K3 indicator

From Memory Alpha, the free Star Trek reference

Among the instruments of the 23rd century biobed was the K3 indicator. This instrument, not normally active, measured the level of pain experienced by a patient. While treating Spock for neural parasite infiltration, Doctor McCoy activated this indicator to illustrate to Captain Kirk how much pain the parasite was inflicting. The indicator rose to about 75% of its maximum; McCoy commented that it was "no wonder the poor devils go mad"; implying that this was a considerable degree of misery. (TOS: "Operation -- Annihilate!")