Thats a great question, and can be pretty easily broken down.
You could say, once lacking the equipment or skills to treat- they need to see someone of higher level of care/ not field treatable. That will change w/ each person, maybe they are dehydrated and just need an IV, an EMT basic could not do that, but an advanced, or paramedic could.- for example. Just knowing the limits is good. Your limits, and the limits of others, limits of equipment on hand, transport times, and communication. Availability of an EVAC.
We also have to recognize that there are really three separate conditions being mentioned above. And all are treated differently, and varies based on the severity.
We have Dehydration: Pretty easily treated in the field - water, fluids, IV, etc some sports drink, that diluted. at least 50/50 with water.
We have; Heat stroke, when dehydration is severe enough it causes altered mental status, and an overheating in the body. Multiple problems here; PT needing to be actively cooled down, AC in truck, ice packs to groin, and armpits. As well as treated for the dehydration. remembering ice cold fluid fast will cause them to vomit. ambient temp fluid, sipped slowly.
We have hyponatremia: Honestly, not very field recoverable w/o advanced intervention.
Not feeling better after hydration and 30mins of shady rest, closer to heat stroke, or hyponatremia. Get em out
Not holding fluid/food down' ( unless its due to fast intake ) get em out.
With any of them, or anything for that mater; Altered Mental Status- needs to leave the field to be evaluated and get treatment- ASAP. This is late sign, getting dangerously close to loosing your PT.
JGAZ, thats a good chart! I take almost all my notes that way now.
Not a super clear answer, i know. Its a muddy water kind of thing, with numerable external factors, that subjectivity change things.