I've been keeping an eye on this eBay computer-based EKG, 1/3 the cost of Midmark... it's a "Buy it Now" for less than $800 (including shipping):
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=220403883499&_trkparms=tab%3DWatching
AC interface? Don't know...
Al
