rlake,

Thanks for your question.  For the most part Shamus is correct.  Doctors consider a lesion of 70% or more to be significant.  But this varies depending on the location of the blockage, the length of the occlusion, the type of vessel involved, and whether this blockage is causing the patient symptoms.  If the interventional cardiologist is convinced that a 60 % occlusion is definitely causing symptoms, and the plaque is rather long, in a narrow vessel that is supplying an area that is not well perfused by other vessels, he may very well attempt to open this vessel and then place a stent, often a drug eluding stent that is impregnated with medication to help prevent re-occlusion. 

Again, decisions to place a stent are not always clear cut.  Each patient's situation is unique and presents many different variables.

I hope this has been helpful.

Martin Cane, M.D.