If students have a heart condition, you should encourage them to speak with their doctor first to ensure that yoga will benefit them, rather than increase their risk. You want them to be healthy, and sometimes even starting slow can aggravate the heart. By getting the okay from the doctor, you are ensured that your students are going to be okay during exercises, but you are also protecting yourself if a student were ever to have a heart attack in your class.
Imagine a pill proven to release neurotransmitters, promote feelings of good will, release tension, and improve health. We would be standing in line to buy it. Why not try Yoga instead?