Why is single ventricle fatal?

Baby's needs in air. A single ventricle is adequate for fetal development when mom is doing most of the work. After air breathing starts, the pressures of the heart change, add lungs, kidney & liver work along with regulating body temperature. The extra pressures needed to push blood & do the extra work cause the single ventricle to fail & the mixed poorly oxygenated blood it pushes can't feed the tissue .