Gas mixing apparatus for a ventilator

A method for mixing gas for a ventilator system for supplying mixed gas to a patient airway during an inspiratory portion of a breath cycle, said ventilator system including a pump chamber having a gas delivery portion for receiving mixed gas, a moveable gas displacement member disposed within said pump chamber, means for moving said gas displacement member between extended and retracted positions, gas mixing apparatus for mixing a first selected gas with a second selected gas, a source of said first selected gas, a valve in fluid communication with said source of said first selected gas and said gas mixing apparatus for regulating flow of said first selected gas to said gas mixing apparatus, and control means for controlling said valve to admit said first selected gas to said gas mixing apparatus during at least one interval during at least a portion of the breath cycle, the method comprising the steps of:

• providing a source for the first selected gas to the gas mixing means;
• providing a source for the second selected gas at ambient atmospheric pressure;
• mixing said first selected gas with said second selected gas in the gas mixing apparatus to provide mixed gas for the pump chamber;
• extending the gas displacement member to deliver a flow of mixed gas from the gas delivery portion of the pump chamber to the patient airway;
• retracting the gas displacement member to draw mixed gas from the gas mixing means into the gas delivery portion of the pump chamber;
• and
• admitting the supply flow of the first selected gas to the gas mixing apparatus at a maximum mass flow rate during at least one interval of time during the breath cycle to obtain a predetermined proportion of said first selected gas in the mixed gas.