Jo – 12kg H-Snatch Workout

Today I attempted to move up to the 12kg for part of my snatch workout. The plan was to go as far as I could with good form, or until it just got too difficult, then switch back to the 8kg. At 1:00R/1:00L, and 12 RPM, I only completed 2 rounds, for a total of 4:00. I wasn't happy with that, but that's what I did.
The last 13 rounds with the 8kg were at 15 RPM, and my avg. HR was 172.
Britt, I was barely able to complete the 30:00 workout, due to using the 12kg in the beginning; should I just stick with the 8kg, so I can complete my full 40:00 workouts, or more? That 12kg really zapped me!