Hello, @AlessandroCarotenuto
You pick one great topic for your Master’s thesis. We would really like to hear about it as soon as it’s done, because biomedical ECG signal records are not easy to tackle 
. Check this and this topics (there are more).
Regarding UMLS - you need UMLS credentials to work with CPT4 vocabulary. Most vocabularies are available for free without any license. So in your case maybe you don’t even need UMLS. Once again - maybe.
You don’t need to repeat the whole process of vocabulary creation on your side. Vocabularies can be dowloaded from Athena. If I were you, I would start with the Book of OHDSI. I put a link to ETL chapter, but please don’t skip the first chapters, they are very important and interesting to read.