Merging Electrocardiogram and Electrocardiograms is simple: just change the topic name in the Index panel. The same for ECG and ECGs. Then regenerate the index (all this assuming that you use InDesign's index). But then merging ECG and Electrocardiogram is another matter, that's not trivial and requires a script.
If there aren't too many of these you can the following. In the generated index, starting from this:
ECG 1, 3, 5
Electrocardiogram 2, 4
change that to
ECG see Electrocardiogram
Electrocardiogram 1, 3, 5
Electrocardiogram 2, 4
The us this script: https://creativepro.com/files/kahrel/indesign/index_update.html to merge the two. You can ru the script against the whole index.
It's pssible to do a script that does this this in one go, but as I said, that's not trivial.
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