You should ask your physician about the cardiac effects of both. Amitriptyline is cardiotoxic and can cause serious arrhythmias and is usually contraindicated in anyone with recent heart surgery or disease. Venlafaxine can increase blood pressure by increasing norepinephrine, also not great after valve replacement. Definitely seek proper advice from a medical professional about the risks and benefits considering you’re coming off a serious procedure.
Agree with Joeshmoe.
I found amitriptyline ( maybe could be described as an old fashioned medicine) not as effective as venlafaxine. I would want to start with a small dose, see how it goes. I am thinking of asking the doctor what he thinks about citalopram. Usually they let me try anything reasonable. I ran over the electric cord with my electric lawnmower and could not remember my bank password with mexiletine so had to give it up. Grass doing better now and can access my bank. All the best.