Security let you through, but may not have known what the device was. I'd suggest for the future, if the airline doesn't have an answer if a Defib can go as Carry-On, contact the Federal Aviation Administration, and have a letterhead hard copy of the answer (if yes) to show security, and if need be, the cabin and flight crew.
If you're coming back to the US from a foreign country, I'd further suggest, via that country's embassy here prior to leaving, that you ask them to inquire of their version of the FAA, and request, in both the other country's language and English, a letterhead hard copy.
In general, keep those hardcopy letters saying permission to carry the Defib be kept with your passport.
Of course, all that was just my opinion. It may either not be needed, might not be enough, or will be somewhere in between. I still say ask the FAA, the individual airlines used, the embassy or foreign council's offices, and security working the boarding gates in either direction you're travelling, BEFORE you're scheduled to travel, and carry hardcopy of all affirmative answers with you.
Just mentioning in passing, when NYPD Aviation does medevac flights, they have been known to request the batteries for both the Defib and portable radios, as there is possibility of the helicopter's electronic navigation equipment being interfered with by the medical equipment. I have not heard if they ask for EMS crew's cell-phones for the same reason.