It can be hard to follow an interval workout using a heart rate monitor alone.
This is because your heart rate takes a few minutes to fully reflect what you're actually doing. It's the same for everyone. For example you might speed up to a fast pace, but your heart rate will remain low for until it builds up to reflect your current intensity. And vice versa.
For this reason, heart rate is great for steady state runs but not so good for interval sessions with frequent changes in intensity. Pace and power output are better measurements for interval sessions (followed by Rate of Perceived Exertion or RPE).