Use one of these SNMP probes, which allow the user to specify the critical interface using interface ifIndex from the Interfaces window:

SNMP - High Traffic
SNMP - High Utilization

The probes tab in the Info window can be used to add the probe. When the interface status changes from "Up" to anything else, the device status will automatically be changed to "Down".

Use case is monitoring for QOS compliance, alerting to network connection failure, etc.