What is an older  cardiac medication that works via covalent bonding?

"Covalent bonding" simply refers to the bonding of atoms to form
molecules via the sharing of electrons.  Heparin is one drug which is
covalently bonded to a variety of items used in the treatment of heart
disease.  See, e.g.,:
http://www.investor.jnj.com/releaseDetail.cfm?ReleaseID=66453&year=2001.

Similarly, heparin sodium
(http://www.rxlist.com/cgi/generic/heparin.htm) is an anti-coagulant
used, inter alia, during cardiac surgery to prevent clotting.  The
heparin solution so used is porcine (pig) based and used by injection.

Heparin as an anti-clotting agent has been used for many years.

Hope this helps!

Generally, in pharmacology, covalent bonding leads to irreversible
actions on a target (be it receptor, enzyme etc.).  So your answer
will be an irreversible antagonist or something of that ilk.

An example would be phenoxybenzamine (irreversible at
alpha-adrenoceptors) in hypertension (not strictly cardiac, but
close!).
http://www.uic.edu/classes/pcol/pcol425/restricted/Ye/AHD2.pdf