There is a Maternity Center in a place not far from Villa Ballester, called Florida, Vicente López. At this Medical Center I found a Cardiologyst named Dr. Mario Portnoy, who surely may be a descendent or a close relative of the late Dr. Mario Portnoy you are looking for. So, you can e-mail him at this adress:
INSTITUTO DE MATERNIDAD SANTA ROSA. M.J.Haedo 4150. Florida, Vicente López. Provincia de Buenos Aires.