When I worked ICU there were many times I just said, "hey watch out for my patients while I go _______". If there was anything significant I would let the covering nurse know. To be honest any competent ICU nurse should be able to walk into a patient care situation a respond accordingly. the patients are monitored and there are protocols in place.
I did have one instance where I went to the bathroom and had the nurse next to me keep an ear out. I returned to find my patient was out of bed and he had removed all his leads. She and I got him back to bed and then he flipped himself over on his stomach (he was a 380 pound + guy). We thought (crap now we have to flip him back over). My friend looked at me and said, "he isnt breathing". We coded him and he didnt make it, but nothing would have changed no matter who was covering.