Is providing a temporary drip for dehydration for a 90 year old man with dementia considered to be an extraordinary measure ?

Drip: Not a question that can answered with such minimal information. Age alone isn't an issue as long as he is conscious. We can't judge lifespan although there are concerns over limited resources..Does he have an advanced directive? I recommend a request be made to the institution's medical ethics review committee for help here. There are stakeholders to address and complexities to sort through.

Probably not: In most cases, temporary IV rehydration measures would not be considered "extraordinary." but if the person has previously signed advance directives indicating s/he wants absolutely no iv's in such an instance -- and provided his/her physician with a copy -- the medical team could follow those directives.

A basic thought: You want to avoid doing something that prolongs the process of dying but most want to avoid discomfort during the process. Thirst and air hunger are 2 real sensations most would wish to avoid. IV drips can help avoid this discomfort. The final decision rests on deciding if the patient can "feel" discomfort, if so it is not extraordinary. If not it prolongs the process without benefit.