Having cardiac chronic problems and need other checks, can I ask extension of 30 day of my visa?

Ask.

You probably have already found the solution to your question by now. Still: based on my own experience, they can give you a S2 visa, usually for 2 months, possibly 3, but they might at one point ask you to present a letter from a hospital advising that you stay in China for a given period of time. For this, you need to go to a 3-star hospital, hospitals that have less than 3 stars can't do this kind of letter. The hospital administration has the form (病情证明单 - you might want to go there first and ask for it), then see a doctor (in the heart section of the hospital, it doesn't need to be a specialist, an ordinary doctor （普通）should work) who will fill in the form for you if he/she agrees. You probably won't have to do more exams, just present the medical reports you have, and it might help to give him/her the letter that you made for Immigration, it will save time and get to the point. Once filled by the doctor, go back to the administration and they will put a stamp on the letter. In my case, the doctor understood the situation in Europe, and based on my own health circumstances, wrote a recommendation that Immigration followed to give me another S2 visa.