ead ▁of ▁the ▁normal ▁d rain age ▁of ▁the ▁pul mon ary ▁ve ins ▁into ▁the ▁left ▁at rium ). ▁ ▁Common ▁or ▁single ▁at rium ▁Common ▁( or ▁single ) ▁at rium ▁is ▁a ▁failure ▁of ▁development ▁of ▁the ▁emb ry olog ic ▁components ▁that ▁contribute ▁to ▁the ▁at rial ▁sept al ▁complex . ▁It ▁is ▁frequently ▁associated ▁with ▁heter ot axy ▁synd rome . ▁ ▁M ixed ▁▁ ▁The ▁inter at rial ▁sept um ▁can ▁be ▁divided ▁into ▁five ▁sept al ▁zones . ▁If ▁the ▁defect ▁involves ▁two ▁or ▁more ▁of ▁the ▁sept al ▁zones , ▁then ▁the ▁defect ▁is ▁term ed ▁a ▁mixed ▁at rial ▁sept al ▁defect . ▁ ▁Pres entation ▁ ▁Comp lications ▁Due ▁to ▁the ▁communication ▁between ▁the ▁at ria ▁that ▁occurs ▁in ▁AS D s , ▁disease ▁entities ▁or ▁comp lications ▁from ▁the ▁condition ▁are ▁possible . ▁Pat ients ▁with ▁an ▁un correct ed ▁at rial ▁sept al ▁defect ▁may ▁be ▁at ▁increased ▁risk ▁for ▁developing ▁a ▁card iac ▁arr h ythm ia , ▁as ▁well ▁as ▁more ▁frequent ▁resp ir atory ▁in fe ctions . ▁ ▁De com pression ▁sick ness ▁AS D s , ▁and ▁particularly ▁P FO s , ▁are ▁a ▁pre dis pos ing ▁ven ous ▁blood ▁carrying ▁in ert ▁g ases , ▁such ▁as ▁hel ium ▁or ▁nit ro gen ▁does ▁not ▁pass ▁through ▁the ▁l ungs . ▁The ▁only ▁way ▁to ▁release ▁the ▁excess ▁in ert ▁g ases ▁from ▁the ▁body ▁is ▁to ▁pass ▁the ▁blood ▁carrying ▁the ▁in ert