Why unstable angina is a contraindication for stress test?

Unstable angina: A stress test forces the heart to work more and it needs more oxygen.Blood vessels dilate to allow more flow . In unstable angina the vessels have limited capacity to dilate in response to the increase need for oxygen so you can precipitate a low oxygen state, muscle wall ischemia/death , i.e.a heart attack. In angina you do a catheterization and if indicated stent or CABG

Test stop with pain: I presume you are scheduled for an Exercise Stress Test or a Treadmill Test, where an EKG is performed while you walk on a treadmill. In this stress test, the test must be stopped as soon as you experience chest pain. Therefore, the test is not useful (and thus contraindicated) in patients with unstable angina. Less invasive tests such as a nuclear medicine scan may be used instead.