Why do they just ecg when u r at emergency if ecg just shows limited information?had chest pain and they say they can c if its somethin acute on ecg?

ECG is good: You are right, all tests are limited, neither 100% sensitive (fail to show the disease) or specific ( show abnormality without disease present). One has to remember " first do no harm" principle, ECG is non-invasive (does not hurt patient), yet quite capable to document changes that may accompany sizable heart attack / coronary ischemia.

ECG yields good info: It is true that the ECG can not tell you all. But it does yield a lot of great information. Heart arrhythmias, conduction abnormalities, clues to electrolyte imbalances, and the vast majority of acute cardiac ischemia will show up . A normal ECG in combination with the clinical scenario goes a long way in ruling out many conditions. While reassuring that you have a normal ecg, u may need more..