A major heart attack would always show up on ekg ?

Never say always : Major heart attacks are the result of either occlusion of blood flow to key areas of the heart or serious electrical dysfunction creating an unstable heart rhythm. While there may be some occult or exceptional cases of sudden death that may not have time to register on an ekg, most, if not all, major heart attacks would be apparent on an ekg.

Not always: For example, with balanced ischemia, the changes in the ekg for one area are balanced by changes in another. But as a general rule, major mi does have significant changes.