Pathophysiology
Clinical meaning
Persistent Pulmonary Hypertension of the Newborn occurs when pulmonary vascular resistance (PVR) fails to decrease after birth, maintaining fetal circulation pattern with right-to-left shunting through PDA and foramen ovale. Normally, increased PaO2 and decreased PaCO2 stimulate endothelial nitric oxide synthase (eNOS) to produce NO, which activates guanylate cyclase to produce cGMP, causing smooth muscle relaxation. In PPHN, this pathway is impaired. Phosphodiesterase-5 (PDE5) degrades cGMP.