There are some basic rules you should know when you start to use a [Polar] heart rate monitor. First of all it is important to know that your heart rate is affected by several internal and external parameters. It takes months (if not years) to get enough experience to understand how the heart rate reacts.

Your [Polar] heart rate monitor works best for pacing at aerobic cycling, since there is a slightly delay on the pulse frequency. If you accelerate the bike to a new steady state pace, you will reach the steady state pulse a few minutes later. That is really important to know, and I guess that is why many riders push too hard in the beginning of intervals, because they are trying to get their heart rate up in the target zones. That is a big mistake that could easily be avoided if these riders used a power meter.

In short intervals with a length of less than 2 minutes, I will recommend that you don't use your heart rate monitor for pacing.

There are several formulas around on the internet that can be used to calculate target heart rate zones. I have seen formulas with more than 6 different target zones, but I normally prefer to use less. You can easily define your own target zones when you use the Karvonen formula. Remember that there are several factors influencing on the pulse, so don't make the intervals too small.

Karvonen's formula: Target HR = Intensity% x (Max.HR minus Min.HR) + Min-HR

Intensity=75%, Max.HR=190, Min.HR=54

T-HR = 75% x (190 - 54) + 54

-HR = 156