transposition of the great arteries
In transposition of the great arteries, the two main vessels leaving the heart are swapped: the aorta sprouts from the right ventricle and the pulmonary artery from the left, the reverse of normal. Imagine two parallel loops of plumbing that should cross over but instead run side by side, so neither circuit ever exchanges water with the other.
The result is two separate, parallel circuits. Oxygen-poor blood returning from the body is pumped straight back out to the body without ever passing the lungs, while oxygen-rich blood circles uselessly between the lungs and the left heart. This is incompatible with life unless some mixing point, such as a hole or open ductus, lets the two circuits exchange blood.
Newborns typically turn deeply blue within hours to days of birth. A medication is often given to keep the ductus open, and a catheter procedure may enlarge an atrial opening to buy time, followed by a definitive arterial switch operation that surgically returns the great vessels to their correct positions.
A newborn becomes profoundly cyanotic in the first day of life; echocardiography shows transposition of the great arteries, and prostaglandin is started to keep the ductus open before surgery.
Deep cyanosis in the first day of life raises concern for transposition.
Transposition is one of the few congenital emergencies where keeping the ductus arteriosus open with medication is genuinely lifesaving, because that channel may be the only place the two parallel circuits can mix.