Cardiac drift is the progressive increase in heart rate observed during sustained submaximal exercise, even when pace and perceived effort remain constant. The term was coined to describe the drift away from steady-state that occurs after the first 10–20 minutes of a long run.
The primary causes: 1. Dehydration: reduced plasma volume increases blood viscosity and requires the heart to beat faster to maintain cardiac output 2. Cardiovascular drift: as core temperature rises with exercise, skin blood flow increases to enable cooling, reducing blood available for muscles and requiring higher heart rate to compensate 3. Glycogen depletion: as carbohydrate stores are depleted, fat metabolism requires more oxygen per unit of energy, increasing the cardiovascular load at constant mechanical output
Cardiac drift is why you can't simply maintain a target heart rate pace through a long run — your heart rate will rise even if you're running at the same objective pace. Long runs should generally be paced by effort or by GPS pace rather than by fixed heart rate targets.