APA style prefers full words, especially in the first mention.
Here's why and what you should use:
* Full words are clearer: "Electrocardiogram" is more easily understood by readers than "ECG."
* Consistency: APA emphasizes consistency. Once you use the full term, you can then abbreviate it as "ECG" in subsequent uses *within the same document.*
Example:
"The electrocardiogram (ECG) showed abnormalities in the patient's heart rhythm."
Important Note: Always check specific journal guidelines as they may have their own preferred abbreviations.