Think about this. When you ask an HIE for a document for a patient, are you trying to make a business (in this case, a health related care) decision? Yep. Is your use of this information part of the day-to-day operations where you need transactional controls? Probably not, though you might think you want up to the minute data.
Arguably, HIEs aren’t in the business of providing “up to the minute data”. Instead, they are in the business of providing “most recent” data within a certain reasonable time frame. So, if the data is basically available, and eventually consistent within say, an hour of being changed, that’s probably sufficient. This is BASE: Basically Available, Soft (possibly changing) state, with Eventual consistency.
On the other hand, when you use an EHR, do you need transactional controls? Probably, because at the very least you want two people who are looking at the same record in a care setting to be aware of the most current state of the data. In this case, you need Atomic, Consistent, isolated as well as Durably persisted changes. This is ACID.
BASE scales well in the cloud with NoSQL architectures. ACID not so much. There are a lot of good articles on the web describing the differences between ACID and BASE (this is a pretty basic one), but you can find many more. If you haven’t spent any time in this space, it’s worth digging around.