Why is electrocardiogram abbreviated EKG instead of ECG?

There is no K in electrocardiogram. So why is it often referred to as EKG instead of ECG?

(ECG is an acronym for electrocardiogram, and it appears to be used twice as often in English literature as EKG. The question is why EKG is ever used at all.)

The problem with using "ECG" for electrocardiogram is that it sounds very similar to "EEG," which is the abbreviation for electroencephalogram. The former is a graph of heart activity; the latter graphs brain activity.

Then there's the potential acronym collision with the other more recent heart scan: the echocardiogram. (To avoid confusion that one is never referred to by an acronym at all; it is instead shortened to "echo.")

In order to avoid confusing ECGs with EEGs and echoes we often use the abbreviation for the German spelling of electrocardiogram: Elektrokardiogramm.

(Sources for this explanation are plentiful.)

Addendum: This seems to be a practice particular to American English. In British English "EKG" is used only about 5% as often as "ECG".

Perhaps because Einthoven, who developed the first practical ECG, spoke Dutch. In 1924 he was awarded the Nobel Prize in Medicine for his design.