Rprego is correct with stating that HR lags so for shorter intervals it will be behind the effort you producing. RPE is your next best guide if you don’t have a power meter. Review you workout, and the zones and their descriptions and then use your perception to determine how hard you are working. Don’t think this is not as powerful as having gadgets, learning how to feel your efforts and then cross checking with the gadgets is your most powerful metric tool!
Leyla