Absolute

A recent significant change in the resting ECG suggesting significant ischemia, recent myocardial  infarction (within 2 days) or other cardiac event

Unstable coronary heart disease

Uncontrolled cardiac arrhythmias causing symptoms or hemodynamic compromise

Severe symptomatic aortic stenosis

Uncontrolled symptomatic heart failure

Acute pulmonary embolus

Aacute myocarditis or pericarditis

Suspected or known dissecting aneurysm

Acute infection

Relative

Left main coronary stenosis

Moderate valvular stenosis

Electrolyte abnormalities (hypokalemia, hypomagnesemia)

Systolic blood pressure at rest over 200 mm Hg, diastolic over 120 mm Hg

Tachyarrhythmias or bradyarythmias

Hypertrophic cardiomyopathy or other forms of outflow tract obstruction

Neuromuscular, musculoskeletal, or rheumatic disorders that are exacerbated by exercise

High degree atrioventricular block

Ventricular aneurysm

Uncontrolled metabolic disease (diabetes, thyrotoxicosis, myxedema)