Trigger question.. (Heaptuple struct)

A programming question, forgive me :-)

In a trigger I see

HeapTuple rettuple = NULL;

[...lala]

rettuple = CurrentTriggerData->tg_trigtuple;

oid = rettuple->t_data->t_oid;

..Which get the OID of the column that pulled the trigger...What I need to
do is get another value out of the record that called the trigger. It's in a
field called app_id. Now I'm guessing that more is to be seen in the
rettuple->t_data struct, but I can't find it so any pointers would be great
.. I've looked through the documentation and couldn't find the answer,
forgive me if I've simply been looking with my eyes closed.

Thanks!!!

-Mitch