For interval training, you need to use your “instinct” and sense of pace in conjunction with your heart-rate monitor. When moving at a set pace/ speed, heartrate takes about 3 minutes to rise then stabilize. If you are only doing 2-3 minutes intervals then you can see that heartrate will not be the correct judge of effort for the interval. So, instead when you start the interval either ride at the power or run at the pace associated with the target training zone and continue to monitor heartrate.
Over the first few intervals you will see the HR gradually start to come in to zone and stabilize earlier in each interval. Then as your session continues, watch to be sure that the heartrate does not go over the upper target for the zone. If the heartrate goes too high then lactate production will increase and this will shift your fuel usage and undermine the goal of the training session. This might well mean a slight easing off in pace/ power to keep the heartrate “in zone”. As you get fitter you will find that you won’t need to ease off the pace and that you are able to complete more intervals at that same effort.