This app was mentioned in 1 comment, with an average of 1.00 upvote
Huh. At first glance, one would think that would be a fairly simple and easily-available app. After a quick search, it seems that all the monitor devices are dedicated to their own branded app. I don't use Apple stuff, though, so I'm not familiar with their App Store. (Why is there no fucking app-search function on this page? Why do you make me hate you every time I interact with you, Apple?)
Android seriously is the superior platform. I very easily found plenty of heart monitoring stuff... although anything I saw that detected arrythmia seemed to require you to hold your phone the entire time. Maybe you could try something like this app that teaches you how to interpret the data yourself, in whatever recording software you're currently using?