You could configure Escalation Rules when an agent has not sent a first response within a set time. This could be done under the Admin-->SLA Policies-->Edit-->"What happens when this SLA is violated?"-->"Set escalation rule when a ticket is not responded to on time".

You could also configure a Supervisor Rule(under Admin-->Supervisor-->New Rule). A sample rule would be

Conditions: Hours since First Response Due

Actions: Send email to agent-->Escalation Agent Name