We report a case of a 15-month-old female presenting with a large aortopulmonary window during a medical mission. The optimal surgical correction for this patient would be based on an estimation of her pulmonary vascular reactivity. This report details the use of antegrade/retrograde flow ratios using Doppler measurements in the proximal descending aorta at baseline and while receiving 100% oxygen. A large increase in the proportion of retrograde flow under the 100% oxygen condition suggested a reactive pulmonary vascular bed. She underwent a complete repair with patch closure of the aortopulmonary window. A postoperative echocardiographic assessment showed near normal right ventricular pressures.