 respiratori chang open-heart surgeri patient undergo cardiac surgeri extracorpor circul develop alveolar-arteri oxygen tension differ venou admixtur earli post-oper period 50 abnorm anatom left shunt lung remaind ventil-perfus inequ chang appear complet revers cardiopulmonari bypass procedur respons initi underli patholog chang magnitud cardiothorac surgeri patient techniqu requir 