we've created an iOS app called HeartBreath.
It is a biofeedback tool that addresses the relationship between respiration and heart rate.
RSA - respiratory sinus arrhythmia.
Paced breathing and HRV measurements from the iPhone camera or a bluetooth HRM.
Also inspired by the work of Dr. Richard Gevirtz.
The app is published on the App Store.
If anyone is interrested in free testing, I can post a TestFlight link here.
In addition whe have an experimental version that integrates with a Faros (180) device (live display of ECG and more).