A heart rate was a term used to refer to the number of heartbeats in a humanoid.
In 2371, during The Doctor's delusion on the USS Voyager's holodeck, he scanned himself and discovered he had a heart rate. (VOY: "Projections")
In 2373, after scanning Lieutenant Tuvok in sickbay, The Doctor noted that his heart rate was at three hundred beats per minute after he was found to be suffering from a severe panic attack on the USS Voyager. It was recalling an apparent memory of a girl falling from a precipice triggered his accelerated heart rate. Tuvok's condition was later discovered to be caused by a memory virus unknowingly placed into his brain by his former crewmate on the USS Excelsior, Lieutenant Commander Dimitri Valtane shortly before he apparently died in 2293. (VOY: "Flashback")