A pathologist testifying as a defense expert said today that he doubts 8-year-old Bianca Cartagena died of asphyxia, a key component in the murder charge against her mother, Candice Cartagena.Dr. Jonathan Arden of McLean, Va., said that after reviewing autopsy results, photographs, slides and other information, he believes Bianca died of natural causes. ???A sudden cardiac death that resulted from