There are some basic rules you should know when you start to use a 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.

Heart rate monitors work 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.

Read how to determine your target heart rate