There is a brief explanation of the ICDA-9 codes on the West Virginia Vital Statistics webpage entitled Health Statistics Center "1998 Vital", where they provide the following explanation:
Cause of Death Classification
"The cause of death is coded from information contained in the medical certification section of the death certificate. This portion of the certificate is completed by the attending physician. In the case of unattended death, the medical examiner is responsible for investigation and certification. As a general rule, data preceding 1949 cannot be accurately compared with those of later years due to the extensive revision of the coding rules made at that time; however, large causal categories can be compared. See Appendix B, "Groupings of ICDA Codes," for a listing of the selected causes."