I work night shift in a 16 bed Surgical-Trauma ICU. We have a couple of techs (usually EMTs) that really watch the patients while the charge nurse rounds on them frequently. The techs know to notify the charge nurse or us (our breakroom is in the icu) for any acute changes. If it's a sick patient, the charge nurse will watch them. We usually have 8 RNs plus 1 charge, plus two techs. There is no definitive policy. The charge nurse just uses his or her judgement.