What types of heart diseases can an ECG detect?

EKG just records the electrical activity of the heart. Any type of heart disease can cause an abnormality of the EKG if it has affected the generation or conduction of electrical impulses in the heart. But the same type of heart disease need not always affect the ECG. For example, there can be significant narrowing of one or more blood vessels supplying the heart without any change in the EKG. Sometimes a stress EKG will bring out the changes in those cases. Even a stress EKG can be negative in rare instances and we need a coronary angiogram to locate the narrowing of the blood vessels, in a person with typical symptoms. EKG is only one tool for evaluation of the heart among the so many available. It has its own advantages and limitations. Most important advantages are the relatively lower cost and ubiquitous availability.

Related article