When I questioned why I had not been offered another cardioversion after two previous ones had lasted over a year until another medical procedure had disturbed my vagus nerve. I was told that the echo had shown that my right atrium size showed it as unlikely that the cardioversion would be successful. It can be either atrium that is the culprit. However if they first do a cryoablation it will change the size of the right atrium and they can then do a cardioversion.
There are quite a few online studies on this.
sciencedirect.com/science/a...
ncbi.nlm.nih.gov/pubmed/901...
cardiovcl.sld.cu/corsalud/2...
circ.ahajournals.org/conten...
jcmr-online.biomedcentral.c...