The phrase "heart racing" means to feel one's heart beating very fast, as if it is pounding or thumping in their chest. It can also refer to feeling a strong sense of excitement, anxiety, fear or passion that causes the heart to beat rapidly.
Usage examples
I felt my heart racing as I nervously approached the stage to give a presentation.
The intense workout had my heart racing and adrenaline pumping.
The suspenseful plot of the movie kept my heart racing throughout.
Running late for an important meeting, I could feel my heart racing as I rushed to catch the train.
The close match had my heart racing, as I cheered on my favorite team.
During the horror movie, the unexpected jump scares had my heart racing in fear.