Cardiovascular (cardiac) drift is the gradual upward creep in heart rate during prolonged steady-state exercise, even when pace and effort remain constant. It's caused primarily by progressive dehydration reducing plasma volume, and rising core body temperature increasing the cardiovascular demand of cooling. Cardiac drift means heart rate becomes a less reliable pacing guide late in long sessions — a runner holding the same pace may see heart rate rise 10-20 bpm over a couple of hours. Good hydration and heat management reduce drift, and it diminishes with improved aerobic fitness.
Key points
- Heart rate rises despite constant pace
- Caused by dehydration and rising core temp
- Makes HR less reliable late in long sessions
- Reduced by hydration and aerobic fitness