SDP - Business Rule to trigger when a high priority incident is logged

I have created a business rule to generate an email notification to IT managers when a High Priority incident is created. This works fine if the Incident priority is set to High at creation time. However if the Incident is originally logged at a lower priority and only later escalated to High, the business rule does not trigger.

I have tried setting the business rule to trigger on "Create and Edit" but this generates the notification any time a change is made to the Incident record after its priority is changed to High (Including when it is closed).

Is there any way around this? The only possibility I can think of is to create a Custom Trigger which calls a script to generate the email, but that is a bit clunky.

Thanks,

Roy