There is a rule about "When an approver approves the request"?

[David Jiménez]

I need to assign the responsible after that the approver approves the request. 

[Amanda Lenczewski]

There isn't a "When approved" rule, however you could use the "Issue Transitioned" trigger and assign the issue when it transitions to the next status after being approved. This can also be done in a post-function on the transition itself.