Why is transposition of the great vessels so deadly?

Blue newborn!: Tga is a very severe cardiovascular anomaly because it can cause very low oxygen concentration in the blood going to the coronaries and carotids and prove lethal if untreated promptly and expertly. If a ventricular septal defect is present, the newborn can be more stable and interventions can be more planned and less emergent. Fetal life is ok - trouble starts as soon as the cord is clamped.

Tga: there are 2 types , one D TGA needs immediate surgery in first days of life. Then you have good life expectancy, not necessarily if it is fixed in time. Other is L-TGA or corrected TGA and that one may not ever need surgery, however sometimes needs surgery and sometimes things may go worse since aorta is supported by a weaker ventricle-RV.