This program was recently on the first lines in the appStore. Judging by the description, it measures the heart rate using a microphone, both built into the iphone and a headset. However, my attempts to get any result close to reality did not lead to anything. Each new value was different at times. Apparently the microphone sensitivity is not enough for these purposes. There is also a semi-automatic mode in the program. Signals indicate a time interval for which you yourself must count the number of heartbeats, then enter this number and get the result. This is probably the most accurate way. 
