tPa is used to dissolve clots only in those who meet certain criteria. first of all, if the hospital has a cardiac catheterization lab on staff who can perform percutaneous transluminal coronary angioplasty, that should be the choice over tPa (so says the latest clinical studies after comparing tpa vs ptca and mortality). tPa is associated with negative side-effects. In order to even be considered for treatment with tPa's, you have to meet certain criteria such an indication of a bundle branch block and history suggesting acute MI, and an ECG finding that is very indicative of an MI (ST elevation for those that care). Fibrinolytics (Thrombolytics) are used not only for acute MI, but also in some instances for established, serious pulmonary embolism and deep vein thromoses.
Also, ischemia is not massive cell death (as gleevec indicated above). Ischemia is descreased oxygen supply to tissue along with a decrease in clearance of metabolites from tissue. this is usually due to poor perfusion. Ischemia can be the result of angina, either due to stable plaque formation, unstable plaque ruptures, vasospasm, etc. A myocardial infarct is a heart attack, which is death of cardiac tissue. Ischemia usually precedes an infarct, though the duration and severity of ischemia can vary. Sometimes they are difficult to differentiate (i.e. unstable angina versus an infarct), but usually lab tests are ordered that help to narrow down your differential (there may also be distinctive changes in the ECG).
tPa therapy was accurately described above, it just helps convert plasminogen to plasmin, which will degrade fibrin scaffolds and help to decrease clot formation. the recombinant tPa's a bit more specific compared to strep, for they also bind to fibrin (which aids in selective activation of plasmin only in (or most likely in)areas where there are clots).
Sorry for this long winded reply, but we are neck-deep in our cardiovascular block and all of these thoughts are floating around my mind. Selfishly, I figured that an attempt at gathering my thoughts would be helpful to not only you, but also to me.
