First of all, we have our ICN separate from our ICU. ICN babies can be on a NC or have an IV, but the IV can't be going at a fast rate and they can't be having excessive A/B's. So there would never be a situation where you would have 3 or 4 ICN babies AND be up for admit.
When we're short-staffed, we usually end up putting as many ICN babies as possible in 4 baby assignments. All ICU babies are in 2 baby assignments, and the nurse may still be up for admit as long as the 2 babies are not too heavy. They will not put a nurse with even one vent up for admit, NCPAP is okay though. We all know that sometimes a 2 vent assignment is more stable and easier than a NCPAP assignment or an admit, so we all pitch in to help our busier coworkers. The charge nurse may take an assignment, but not always since she is probably frantically calling staffing for more nurses! We also use registry nurses and we may get a nurse or two floated down form PP to work with ICN babies if PP isn't too busy (they usually are, though). The staffing office will then go about calling everyone at home... if we are really in deep, they will be authorized to offer crisis pay (DT), which usually gets nurses to come in. We don't have mandatory OT.
If they still don't have enough staffing and we're getting close to a shift change (most of use do 12hrs, but there's a few 8hr people), or we have nurses in truly unsafe assignments (a 1:1 baby in a 2 baby assignment for instance), they will call the manager and all the leads and ask them to come in. If it's day shift, they are usually already there and will come and help (most of them keep a set of scrubs
with them for just this reason!). This can get us an extra 3 or 4 nurses. If our unit is close to full we will close to outside admits, but if our problem is merely not having enough nurses, we don't generally close. But usually not enough nurses=unit is hoppin' full!