I wouldn't count on it. During a heart attack, it's the blood flow to the heart muscle that's blocked, but the lungs are still working and oxygenating the blood. So these oxygen meters will show a normal value, since your finger is still receiving the normal amount of oxygen in the blood.

For a pulmonary embolism, on the other hand, I'd expect them to be useful. And always keep in mind that they can't detect carbon monoxide poisoning, even though the blood oxygen is low in that case.