Why is electrocardiogram abbreviated EKG instead of ECG?

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".