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."
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.