The term "silent myocardial infarction" refers to a painless (or almost painless) heart attack, as opposed to the classic extreme central chest 'heaviness'. It is actually frighteningly common: as much as 25% of myocardial infarctions (MIs) may be 'silent'. This is why you should do an E.C.G. (E.K.G.) and pull blood for cardiac markers if you are even slightly concerned about an MI.
'Silent' MIs are typically found in three groups of patients: the elderly, diabetics, and heart transplant patients. In the latter case, it is easy to discern the pathogenesis: the nerves from the donor's heart are simply never properly connected to the recipient. However, in the case of the elderly or diabetics, the pathogenesis is controversial. One influential theory favours an autonomic neuropathy secondary to diabetes, although other authors even contend that the difference is a statistical artifact - there are more heart attacks in diabetes anyway, so there are more 'silent' MIs too, without there being an increased incidence of them.
I'll leave you with a case that I saw today. A 58 year old diabetic and hypertensive woman developed sudden shortness of breath, and on examination wheezes were prevalent bilaterally. Had she been a chronic smoker, I may have put this down to a COPD exacerbation, but she hadn't so much as touched a cigarette in her life, and nor did she have a history of asthma. There were no overt signs of cardiac failure, but both the ECG and troponin-T revealed her to have had an MI. The wheezes were evidently from mild left-sided cardiac failure, secondary to her MI. Makes you think, doesn't it?