Anatomy. In critical PS, the TV is often mildly hypoplastic and the RV may be muscle-bound and restrictive, but there is at least a trickle of antegrade PBF through the PV, distinguishing it from PA/IVS. Critical PS and PA/IVS are prognostically different in that nearly all patients with critical PS are able to achieve a biventricular circulation in the neonatal period, which is not true for all patients with PA/IVS. In PA/IVS, the anatomy of the TV, RV, and PV may vary from mildly to severely hypoplastic. Interestingly, severe pulmonary artery hypoplasia is uncommon. If the TV Z score is <2.5 (a surrogate for RV size and function), the likelihood of achieving a biventricular circulation in the newborn period is low. In cases of severe RV hypoplasia, the coronary circulation may be RV-dependent (see next pages).
Physiology. Patients with both critical PS and PA/IVS are PGE-dependent after birth (i.e. discontinuing PGE leads to critical hypoxemia); a patient with a high degree of pulmonary stenosis who does not become hypoxemic has severe PS rather than ‘critical’ PS. Hypoxemia occurs due to right to left atrial level shunting due to restrictive RV physiology which causes decreased PBF due to right to left atrial shunting. This diminution of PBF leads to decreased LA return and encourages the right to left atrial shunting. In most cases, following echo confirmation, the first step is most commonly a cardiac catheterization for coronary assessment and a balloon valvuloplasty of the PV.