@ peregrine: I was trying to find something that was more specific than just feed to forestall the argument that feeding doesn't necessarily mean biting.
@ Snark Knight: There is a reason I was equivocal with all the may have's.
The reason I've always thought that it was more than just a spell is that the fire behaves weirdly, and it stops Harry's heart. Then Michael beseeches the Lord on Harry's behalf. Michael says "[Harry] deserves better than to die here, Lord!" Then a little later Harry as narrator says "[a]nd then my heart lurched and began to beat again," which could just be a case of CPR standing for "Clean, Pretty, Reliable."
There is a lot of rules inconsistency in the earlier books. This whole scene may be operating under slightly different rules than later in the series, and that explains the oddness of it. Or there is something more going on.
Furthermore, an otherwise healthy wizard might be able to throw a death curse and have a doctor on standby to get them going again. We don't know and neither do wizards. Luccio thought her wound was fatal, but a doctor who doesn't actually practice medicine, let alone trauma surgery in the field, was able to keep her from dying.