Thank you for any assistance.
Diagnostic is an adjective, describing something. So, for example, you’d say “musical instruments,” “expensive instruments,” and “diagnostic instruments.”
Diagnostics is a noun – like any specialized field. So you’d say “I study English,” “I study Linguistics,” and “I study Diagnostics.”
So, the question is, are you using the word “diagnostic” to describe the cardiac unit? Or are you specifying a field, which is named “Cardiac Diagnostics?” Honestly I think you’re probably ok either way; it’s not that one is right and the other is wrong – just a question of style and (very subtle) implication.
Personally, I think “Cardiac Diagnostics Unit” is slightly easier to say aloud, if that matters.
I’m glad you asked! Now we can rename our department the “Grammatic Diagnostics Unit.” ;) I also do comma splices.