how to retrieve item value prior to an event in the template's trigger

[ali136]

Hello everyone
I need to know the value of item prior to event.
here is my scenario:
lets say I have an alarm system that informs me about events. and I have BRAS for pppoe. whenever more than 50 PPPOEs drop I will have an alarm. so far everything is clear.
what I need is to know ho many PPPOE clients has been dropped
I used to use it in earlier versions of Zabbix like this:

{{HOST.HOST}:{ITEM.KEY}.avg(10m,3m)}

it's still possible to do it for host in v6:

{?avg({host.host}/item.key,1h)}

but I couldn't do it in template(there are more than 200 BRASs) .

Thanks in advance
Ali.