The sub-ambient pressure alarm is intended to identify a breathing system pressure of <-10 cmH2O. Common causes of negative pressure include spontaneous respiration by the patient with an obstruction in the circuit, scavenging system problems, or application of a suction device into the circuit (suction catheter in the endotracheal tube). This alarm would function if an OG or NG tube was mistakenly placed in the trachea and put to suction.
In terms of scavenging systems, in active disposal systems a vacuum is applied to move waste gas from the circuit to the scavenger. Both excessive scavenger flow (via the vacuum) and a valve stuck in the “open” position (resulting in the vacuum from entering the breathing circuit) may result in sub-ambient pressure in the breathing circuit.