I'm new on this forum and I'm developer of Hypoxic, an app to provide controlled breathing exercises with possibility to track and measure progress in doing exercises (with using a bluetooth heart monitor HxM™ Zephyr).
This app zeroed in on hypoxic training (but not only!), which means adaptation to body's deprivation of adequate oxygen supply. The idea of hypoxic training comes from swimming where sportsmen simulate limiting oxygen intake for improving their performance. During hypoxic training body begins to adapt to the lower levels of oxygen by producing more red blood cells in order to use more of the oxygen from the air that you breathe in. It has positive consequences for state of health, physical performance and mood.
In addition to aforesaid, tracking HRV (Heart Rate Variability) during breathing exercises allows estimate person's fitness level and it's progress. Application supports tags on records, so, for example, one can figure out, how changes Stange's test outcome before and after, say, workout.
So, if you have questions, feedback, bug reports, feature requests, etc, please, feel free to reply!
You can download app here: Google Play