CCD and HL7 V3 / V2

Question 1

If you're going to be an HIE, you're going to need to handle both HL7 v2 and v3. My experience is that almost all "real life" data transmission is v2 (I think this is probably because most systems capable of v3 transmission also allow v2 formatting, whereas the older systems cannot handle v3, but this is just speculation). There's a very superficial overview of the core differences between v2 and v3 here, which talks a bit about what fundamental changes are being made besides format (delimited vs xml)

The second part of your questions doesn't really have a definitive answer, and ties in with your fourth question. CCD can be HL7v2 compatible, but it's a lot more difficult and relies on the sender and receiver both having a common understanding on a number of issues (hl7v2 is a very loose "standard"). Here's HL7standard.com's thoughts on the subject:

The short answer is "yes" with the long answer being "it depends."

There are different levels of CDA/CCD documents. It probably would be possible to create a Level-1 CDA document from an HL7 v2 message, assuming you had a fairly robust HL7 v2 report of some flavor (MDM or ORU type of message most likely).

It would be harder to take an HL7 v2 message and convert it into a Level-2 or Level-3 CDA document. More data is required in those formats, and that data needs to be structured, not just free form text.

The CDA is based on the HL7 v3 RIM, and compliant CDA documents will follow that data model. Mapping from one data model to another can be challenging.

All CDA documents need to have a responsible party or person who 'signs' the document. At a minimum, the HL7 v2 message would have to have this information.

Converting a CDA document into an HL7 v2 message is possible, but you run the risk of losing the 'context' of the data that is in the original document if it is a highly structured Level-2 or Level-3 document. If the CDA contains nothing more than a textual report and some patient demographic information, then it can be converted into a comparable v2 message.

For getting a full list of fields, I'd just purchase the full HL7 standard... and maybe have a look over at bluebuttonplus.org... though keep in mind that v2 can result in some confusion, as message sub-segments aren't always consistently used between different actors. Anyway, I don't think that there are clean answers to most of what you're asking, but at the very least you will really need to handle both formats. I'd also throw in that you may want to look at a product like Mirth or InterfaceWare to handle some of your parsing and testing.

Question 2

You'll definitely want to take a look at HL7.org. The HL7 standards are now available for free, and a large part of it is licensed at no cost. As far as HL7 version, 2.5.1 seems to be the standard for the v2 space. As stated in the other answer, you'll want to support both V2 and V3 HL7.

If you are looking in to sending the CCD, looking at the same HL7 site search for QRDA, the Quality Document Reporting Architecture, an XML HL7 standard for the transmission of the CDA.

Two other notes on this, you'll want to look at the emerging discussions around QRDA I and QRDA III that are taking place now with ONC, and the revised standards for the CCDA.

Here's a direct link to the, I believe, most recent CDA standard on HL7.org HL7/ASTM Implementation Guide for CDA Release 2 -Continuity of Care Document (CCD®) Release 1

Finally, you will need to have conversations with the major EHR players such as Epic, Cerner and Meditech. They'll need to be on board with your solution and can guide you through the implementation process. In addition to that, they will need to know how best to set up their systems to talk to yours so they can guide their clients. Without this I'm afraid you'll get no traction.