PHYS-11 · Principle

Why does my heart rate creep up on long rides at the same watts?

That is cardiovascular drift: after about 10 minutes heart rate rises as stroke volume falls. Heat and fluid loss make it bigger, and with them the effort really does get harder.

How the body works, agreed across the reviews below; a study entry shows what one experiment measured.

established physiologyApplies to continuous efforts beyond about 10 minEstablished — the drift and its enlargement by heat and fluid loss; well supported — the fall in VO2max with it, above all in the heat (one group's review and trial, REF-3495, REF-3496); contested — the main cause…

Practical Application

Why does my heart rate creep up on long rides at the same watts? That is cardiovascular drift: after about 10 minutes heart rate rises as stroke volume falls. Heat and fluid loss make it bigger, and with them the effort really does get harder.
What it means for training: compare heart rate at fixed watts only between like sessions; cap easy work in the heat by heart rate, talk test or RPE, late in a long cool ride by talk test and RPE (SYNTH-38).
What it does not mean: that drift alone diagnoses dehydration or low fitness; +11% vs +2% at 35 vs 22 °C is REF-3496's result in 10 men, not a norm.

Mechanism

After about 10 min heart rate climbs while stroke volume and VO2max fall, most in heat: the same watts become a larger share of a smaller maximum (REF-3495); heat and dehydration can change the heart rate-VO2 link profoundly (REF-1341).
Heat brings hyperthermia, hypohydration and haemodynamic strain (REF-002); cardiovascular strain is the "lynchpin" of a fluid deficit's cost in the warm (REF-062); dehydration lowers muscle and skin perfusion (REF-228).
Whichever cause holds (REF-1340 argues one side), in the heat the same work is a larger share of a smaller capacity (REF-3495).

Common misreading

"Decoupling under 5% means the base is done" (a house cut-off, REF-3495); "big drift means I drank too little" (heat alone did it with fluid replaced, REF-3496).

Backs

SYNTH-38 "Heart rate drifts upward at a fixed workload" · SYNTH-16 "Cardiovascular drift; skin-blood-flow steal" (the steal is one side of a contested cause) · SYNTH-68 "Do not diagnose dehydration from heart-rate drift" (alone)

Sources

Wingo et al. 2012 REF-3495 · Paper ↗
Périard et al. 2021 REF-002 · Paper ↗
Coyle & González-Alonso 2001 REF-1340 (B+) · Paper ↗
REF-1341 (B+)
Souissi et al. 2021 REF-1370 (C+) · Paper ↗