Conotruncal Rotation #4 — Posterior TGA (NGA Type)

“Posterior TGA” represents an advanced step along an NGA-referenced conotruncal rotation spectrum in which the aorta remains the posterior great artery and the pulmonary artery remains the anterior great artery, yet lateral great-artery shifts plus outlet-septal (infundibular) realignment produce TGA-type ventriculo-arterial discordance physiology. A classic anatomic description emphasized the posterior/rightward aorta, distinctive conal development, and the feasibility of arterial switch despite this unusual geometry. [1] (PubMed)

1. Core morphologic signature (defining anatomy)

Posterior TGA can be summarized by three coupled anatomic changes:

Great-artery “shift” within the NGA reference

Posterior Ao → rightward shift (+++)
Anterior PA → leftward shift (++)

Further rotation of the infundibular fold (IF)

The IF rotates further and tends to attach toward the tricuspid-side ventriculo-infundibular fold (right-side VIF).
Geometric cue: IF ≈ IVS (nearly parallel), suggesting a “late/advanced” outlet rotation rather than mild malalignment.

VSD becomes subpulmonary (often with anterior extension)

The VSD lies beneath the anterior PA, typically functioning as a subpulmonary “window.”

2. Conal–septal logic (why it behaves like TGA)

A helpful mental model is to separate two questions:

Spatial relationship: Which great artery is posterior vs anterior (NGA reference)?
Ventriculo-arterial commitment: Which ventricle is functionally aligned to each outflow, determined by IF/IVS geometry and the VSD route?

In posterior TGA, the posterior Ao is displaced rightward while the IF rotates toward the tricuspid-side VIF, so the posterior outflow becomes more RV-committed, whereas the LV preferentially streams toward the anterior PA through a subpulmonary VSD—creating TGA-type physiology despite the “posterior Ao / anterior PA” relationship. (Historically, this anatomy was described with a hypoplastic subaortic conus and a well-developed subpulmonary conus, underscoring why the VSD and streaming patterns are so characteristic.) [1] (PubMed)

3. Imaging “reading tips” (fast recognition)

When reviewing echo/CT/3D datasets, look for a three-part pattern:

Position cue: the posterior aorta sits unusually rightward (often strikingly so). [1] (PubMed)
Septal cue: IF and IVS are nearly parallel, consistent with advanced outlet rotation.
VSD cue: the VSD clearly opens subpulmonary, i.e., under the anterior PA, sometimes with an anteriorly extended channel.

4. Hemodynamics and clinical implications (high-yield)

Because the VSD is subpulmonary, physiology frequently resembles Taussig–Bing–type streaming:

LV → PA preference through the subpulmonary VSD (risk of pulmonary overcirculation).
RV → Ao dependence for systemic output.
Clinical severity varies with:

VSD size/restriction and adequacy of mixing (ASD/PDA),
presence of LVOTO/RVOTO, arch obstruction, and AV-valve relationships.

5. Surgical meaning (decision anatomy you can measure)

Posterior TGA should be approached as a morphology-driven repair selection problem, where the VSD route, outflow obstruction, and valve–artery relationships determine feasibility of each strategy.

5.1. VSD pathway and “commitment”

Is the VSD truly subpulmonary and nonrestrictive?
Does the VSD allow a short, non-turbulent LV pathway to the intended systemic outflow (if intraventricular rerouting is contemplated)?

Large surgical series of DORV emphasize that VSD location, associated lesions (e.g., coarctation/outflow obstruction), and overall complexity strongly influence both strategy selection and outcomes. [3] (PubMed)

5.2. Valve–artery geometry: the “distance rule” concept

A particularly practical anatomic criterion is the distance between the tricuspid valve and the semilunar valves, which correlates with the feasibility of intraventricular rerouting and helps triage toward alternative repairs when the geometry is unfavorable. [2] (PubMed)

(Conceptually: when intraventricular routing is not “straightforward,” strategies that incorporate outflow translocation or arterial switch with VSD closure become more relevant.)

5.3. LVOTO as a strategy pivot (ASO vs Rastelli/Nikaidoh-family choices)

If LVOTO is present, the repair choice becomes even more anatomy-dependent. An anatomic study of TGA with LVOTO (including TGA/VSD and Taussig–Bing specimens) demonstrated that:

ASO with LVOTO relief can be feasible in many anatomic substrates,
but specific obstructive morphologies (e.g., certain papillary muscle configurations) may limit straightforward relief and influence the operative plan. [4] (PubMed)

Key takeaways (one-slide summary)

Posterior TGA = posterior Ao/rightward +++ and anterior PA/leftward ++, with advanced IF rotation (IF ≈ IVS) and a subpulmonary VSD.
The physiology is “TGA-like” because streaming and ventricular commitment are dictated by IF/IVS alignment + VSD pathway, not by the posterior/anterior labeling alone.
Surgical planning is geometry-first: VSD relationship, TV–semilunar distances, and LVOTO substrate guide whether ASO, intraventricular rerouting, or alternative pathways (REV/Rastelli/Nikaidoh-spectrum) are most coherent. [2–4]

References (PubMed-verified)

[1] Tam S, Murphy JD, Norwood WI. Transposition of the great arteries with posterior aorta. Anatomic repair. J Thorac Cardiovasc Surg. 1990;100(3):441-444.

[2] Sakata R, Lecompte Y, Batisse A, Borromée L, Durandy Y. Anatomic repair of anomalies of ventriculoarterial connection associated with ventricular septal defect. I. Criteria of surgical decision. J Thorac Cardiovasc Surg. 1988;95(1):90-95.

[3] Kleinert S, Sano T, Weintraub RG, Mee RB, Karl TR, Wilkinson JL. Anatomic features and surgical strategies in double-outlet right ventricle. Circulation. 1997;96(4):1233-1239.

[4] Hazekamp M, Portela F, Bartelings M. The optimal procedure for the great arteries and left ventricular outflow tract obstruction. An anatomical study. Eur J Cardiothorac Surg. 2007;31(5):879-887.

Conotruncal Rotation #4 — Posterior TGA (NGA Type):