Because of the limitations of pulse oximetry I recommend you start with a Polar H7 heart rate belt. It is affordable (about $79) and available on Amazon or a local sporting goods store. If you don’t choose an H7, ensure your heart rate belt is Bluetooth Low Energy (BLE) so it syncs with the updated iPhone or Android device. Garmin uses a different standard (Ant+) and won’t work with what you want to do.
There are many, many apps that will take the readings from your Polar H7 and convert the data. The three best are iThlete, SweetwaterHRV and Marco Altini’s Heart Rate Variability Logger. iThlete is limited in that it is for a daily, short reading to determine your state of physical training. SweetwaterHRV is a good app made by very nice people and it is a bit difficult to extract data (you download and email it). It has a lot of interpretations of the data in the app.
Marco Altini’s app gives nice readings in real time and has a dead simple Dropbox or iCloud download feature. You take a reading, press a button on your device and the .csv file is in your file folder. That simple. Once you have a .csv file you can import it to excel and play with the numbers. You can also see the derivations and start looking there.
Find out more by reading about Setting Up What You Study.