What is the highest possible heart rate of a human being?

I am a professional free diver which means I dive without an oxygen tank. It's more of a sport than a profession and so you need a lot of training, as with any sport. One of the main areas of training is obviously your breathing and how long you can hold your breath, as the longer you hold your breath, the longer you can stay under. To help with holding our breath we also train to bring our heart rate as low as possible. At the moment my lowest achieved heart rate is around

9bpm, however I once met a man at a convention in Atlanta who claimed he could bring his down to 4bpm! Hope this helps