First of all i am a med student but not a very good one, so count my statement like any other guy
i will try to explain how both CO (carb monox) and nicotine can kill a patient with a heart condition
it would be better if you state which specific heart condition you were diagnosed with, and some lab results for example, your ejection fraction, if you have done a coronary angiography the % of stenosis etc etc
A patient with heart disease, should avoid nicotine at all cost, because like another member already mentioned it constricts patient's blood vessels, this will increase the work of his heart mainly due to increased resistance and i can think of at least 3 other mechanisms and i'm not even that good of a student
increased workload of the patient's heart will require more oxygen delivered to his heart muscles, and Carb Monox comes into play here, it will decrease his oxygen delivery to the heart muscles which is working too hard already, and while nicotine constrict the heart blood vessels the patient can experience a myocardial infarction.
because patients already have a narrow blood vessel due to fat deposit and hardening of their heart arteries, and nicotine makes them even smaller, while CO decrease your red blood cell ability to carry and deliver oxygen to patient's heart muscle, at the same time the muscle is over working and overworking cause it to demand more oxygen.
you see it all works against the patient, everything together, so there's no excuse to keep any of it. exercise, eat healthy, take prescription drugs, see your doctor regularly, and exercise some more.
i can go into detail if you'd like, i kinda enjoy this, but please provide the specific diagnosis and lab results then i can explain more.