You've set yourself a huge task here, guest! A quick google search on "English language medical journals" returned 970,000 hits!
You might try contacting the British Library, which might be able to tell you about BrE Journals, and AmE journals published in UK
http://www.bl.uk/
There is probably an equivalent organistation in US, but i don't know what it is. Probably Library of Congress.
http://www.loc.gov/rr/
If you can get a list of all the different specialities and colleges (eg College of Surgeons etc.) they might be able to help you with titles / numbers of their specialist journals. Every specialty has
at least one journal.
Good luck!!