an unstable pulse
unstable
/ˌən-ˈstā-bəl/
adjective
not stable : not firm or fixed : not constant: such as
not steady in action or movement : irregular
wavering in purpose or intent : vacillating
inconstant fickle capricious mercurial unstable mean lacking firmness or steadiness (as in purpose or devotion). inconstant implies an incapacity for steadiness and an inherent tendency to change. fickle suggests unreliability because of perverse changeability and incapacity for steadfastness.
noun
angina pectoris characterized by sudden changes (such as an increase in the severity or length of anginal attacks or a decrease in the exertion required to precipitate an attack) especially when symptoms were previously stable