Restricting outbound Idocs only for specific field changes of PO

I have a requirement in which IDOC for message type - ORDERS &  basic type ORDERS05 ( with an ehancement ) needs to be sent. For this a custom output type has also been setup.

This IDOC needs to be sent only when a new PO is created or some specific fields ( article, quantity, unit of measure, delivery completion ) of a old PO have been changed.

What steps needs to be followed to meet requirement of sending IDOC only on some specific field change of PO ?

Thanks