ratio of forward to reverse flow in the pulmonary veins by pulse wave Doppler evaluation (Fig 10.9).61–63 Candidacy for this procedure is primarily based on high-risk physiology as opposed to discrete anatomic features; however, fetuses with a hypoplastic left atrium and exceptionally thick atrial septum may not be candidates for technical reasons FIG 10.9 Pulse wave Doppler of a pulmonary vein in a fetus with hypoplastic left heart syndrome and intact atrial septum prior to intervention Technical Aspects An 18- or 19-gauge cannula is inserted through either the right or left atrium, and the stylet or a 22-gauge Chiba needle is used to puncture the atrial septum A balloon and/or balloon-mounted stent are inflated across the septum, with the goal of creating an atrial communication of 3 mm or greater (Video 10.5) Fig 10.10 demonstrates left-to-right flow across the stent following successful deployment Bradycardia and ventricular dysfunction are uncommon with atrial access; however, hemopericardium can occur after the cannula is removed With stent placement, there are additional risks of malposition or embolization.59,64,65 In the Boston series, 19 of 21 procedures were reported as technically successful, with two cases of fetal demise within 24 hours of the procedure.64 In 2017 the International Fetal Cardiac Intervention Registry, which includes data from multiple centers around the world, reported technical success in 36 of 47 cases (77%) treated from 22.9 to 36.1 weeks Although no maternal complications were reported, there were six cases of fetal demise.66 FIG 10.10 In this fetus with hypoplastic left heart syndrome and intact atrial septum, a stent is visualized across the atrial septum with left-to-right flow Postnatal Outcome and Management In the series of patients reported from Boston, fetuses with atrial communications of 3 mm or greater had significantly higher oxygen saturations and a decreased need for emergent left atrial decompression at birth However, survival following stage 1 surgery was not significantly different as compared with fetuses with smaller atrial communications or unsuccessful fetal cardiac intervention Similarly, the International Fetal Cardiac Intervention Registry found that fetuses with successful fetal cardiac intervention were less likely to be delivered via cesarean section and require immediate postnatal intervention or resuscitation.66 However, survival of live-born fetuses to discharge was similar between patients who underwent technically successful fetal cardiac intervention versus unsuccessful (or no) fetal cardiac intervention.19 The registry has incomplete data through 1 year of age; however, the subset of fetuses who had a technically successful fetal cardiac intervention with a nonrestrictive atrial communication at the time of birth appeared to have better survival as compared with fetuses who did not undergo fetal cardiac intervention.66 Beyond fetal cardiac intervention, such patients may be managed with staged reconstruction, with the choice of neonatal procedure (hybrid or stage I Norwood) largely depending on institutional preference (see also Chapter 71) Although outcomes with the hybrid procedure may be marginally better for such high-risk patients,58 the overall prognosis of HLHS with IAS remains poor, with mortality rates in excess of 50% Survival may perhaps be improved by not only a more aggressive interventional approach performed earlier in gestation but also by gaining a better understanding of the complex, secondary pulmonary complications