How to handle shipment block reason change in a delivery note?

Hi Experts,

   Whenever shipment block reason is changed and save button is pressed I want to trigger a PI interface. The field is TRSPG in shipment screen. tcode VL02N (update DN)

   In the interface I want to capture the updated shipment reason and send it to external system.

   The problem is, in ECC how to handle this event? Is there any user exit? In the user exit how do I know that shipment block reason is the field that was changed?

   Please help.

Thanks

Gopal