Anesthesiology is sort of a mixed bag on the PGY1 or CBY year - both terms used interchangably. You will find that approx 25% of programs will provide the CBY year as part of their program - a 4 year gas program. Another 25% will not offer anything but PGY2 slots, where you must secure your own prelim year. Finally, about 50% of programs will offer some slots that include the CBY & others that do not. So, the first thing you will need to learn about programs as you begin to compose you list of programs to research & consider is how many CBY slots and how many PGY slots they have.
Now, all gas programs will accept either 1 year of surgery or 1 year of IM or 1 year of an AOA internship (for the DOs-to-be in the crowd) as your prelim year. I came across a few programs who would accept essentially the first year from essentially anything other than psych or path as a prelim year...but those appeared to be the exception based upon my experience.
I would be wary of many surgical prelims. I interviewed at several places where I checked out the inside poop from folks who had done a surgical prelim. Frequently, as a prelim, you are the bitch-boy doing rectals, scut, notes, admissions & rarely, if ever, seeing the inside of the OR. However, there are some surg programs who flatly state that they do not make any differences. I am doing a surgical internship at Dartmouth, before I start gas there. During my surg interview, their position on prelims was flatly stated to be that they made no difference, the rotations were the same, the expectations were the same and that you would all have comparable quantities of time in the OR. I spoke with a couple of residents who independently confirmed that this was a reality.
IM programs - what can I say? Pre-pre-round, to pre-round to round, re-round and then round again!!! Sprinkle a couple of conferences & lectures in there....with a smattering of patient care...and you have a day in the life of an IM prelim!!!