The exhalation valve opens the expiratory flow path in response to the difference signal having a value indicating that the measured tidal volume is equal to or greater than the selected volume limit, as long as the proximal pressure exceeds a selectable reference pressure.
An operator control produces a volume limit signal, indicating a selected volume limit, that is compared with the measured tidal volume signal to yield a difference signal. The flow rate signal is integrated to derive a measured tidal volume signal. The volume-cycle override system includes a flow sensor that outputs an inhalation flow rate signal. A pressure-limited, time-cycled pulmonary ventilator includes a pressurized gas system that supplies respiratory gas to a patient at a selected inspiratory flow rate an exhalation valve that (a) opens an expiratory flow path in response to the elapsing of a selected inspiratory time period, and (b) maintains a selected proximal pressure limit during the inspiratory time period and a volume-cycle override system that actuates the exhalation valve so as to open the expiratory flow path if the patient\s tidal volume reaches or exceeds a selected volume limit during the inspiratory time period.
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