Yes, it is not possible to hit a specific Hr and maintain it precisely. But in the suggested interval, regard the LTHR as the minimum for the "hard" part of the interval. In other words, do the three minutes at around 130 or slightly above, then do the two minutes without allowing your HR to drop below 145. Be prepared for the fact that there is a lag between making an effort and your HR catching up, so time the interval from when you start going at the right intensity, don't wait until your HR has climbed to start timing. You'll find with experience that you can judge the effort required, and become used to feeling "at this level of effort, I know my HR will come up above 145 within 30-45 seconds".

I hear what you say about the drugs. But drugs or not, we now know that your heart can physically beat more than 170 times per minute without you dropping dead.