Heat Stress is a trade-off gear perk that greatly improves blast defense but slightly lowers your stamina capacity. It is for fights full of grenades, rockets and explosive ammo.
Explosions hurt you much less, but you can sprint a little less before running out of stamina.
Good against rocket-heavy enemies and helicopters. Pick Major Blast Protection gear instead if you have a free slot for it.
Marathon Runner or Large Catch Your Breath offset the stamina loss.