A ventilator system comprising: a pressure generating system that generates a flow of breathing gas including a fixed base flow; a ventilation tubing system including a patient interface for connecting the pressure generating system to a patient; an exhalation valve connected to the ventilation tubing system; a plurality of sensors operatively coupled to at least one of the pressure generating system, the patient, and the ventilation tubing system for monitoring inspiratory pressure, inspiratory flow, exhalation pressure, exhalation auxiliary pressure, and exhalation flow to determine a monitored inspiratory pressure, a monitored inspiratory flow, a monitored exhalation pressure, a monitored exhalation auxiliary pressure, and a monitored exhalation flow; an exhalation flow estimation module, the exhalation flow estimation module estimates the exhalation flow based on the monitored exhalation pressure and the monitored exhalation auxiliary pressure to determine an estimated exhalation flow; a driver, the driver controls the exhalation valve to deliver ventilation to the patient based at least on at least one of the monitored exhalation flow, and the estimated exhalation flow determined based on the monitored exhalation pressure and the monitored exhalation auxiliary pressure; a main trigger module, the main trigger module triggers an inspiration based on a first of at least one of the following events to occur: detection of a first trigger condition based at least on the monitored exhalation flow, and expiration of a predetermined amount of exhalation time; a backup trigger module, the backup trigger module triggers the inspiration based on a first of at least one of the following events to occur: detection of a second trigger condition based at least on the estimated exhalation flow, and expiration of the predetermined amount of exhalation time; a controller, the controller determines an absence in the monitored exhalation flow and switches from the main trigger module to the backup trigger module. a pressure generating system that generates a flow of breathing gas including a fixed base flow; a ventilation tubing system including a patient interface for connecting the pressure generating system to a patient; an exhalation valve connected to the ventilation tubing system; a plurality of sensors operatively coupled to at least one of the pressure generating system, the patient, and the ventilation tubing system for monitoring inspiratory pressure, inspiratory flow, exhalation pressure, exhalation auxiliary pressure, and exhalation flow to determine a monitored inspiratory pressure, a monitored inspiratory flow, a monitored exhalation pressure, a monitored exhalation auxiliary pressure, and a monitored exhalation flow; an exhalation flow estimation module, the exhalation flow estimation module estimates the exhalation flow based on the monitored exhalation pressure and the monitored exhalation auxiliary pressure to determine an estimated exhalation flow; a driver, the driver controls the exhalation valve to deliver ventilation to the patient based at least on at least one of the monitored exhalation flow, and the estimated exhalation flow determined based on the monitored exhalation pressure and the monitored exhalation auxiliary pressure; a main trigger module, the main trigger module triggers an inspiration based on a first of at least one of the following events to occur: detection of a first trigger condition based at least on the monitored exhalation flow, and expiration of a predetermined amount of exhalation time; detection of a first trigger condition based at least on the monitored exhalation flow, and expiration of a predetermined amount of exhalation time; a backup trigger module, the backup trigger module triggers the inspiration based on a first of at least one of the following events to occur: detection of a second trigger condition based at least on the estimated exhalation flow, and expiration of the predetermined amount of exhalation time; detection of a second trigger condition based at least on the estimated exhalation flow, and expiration of the predetermined amount of exhalation time; a controller, the controller determines an absence in the monitored exhalation flow and switches from the main trigger module to the backup trigger module.