You need to define it with first use, and then you may use the abbreviation throughout the rest of the paper i.e. electrocardiogram (ECG). This is due to the rule that you cannot assume that your audience will inherently know what ECG means.
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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.