@MasterPannya Cardiac arrhythmia is just general term for a heart with abnormal rhythm(fast, slow, irregular) so you are technically correct the best kind of correct. I don't think it's specified for the MC from Katawa but in this it can be inferred to be a form of bradycardia(slow heart).
argh damn, i don't know if i like or hate this manga. Always got a strange feeling reading this. Probably because the MC is just a wuss. And those hints from the author (where she looks from the window at him)... gnargh