The underlying cause of death is the condition giving rise to the chain of events leading to death. Between January 1, 1979 and December 31, 1998, the underlying causes of death were classified in accordance with the Ninth Revision of the International Classification of Diseases (ICD-9), a coding structure developed by the World Health Organization. Starting January 1, 1999, causes of death were classified using the Tenth Revision of the International Classification of Diseases (ICD-10). With each revision there are differences in classifying the underlying cause of death. Therefore, health statistics based on one revision are not directly comparable to the other revision without the use of comparability ratios.

Heart Disease Deaths: Before 1999, deaths were classified with ICD-9 codes 390-398, 402, 404-429. Starting in 1999, deaths were classified using ICD-10 codes I00-I09,I11,I13,I20-I51.