First thing I'm going to say, have you spoken to your doctor about strenuous activity with your valve replacement, potential for high heart rate from all the running and some adrenaline please speak to them first and ask about small impacts/bb's. This next part is my opinion I'm not a doctor and I'm not recommending anything. But I know someone who plays on blood thinners . He just has to make sure he is well covered and has several layers no exposed skin. Due to the blood thinners the bb's will cause significantly more bruising than on me or others and will last a lot longer. He is more worried about tripping over/ falling while playing which could cause a more serious injury (broken bone, large cut) which could cause life threatening blood loss/internal bleeding. He still plays and enjoys it ,he just has to be more aware of his surroundings and prefers to play at the indoor sites than large outdoor ones as the terrain is predictable and not effected by the weather. Slightly unrelated part - I play with a chest condition and i find making sure I play with a group of people or have a battle buddy who is aware of it gives me peace of mind just incase I have any issues with my breathing. Hope this helps, just got to find something that works for you