I looked up causes of asphyxiation and none just happen randomly. I could not find where a heart attack would cause asphyxiation. There are a number of similar definitions online. Here is a research paper that proposes a way to classify asphyxia in forensic context. None are fit CD's explainations.
" It is proposed to classify asphyxia in forensic context in four main categories: suffocation, strangulation, mechanical asphyxia, and drowning. Suffocation subdivides in smothering, choking, and confined spaces/entrapment/vitiated atmosphere. Strangulation includes three separate forms: ligature strangulation, hanging, and manual strangulation. As for mechanical asphyxia, it encompasses positional asphyxia as well as traumatic asphyxia. The rationales behind this proposed unified model are discussed." Source med.ncbi.nlm.nih.gov/20561144/
Anyway, the COD came from the kids not the direct source of the autopsy. If they are correct, then it doesn't look good for Chad.