Referencing columns in a record (or dereferencing variables)

In plpgsql, is it possible to indirectly reference a column in a record
object?

For instance, in a trigger function I've got a variable, v_column_name
containing (predictably) a column name in existing in OLD and NEW.  I
want to reference that column.  This of course doesn't work:

    IF OLD.v_column_name != NEW.v_column_name THEN
    ..

Is this possible in plpgsql?  Is there a way to deference v_column_name?