The medical word root will be taken from a different source language, like Latin. Hence, to understand precisely what the word means (or to create a word), you need a grasp of both English language and the Latin language, as well as the rules relating to suffixes like ‘ac’.
‘Kardia’ is the word that relates to the heart, but in English, a patient could not simply suffer a ‘kardia’ event. Instead, to make the word more English and easy to work into language, the suffix ‘ac’ must be implemented in order to make it into a ‘cardiac’ event. The suffix ‘ac’ is there to add to the prefix to make sure that all words that might be taken from a different source language can easily become a part of English, soon becoming professional and official medical terms as they are borrowed from other languages, a sign of lexical change.
The suffix ‘ac’ is just one of many suffixes used in the English language in medical terms, like ‘s’ or ‘es’.