Yes, there are 81 mg children's aspirin tablets. routinely in the ambulance and ER, adults with MI ("heart attack") symptoms are given four 81 mg tabs to chew, because it hits the system more quickly than 325mg pills in the gut. The aspirin reduces the blood's clotting ability, so that if the MI is being caused by a clot collecting in a cardiac artery, the clot is slowed down and allowed to dissipate. Heart attacks are not only caused by clot collection, but also by spasming arteries or simply congested/plugged up arteries, which all result in diminished blood flow to a portion of the heart, which causes the pain. Aspirin won't do anything for congested or spasming vessels- that's where nitroglycerine comes in, which actually dilates the artery to allow more blood through (the cayenne does the same thing). The pain can come and go with exertion because there may be enough blood getting through to satisfy the myocardium at rest, but as you increase the demands on the heart, the blood supply can't keep up, and the ischemia (oxygen deprivation) occurs, resulting in pain. If ischemia occurs long enough, actual damage occurs, which results in the infarct.
The CPMB enzymes are really only released when heart muscle is damaged, so no amount is good to have. The higher the level, the more the damage.
The pericarditis is definitley painful, but shouldn't result in raised CPMB levels. The heart is surrounded by a sac with a small amount of lubricating fluid inside. Any inflammation in the sac results in acute pain, as the heart is continually rubbing against the fibrous sac, resulting in inflammation. Same principle as an engine, without lubricant, you tear it up.
Just my unsolicited .02