PHYS-28 · Principle

Why is my heart rate 15 beats higher in the heat at the same pace?

Blood flow to hot skin and sweating strain the heart and heat shrinks your aerobic reserve, so the same pace is a harder effort, not noise or lost fitness.

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

established physiologyApplies to everyoneEstablished — heat raises heart rate and lowers VO2max and aerobic performance at the same work, more with fluid loss; humid heat cuts evaporative cooling

Practical Application

Why is my heart rate 15 beats higher in the heat at the same pace? Blood flow to hot skin and sweating strain the heart and heat shrinks your aerobic reserve, so the same pace is a harder effort, not noise or lost fitness.
What it means for training: cap easy work by heart rate, talk test or RPE and let pace fall; key race-pace work stays on power, with heart rate read, not obeyed (SYNTH-38); acclimate before a hot race (REF-061, SYNTH-04).
What it does not mean: that a hot session's heart rate is noise to correct away, or that a skin-sensor core temperature can gate the session (SYNTH-91).

Mechanism

Supporting high skin blood flow and sweating strains the heart, and heat, above all hot skin, shrinks cardiovascular and aerobic reserve and raises relative intensity (REF-1352); hyperthermia, hypohydration and haemodynamic strain impair aerobic performance (REF-002); drift grows and VO2max falls at the same watts (PHYS-11, REF-3496).
Fluid loss compounds heat by lowering muscle and skin perfusion (REF-228); a deficit costs more the warmer the skin (REF-062).
At the same WBGT, humid heat cut evaporative cooling and work done (REF-711); the luteal phase raises core temperature, not exercise heart rate (REF-709).

Common misreading

"The watts are the watts": in heat the same watts are a larger share of a smaller maximum.
"The skin steals the muscles' blood": the reviews here give the strain and the smaller reserve, not a split of cardiac output.

Backs

SYNTH-38 "in the heat the rise is real strain, not noise" · SYNTH-16 "Ambient temperature" row · SYNTH-39 "The same watts genuinely cost more in the heat"

Sources

Sawka et al. 2011 REF-1352 (B+) · Paper ↗
Périard et al. 2021 REF-002 · Paper ↗
Racinais et al. 2015 consensus REF-061 · Paper ↗