Why is it called ekg and not ecg?

Answer 1

because in Germany they called it an electrokardiogram. and EKG stuck. but cardio is greek deriviative and in English we say ECG... but its still easier and rolls off the tongue better as EKG so it has stuck.

Answer 2

The reason is that ECG sounds very similar to EEG, which is the abbreviation for electroencephalogram, a test that measures electrical activity in the brain. In order to avoid confusion, it became a convention to use the abbreviation for the German spelling elektrokardiogramm-for the heart test, which is why it is commonly called an EKG.