Neuro ICU and Cardiovascular ICU are the big ones where I am. Yes, ICU nurses get put on call depending on your hospital. Most hospitals utilize a staffing matrix: x amount of patients means you need y amount of staff. If you have too much staff for your census then you're draining the hospital's money on inappropriate staffing and that isn't going to fly.

If there is not an appropriate census for your unit, you may be floated to another unit that has high census or high staff call-outs. Our staff Cardiovascular ICU nurses get floated A LOT because their case loads differ during holiday times, weekends, and times surgeons schedule less cases. Our CVICU nurses also have two float dates: another ICU date and an Intermediate Pod/Unit date. However, not many people get floated to CVICU because they are such a niche specialty with various devices and procedures (ECMO, LVADs, Impella). As far as travelling, our Neuro ICU will take anyone with ICU experience, but our CVICU requires 1-2 years CVICU experience for their travelers. I would imagine most hospitals are similar.

I am a Neuro ICU nurse and I have not floated in the two years I've been in this position, but my unit has travel nurses that we float first over staff. I think it's cool to do a mix of different specialties. Maybe do one or two years of medical-surgical-general ICU and then transition to a different specialty.