Yes, "ECG" is an acceptable abbreviation in APA format as long as it is defined in full the first time it is used in the text. Subsequently, you can use the abbreviation throughout the rest of the document.
Sinus rhythm .
the abbreviation for Electrocardiogram is either EKG or ECG
ECG (electrocardiogram). The initialism "EKG" was introduced because "ECG" was too easily confused with "EEG" (electroencephalogram), when interpreting verbal orders.
FOUR
It stands for "electrocardiogram", a basic read-out of the function of the heart muscle.
LEAD DISPLACEMENT
Either way is fine.
ST elevation
The wave direction indicated whether the electrical impulse from the heart is going towards or away from the ECG lead that is being studied.
EEG is used for the electroencephalogram. It is sometimes confused for the one used for the heart called the EKG (ECG) electrocardiogram.
The abbreviation for the word,"VERSION" is V.