The movie is based on a manga called "Boku no Hatsukoi o Kimi ni Sasagu", the story is approximately this:
"Takuma Kakinouchi (Okada Masaki) has a childhood sweetheart: Mayu Taneda (Inoue Mao), daughter of his cardiologist. Although Takuma feels strongly for Mayu and vice versa, he distances himself from her intentionally, for he doesn't know how long will his heart keep functioning after he turns 20 (a secret she doesn't know yet), and making her cry for his unavoidable fate is the last thing he would do."
And yes, this manga is from the same author of Jun's movie based manga "Boku wa Imouto ni Koi Suru Wo". There is a possibility to Jun to star in this movie too because his character Yori in Boku Imo appears in this manga too... Takuma Kakinouchi is Yori's upperclassman
I also have heart issues (not that serious though, lol) so maybe I can relate to Okada's character. And Mao-channn <3 I hope this movie doesn't have a sad ending. I'll keep an eye out for this, looks good.