There are some fundamental differences between intrauterine and extrauterine circulation. The foetus has very high pulmonary vascular resistance due to amniotic fluid in the developing lung and vasoconstricted pulmonary vasculature. This means that the right ventricle is operating in a higher pressure regime than the left ventricle. Due to the relatively small volume of pulmonary return from lungs and small atrial shunt via patent foramen ovale (PFO), left ventricle (LV) receives relatively less flow during the foetal life (approximately one third), and RV plays the role of a dominant ventricle receiving and pumping on approximately two thirds of the combined ventricular output and operating in a higher pressure regime than the left ventricle. When these high right-sided pressures persist in newborn life, it causes a problem known as persistent pulmonary hypertension of the newborn (PPHN) (Fig. 4.1).