True about TGA –
In TGA aorta lies anterior and to the right of puhnonary artery.
In patients with TGA the oxygenated pulmonary venous blood recirculates in the lungs whereas deoxygenated systemic venous blood recirculates in the systemic circulation --> Pulmonary artery saturation is always higher than aortic saturation (opposite to normal person).
Because oxygenated blood is not available for systemic circulation, survival depends on the mixing available between the two circulation.
So, patients with TGA can be divided into : -


TGA with intact ventricular septum
TGA with VSD.

i) TGA with intact septum

The mixing of blood depends on atrial communication.
Most of the time atrial communication is a patent foramen ovale.
As patent foramen ovale is very small communication, mixing of oxygenated blood with deoxygenated blood is inadequate —> Patients are symptomatic and cyanotic at birth, and CHF may develop during first week.

ii) TGA with VSD

Presence of VSD of adequate size results in good mixing of blood.

a Patients become symptomatic, at 4-8 weeks when fetal pulmonary vasculature regresses maximally —> CHF
develops at this time.

The failing left ventricle and large pulmonary blood flow cause increase in left atrial pressure —> T back pressure Pulmonary venous hypertension and pulmonary plethora.
The presence of a large VSD equalizes the pressure in the two ventricles as well as in two great vessels.
Pulmonary artery carries large flow —) patients with TGA and large VSD develop pulmonary vascular obstructive disease (Eisenmenger physiology) early in life.
Cyanotic disease
Aorta anterior to pulmonary artery
VSD
All of these