A respiratory monitoring system includes (10) a mechanical ventilator (12) configurable to perform a ventilation mode in that includes an inhalation gas delivery phase and provides extrinsic positive end-expiratory pressure (PEEP) at a PEEP level during an exhalation phase. A breathing tubing circuit (26) includes: a patient port (28), a gas inlet line (16) connected to supply gas from the mechanical ventilator to the patient port, a gas flow meter (30) connected to measure gas flow into lungs of the patient, and a pressure sensor (32) connected to measure gas pressure at the patient port. At least one processor (38) is programmed to: compute a proxy pressure comprising an integral of gas flow into the lungs measured by the gas flow meter； and trigger the inhalation gas delivery phase of the ventilation mode when the proxy pressure decreases below a trigger pressure threshold.
