The goal for VO2 intervals is to barely get through them really.
First third of interval, really hard.
Second third, really hard, unsure If I can get through this…
Last third, oh my god, I’m about to blow up but I just might make it to the end…
Your HR will be way up there. After many repeats of VO2 intervals, it will be nearing HR max, hopefully not exactly reaching there.
If. you start to fail them, by giving up before end or back-pedalling , it might be the power targets are slightly too high. It could also be you just need to grind them out and last to the end. Its a mental game as well as a physical one at this % of FTP for short periods.
As for does it have negative consequences?
For me completing all prescribed VO2 intervals results in a great fitness bump a few weeks down the line so not doing them 100% means less of a bump.
Also for me, if I give up in them I tend to develop a habit of giving up too early in rides and races. The mental training aspect of cycling can’t be underestimated and is so important for confidence.