Monitoring if traffic exists in interfaces of appliance fortigate

Hi folks I am Brazilian, sorry for bad English I have an appliance of FortiGate. I need to monitor (with Nagios) two doors that have internet links from different carriers. Must if there is no traffic on one of the interfaces, the nagios returns critical, so I know that the link dropped If posível, give me examples. Remembering that does not help me monitor port status (up or down), because if the internet goes down, the port status is UP, generating a false ok Thank you, already