Your question implies that the setting of the story takes place in the 1960's. If that is the case you have a historic problem with a patient recovering from a cardiac
bypass surgery.
"From 1960 to 1967, several sporadic instances of coronary grafting were reported. All were isolated cases and, for uncertain reasons, were not reproduced."
http://cardiacsurgery.ctsnetbooks.org/cgi/content/full/2/2003/3ck=nck#THE_DEVELOPMENT_OF_CARDIOPULMONARY_BYPASS
So cardiac bypasses did not become a relatively routine procedure until the mid to late 1970's. One relatively common complication associated with post bypass surgery is a
ventricular arrhythmia. Any medication that is pro-arrhythmic could help precipitate the problem - for which a post bypass patient is at risk for any way.
Again in 1960's perspective.
*Tricyclic antidepressants such as amitriptyline
*Too little potassium usually from too much
diuretic such as
Lasix without potassium replacement.
*Antibiotics such as Erythromycin
*Antipsychotics such as
Thioridazine.
Any combination of the above would theoretically have expotential effects when added together. All of the medications I named were around and were used in the 1960's.
In those days, I am relatively sure that they did not have external pacemakers, either.
I hope this helps. Good luck.