Cardiopulmonary interactions play an important role in the pathophysiology of many different types of congenital heart lesions. In hypoplastic left heart syndrome (HLHS), a restrictive or intact atrial septum in utero may lead to maladaptive changes within the pulmonary vasculature, characterized by “arteriolization” of the pulmonary veins and dilation of the lymphatics. At birth, these neonates with HLHS and a restrictive or intact atrial septum have profound disturbances in ventilation and oxygenation. Despite aggressive interventions to open up the atrial septum in the cardiac catheterization lab immediately after birth, postnatal survival rates for fetuses with HLHS with a restrictive or intact atrial septum remain significantly lower than for HLHS fetuses without this risk factor (33 percent compared to 92 percent). Consequently, a tool to assess the integrity of the pulmonary vasculature before birth allows us to plan for an immediate intervention after birth via a scheduled delivery to ensure the highest chance for postnatal survival.
Maternal hyperoxygenation testing
We believe that maternal hyperoxygenation (MH) testing allows us the best chance to predict the health of the pulmonary vasculature prenatally.* In the late third trimester, a scan is first performed in room air and then repeated after at least 15 minutes of MH, in which the mother is administered a non-rebreather mask with 100 percent oxygen at 8L of flow, which effectively provides an FiO2 of 60 percent. With Doppler echocardiography, the branch pulmonary arteries are interrogated at three locations: the segment immediately after the take-off of the ductus arteriosus (PA1), the distal most extra-parenchymal pulmonary artery segment prior to entrance into the lung (PA2), and the first branching point within the lung parenchyma (PA3). The Doppler-derived pulsatility index, defined as the (peak systolic velocity – end-diastolic velocity)/time averaged mean velocity is calculated at each site on room air and then with MH. A normal vasoreactive response is considered at least a 10 percent decline in the pulsatility index with MH testing in the late third trimester.
We are the only fetal echocardiography lab in the country to incorporate maternal hyperoxygenation testing into our routine assessment of all fetuses with HLHS. In addition, MH testing has proven useful in our assessment of fetuses with other congenital heart lesions known to impact the pulmonary vasculature, such as obstructed total anomalous pulmonary venous return. Finally, in fetuses with borderline left ventricles and aneurysmal patent foramen ovales, MH testing in the late third trimester increases pulmonary venous return, resulting in better filling of the left ventricle. Indeed, in many fetuses with borderline left ventricles secondary to an aneurysmal patent foramen ovale, flow will actually change from retrograde during the room air study to antegrade during MH testing, reassuring us of the adequacy of the left ventricle prenatally.
*Szwast A, Tian Z, McCann M, Donaghue D, Rychik J. Circ Cardiovasc Imaging. Vasoreactive response to maternal hyperoxygenation in the fetus with hypoplastic left heart syndrome. 2010 Mar;3(2): 172-8. doi: 10.1161/CIRCIMAGING.109.848432. Epub 2009 Dec 31.