Conotruncal Development and Septal Spiral
TGA with Ventricular Inversion (L-TGA): Clockwise Spiral
D-TGA: Nonspiraling Septum
Tetralogy of Fallot: Incomplete Normal (Counterclockwise) Spiral

Septation of the conus/truncus into recognizable aorta and pulmonary trunk requires four separate processes: looping of the conus/truncus to the interventricular position; septation of the conus cordis into two roughly equal tubes; rotation of the truncal septal plane 90 degrees from that of the interventricular septal plane; and alignment of the conotruncal septum with the interventricular septum.

Pulmonic stenosis results when conotruncal septation results in a smaller diameter for the future pulmonary trunk. It may also occur at the level of the infundibulum if the conotruncal septum is not aligned with the bulboventricular septum, resulting in a more prominent supraventricular crest or a subpulmonic ridge. A membranous VSD may also occur.

Starting in the distal truncus arteriosus, the truncal septal ridges grow and spiral in a counterclockwise (from caudal view) direction, eventually joining to form a solid septum between what will become the ascending aorta and the pulmonary trunk. In L-transposition, the spiral is clockwise, and only encompasses about 45 degrees of rotation. Thus, the systemic conduit is located antero-left to the pulmonary conduit at the level of the semilunar valves. This means the proximal conotruncal septum lies in a different plane from that of the bulboventricular septal margin. This is complicated by the L-looping of the bulbus cordis and consequent anomalous interventricular septal angle. Hence the high frequency of VSD in this anomaly. In addition to ventricular inversion, the atria also suffer laterality problems and may be isomeric. L-Transposition and Ventricular Inversion: 2 Cases shows more examples of the spectrum of lateralizing and conotruncal anomalies in L-TGA.

(Figures from Gurvitz, M. General echocardiographic approach to the adult with suspected congenital heart disease. In Otto CM, ed. The practice of clinical echocardiography [3. ed]. p. 1029, 1030 ©Elsevier, 2007)

The septal ridges of the d-TGA truncus fail to spiral, instead retaining the same nearly-coronal plane from distal to proximal. Hence, the aorta is sequestered into the right ventricular outflow tract, and the pulmonary artery is sequestered to the left ventricular outflow tract. This effectively isolates the two circulations except for the nearly omnipresent patent ductus arteriosus and/or ASDs and VSDs.
(Figures from Gurvitz, M. General echocardiographic approach to the adult with suspected congenital heart disease. In Otto CM, ed. The practice of clinical echocardiography [3. ed]. p. 1029, 1030 ©Elsevier, 2007)

In tetralogy of Fallot, the conotruncal ridges spiral counterclockwise to about 60 degrees, leaving the aortic root "overriding" the crista supraventricularis. It lies more anterior than it should, causing the conotruncal ridges to miss the crista and leaving a VSD. Anterior shifting of the conotruncal ridges also causes pulmonic hypoplasia and stenosis.
(Figures from Gurvitz, M. General echocardiographic approach to the adult with suspected congenital heart disease. In Otto CM, ed. The practice of clinical echocardiography [3. ed]. p. 1029, 1030 ©Elsevier, 2007)