Arrythmia. Basically his heart's weak, and if it's strained too far he could have a heart attack. It's bad enough when he has flutters, because that can completely drain him of his energy and happiness of that day.
No, it's sort of become a meme that he has heart attacks easily. It's supposedly rather common to see someone fainting or having a heart attack or similar from someone being so cute in anime, so instead of fainting poor Hisao gets a heart flutter or heart attack instead.