I did a practice run early today so I can test out my equipment and make sure everything was working right. Ran the nitrogen at 20 LPM, didn't pre-fill the 3M PPeH hood, didn't hyperventilate prior, didn't take deep breaths when I put the hood on, I just breathed normally. I had a fingertip oximeter attached to see the change in oxygen levels so I can confirm if my setup was viable.
First 5-10 seconds were fine, no pain or discomfort, but soon after I saw the oxygen levels quickly drop from 98% to 70%. Panic set in and I started hyperventilating and ripped the hood off. I believe at this point my SI kicked in. Felt fine afterwards.
So at least I know that my setup is working fine. I will have to be mindful of my SI and prepare myself thoroughly for the real run. I know I didn't use the proper techniques, but as I said, this was just to primarily to test out the equipment, familiarize myself with everything, and see how comfortable it would be.