Whenever the player moves around quickly (via holding of [SHIFT] / clicking (L3)), the heart rate will increase. If the player remains still or moves slowly, the heart rate will return to a normal level. The current heart rate is indicated by a dynamic icon in the lower righthand corner of the HUD.
An elevated heart rate will increase aim wobble (the standard movement of the iron cross). This aim wobble can temporarily be steadied via breath holding (via holding of [SHIFT] / clicking (L3)), but this will cause the heart rate to rise. At a certain point, the breath can no longer be hold and the steady aim will be lost.