8 minute intervals

I did 8 minute intervals on Tuesday. I did them last Tuesday too. I do 220-230 watts with a 15 second surge every two minutes. Last week I went higher power but really died at the end. This week I did 220 watts and felt better as the workout progressed. The surges are 280-300 watts. I felt like my pedal stroke was getting better and I concentrated on the recovery part of the pedal stroke.