The ability to control inspired oxygen concentration allows a balance to be struck between the patient's oxygen requirements and gas consumption, and the adverse effects of oxygen. A Venturi within the ventilator can be used to entrain ambient air into a high-pressure oxygen supply. The ventiPAC offers inspired oxygen concentrations of 100 or 45% depending on whether the entrainment port is switched off or on to provide an 'air mix'. The Oxylog 3000 adjusts the entrainment ratio using solenoid valves to provide a continuous range of oxygen concentration. In the absence of a high-pressure oxygen supply, the LTV-1000 uses a different approach. Oxygen supply pressure is measured and a system of valves blends oxygen proportionally with ambient air into a reservoir chamber for delivery to the patient.
Crucially, the inspired oxygen concentration displayed by most portable ventilators is that selected by the operator and not a measured value.