An ECG was first invented in London. It was first shown to be practicable at the Middlesex Hospital, although its discovery is often attributed to the Dutch (Einthoven).
It records the electrical activity of the heart and, therefore, has only a little to do with the heart muscle function and the flow down coronary arteries. Only when these are significantly deranged will the ECG become abnormal.
If the plumbing in a house goes really wrong the electrics might eventually become affected! Patients and many doctors have an undue faith in the ECG.
The problem is that it can look normal, even in the presence of heart disease (a so - called false negative result) and falsely abnormal when there is nothing wrong (a false positive result).