VO2 Max Intervals

I am doing VO2 Max intervals and I have a few questions. But first here is some info:
Age: 49
My LT threshold is 300 Watts at 150 BPM (max HR is 173)
VO2 Max is 60.9 and my MAP is 80%
VO2 Max power range is 330 - 415
The intervals I am doing are 3 sets of 6X90 with 10 minutes of rest between sets.
After doing the intervals I looked at my data and found that while I was pushing the correct power range for the interval, my HR really didn't get out of my LT zone (typically pushing ~155-157). So here are the questions:
1) I understand that HR is slow to react during this time frame however am I still getting the benefit for VO2?
2) If not, should I extend the duration time such that my HR does get up to VO2 Range?
3) If I am increasing the duration should I reduce the number of reps in each set?

Thanks for any help

Conventional wisdom would be to hold VO2 Max efforts for 3 to 5 minutes each. With typical sets being 5x5s with 5 minutes rest between each interval or 8x3s again with equal 3 minute rest periods. There are other theories but most involve working hard enough to achieve maximum cardiac output at least after the first couple of efforts and for the last minute or two of each effort.

IMO you should extend your L5 efforts to 3 to 5 minutes each and ride them hard enough that you hit deep basically uncontrollable breathing by ~2.5 minutes into each effort yet easy enough that you complete the efforts and the sets.

There are other theories about the best way to target VO2 Max improvement but they all involve sufficiently intense efforts to max out your VO2 gas exchange and I'm not sure you'll get there on 90 second efforts unless you limit your rest between efforts and hit the higher end of your listed VO2 Max range on each effort.

-Dave