TGA receives well-oxygenated pulmonary venous return and has higher pressure than the RA, because the LV (which pumps to the PA) operates in a lower-resistance pulmonary circuit
• Therefore the dominant direction is L→R at atrial level, but flow is bidirectional
• This is why balloon atrial septostomy (Rashkind procedure) works: it enlarges the interatrial communication to allow more effective mixing. Without it, the neonate develops profound, refractory hypoxemia
• The atrial level is considered the most important mixing site in TGA with intact ventricular septum

2. Ventricular Level (VSD)

• Net direction: Left-to-Right (LV → RV) in most cases
• The LV in d-TGA drives the pulmonary circuit (lower resistance) and therefore has lower pressure than the RV, which drives the high-resistance systemic circuit
• A VSD allows oxygenated blood from the LV to cross into the RV (L→R), which then gets ejected into the aorta - this directly improves systemic oxygenation
• A large non-restrictive VSD produces more effective mixing and less cyanosis compared to intact ventricular septum
• If pulmonary vascular resistance rises (Eisenmenger), the net shunt can reverse to R→L, worsening cyanosis

3. Great Artery Level (PDA)

• Net direction: Right-to-Left (Aorta → PA) - this is the reverse of a PDA in a normal heart
• In d-TGA, the aorta carries deoxygenated systemic venous blood at systemic pressure; the PA carries oxygenated pulmonary venous blood at lower pulmonary pressure
• Therefore the net flow through the ductus is from aorta → pulmonary artery (R→L), sending deoxygenated blood into the pulmonary circuit
• The oxygenated blood in the PA crosses into the aorta as a smaller bidirectional component - this is the beneficial mixing portion
• PGE1 is given to keep the ductus open, maintaining this mixing site until surgical repair

Summary Table

Note: All communications are bidirectional - the "net direction" is the dominant flow. The counter-flow is the effective mixing component that sustains oxygenation.

The Concept of "Effective" Blood Flow

• Effective pulmonary blood flow (Qep) = deoxygenated blood that actually reaches the lungs (must cross from systemic → pulmonary circuit)
• Effective systemic blood flow (Qes) = oxygenated blood that actually reaches the body (must cross from pulmonary → systemic circuit)
• In TGA, Qep = Qes (must be equal by conservation of flow)
• The larger these effective flows, the less cyanotic the patient
• A large ASD, VSD, or PDA all increase Qep and Qes, improving oxygenation

Sources: Fuster and Hurst's The Heart, 15th ed.; Sabiston Textbook of Surgery, 21st ed.; Tintinalli's Emergency Medicine; Grainger & Allison's Diagnostic Radiology

Shunt Direction in d-TGA — Quick Summary

ECG in d-TGA

Classic Findings

Key Points

• ECG may appear normal for age in the first day or two - because neonatal RV dominance is physiologically normal, so pure RVH blends in
• As systemic RV pressure loads, RVH becomes more evident - tall R in V1, deep S in V5-V6, upright T in V1 beyond the neonatal period

• Biventricular hypertrophy may be seen, as the LV is also pressure/volume loaded
• If large VSD with pulmonary overcirculation - LVH pattern may appear

The axis + RVH in TGA is non-specific - it is the CXR ("egg on a string" + increased pulmonary markings) combined with the ECG that points toward TGA rather than TOF (which has decreased pulmonary markings).

Source: Tintinalli's Emergency Medicine; Fuster and Hurst's The Heart, 15th ed.