A Declare trigger rule does exactly this (call an activity on database update), however the update must be originated by Pega for this to trigger. You need to provide more details for your use case to obtain a suitable solution.
I think you can implement DB trigger on external DB side. That trigger should populate queue. You can implement your own queue or try to use some common used queue like JMS. Inside pega you will have advanced agent that check this queue and set queue item to handle for another Standard agent. Think such approach will be most safe.