The first attempt at fetal aortic valvuloplasty was reported by Maxwell et al in 1991.1 Over the next decade, multiple centers attempted to perform the procedure; however, due to selection of severe cases in the third trimester and technical difficulties, the experience was largely unsuccessful.11 In 2000, investigators at Boston Children's Hospital and Brigham and Women's Hospital embarked on a program to perform fetal aortic valvuloplasty among second trimester fetuses that met specific physiologic criteria of fetal AS with evolving HLHS With technical modifications to the procedure, greater success was able to be achieved.12 The experience has since been replicated at other centers worldwide.13–19 Makikallio and colleagues outlined specific pathophysiologic features of fetuses with severe midgestation AS that reliably predicted evolution to HLHS These features included LV systolic dysfunction, retrograde flow in the transverse aortic arch (Fig 10.2), monophasic mitral inflow, and left-to-right flow across the foramen ovale,20 which have since been validated in a distinct cohort.21 The goal of fetal intervention for this disease is to relieve the severe AS that triggers these hemodynamic alterations in the second trimester, thereby avoiding evolution to HLHS and enabling a biventricular circulation postnatally.12 FIG 10.2 Sagittal image depicting retrograde flow in the transverse aortic arch in a midgestation fetus with severe aortic stenosis and evolving hypoplastic left heart syndrome Patient Selection When selecting candidates for fetal aortic valvuloplasty, the first consideration is whether the fetus has evidence of severe valvar AS and pathophysiologic features that are highly suggestive of evolution to HLHS These features are the hemodynamic findings outlined previously, namely, LV systolic dysfunction, retrograde flow in the transverse aortic arch, monophasic mitral inflow, and leftto-right flow across the foramen ovale This is particularly important because a small subset of patients with midgestation AS may not present with these features and may achieve a biventricular circulation with postnatal aortic valvuloplasty alone.22 The second consideration is whether the left heart is salvageable Some fetuses present with disease that is too far advanced For example, if the LV is already hypoplastic at the time of presentation, then recovery is unlikely Based on fetal aortic valvuloplasty performed in 70 fetuses, McElhinney et al devised a scoring system to help predict which patients would be likely to have a biventricular circulation postnatally.23 The scoring system included LV long-axis z-score greater than 0, LV short-axis z-score greater than 0, aortic annulus zscore greater than −3.5, mitral valve annulus z-score greater than −2, and AS (or mitral regurgitation) maximum systolic gradient ≥20 mm Hg The presence of four or more of these features had 100% sensitivity and 38% positive predictive value for identifying patients with a biventricular outcome In addition, certain patients may present with a normal or dilated LV, but extensive scar tissue, or endocardial fibroelastosis is present Endocardial fibroelastosis is associated with abnormal LV geometry and more severe diastolic dysfunction, which also limits the ability to salvage the LV.24,25 For this reason, the Boston group recently reanalyzed their experience of 123 fetuses In addition to the size of leftheart structures (particularly the ascending aorta and mitral valve) and the estimate of LV pressure, the mitral valve inflow time, which is a marker of diastolic dysfunction, was retained in the model to estimate likelihood of biventricular outcome from birth (Fig 10.3) FIG 10.3 Classification and regression tree analysis for likelihood of biventricular outcome following fetal aortic valvuloplasty among live-born patients AO, Ascending aorta; BV, biventricular; MV, mitral valve; LVP, left ventricular pressure (Modified from Friedman K, Sleeper LA, Freud LR, et al Improved technical success, postnatal outcomes and refined predictors of outcome for fetal aortic valvuloplasty Ultrasound Obstet Gynecol 2017 Epub ahead of print.) Finally, and perhaps most importantly, the mother must be a suitable candidate for fetal cardiac intervention Since the fetus may or may not be of a viable gestational age and carries a significant anomaly, fetal cardiac intervention should not be performed unless there is minimal risk to the mother All mothers should be thoroughly assessed by maternal-fetal medicine specialists prior to offering fetal cardiac intervention Technical Aspects Because the safety of the mother is paramount, fetal cardiac intervention should be performed in an obstetric operating room with maternal epidural anesthesia Once the epidural is placed, the fetal position is determined, and version is often performed to move the fetus into a favorable position As seen in Fig 10.4, the fetus is ideally positioned such that the left chest is anterior and there is a straightforward pathway to the LV outflow tract.12 Once the fetus is optimally positioned, an intramuscular injection of analgesic (fentanyl), paralytic agent (pancuronium), and atropine is given