If you are doing HIIT cardio, and you should be, then heart rate is irrelevant. Just perform the exercise at your absolute highest possible intensity for the allotted 30-60 seconds. You should be going as hard and fast as you can possibly go, for the entire time.
Part of the benefit of HIIT cardio is the constant increase and decrease in intensity levels. It trains your body to not just improve its maximal aerobic capacity under load, but also to slow down your heart rate and recover more quickly once the exercise is over. It's the improvement in recovery and returning to resting heart rate faster that scientists think is maybe the most valuable component to HIIT cardio.