This is the American College of Sports Medicine (ACSM) guidelines for terminating a Fitness Test

ACSM Indications For Termination of an Exercise Test
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Absolute Indications

Suspicion of a myocardial infarction or acute myocardial infarction (heart
attack)
Onset of moderate-to-severe angina (chest pain)
Drop in systolic blood pressure (SBP) below standing resting pressure or drop in SBP with increasing workload accompanied by signs or symptoms
Signs of poor perfusion (circulation or blood flow), including pallor (pale appearance to the skin), cyanosis (bluish discoloration), or cold and clammy skin
Severe or unusual shortness of breath
CNS (central nervous system) symptoms
e.g., ataxia (failure of muscular coordination), vertigo (An illusion of dizzying movement), visual or gait (pattern of walking or running) problems, confusion)
Serious arrhythmias (abnormal heart rhythms)
e.g.: second / third degree AV block, atrial fibrillation with fast ventricular response, increasing premature ventricular contractions or sustained ventricular tachycardia)
Technical inability to monitor the ECG
Patient's request (to stop)

Relative Indications

Any chest pain that is increasing
Physical or verbal manifestations of shortness of breath of severe fatigue
Wheezing
Leg cramps or intermittent claudication Hypertensive response
Pronounced ECG changes from baseline elevation
Exercise-induced bundle branch block that cannot be distinguished from ventricular tachycardia
Less serious arrhythmias such as supraventricular tachycardia

1995 ACSM's "Guidelines for Exercise Testing/Prescription"