Learn from the attached documents about how heart rate zone training works and what your heart rate means.
If you are having a hard time returning to zone 1 during your cool down, check out Coach Matt Fitzgerald's recommendation.
"Because it takes the heart a little time to adjust to changes in pace, it's not always possible to get the heart rate down to zone 1 during recovery periods between intervals and during cool-downs. That's okay. As long as you do these zone 1 efforts at about the same pace that suffices to elevate your heart rate to zone 1 (and no higher) during your warm-ups, then they are serving their intended purpose. It's not necessary to slow all the way down to a walk just to get your heart rate down to zone 1 quicker during active recoveries and cool-down."