My recommendation will be to track HRV (1) and do the high intensity only when you are on or above your baseline. This is to avoid unexpected side effects of HIIT. If you can't get a hrv tracker then best to keep track of your heart rate recovery. It is common to see heart rate stay high after the exercise, it should return to normal in a few hours (3-4 hours). if not then you are over doing it.
1 - Apps like Elite HRV, hrv4training or Devices like Garmin (watch), CoreSense (Elite HRV), Scosche Rhythm+, polar or garmin chest wrap.
1 - Apps like Elite HRV, hrv4training or Devices like Garmin (watch), CoreSense (Elite HRV), Scosche Rhythm+, polar or garmin chest wrap.