Because the ventilator’s driving pressure (65-75 cm H 2O) prevents the expansion of the bellows until breathing system pressures overcome the driving pressure, activation of the flush valve at this time (eg, to reinflate the bellows after a brief disconnection) would direct all the volume and resulting pressure to the breathing circuit. During the inspiratory phase, the ventilator’s driving pressure actively compresses the bellows to deliver a breath, and the ventilator’s exhaust valve is closed. Similarly, if the oxygen flush valve is used during the inspiratory phase of mechanical ventilation, the patient’s lungs may be exposed to excessive pressure and overdistension. Therefore, use of the flush valve while a patient is connected to a nonrebreathing system transmits excess volume and pressure directly to the patient’s airway and lungs. 3,4 A nonrebreathing system (eg, Bain breathing system) has a relatively small inner volume and little compliance. The oxygen flush valve allows oxygen at high pressure and volume into the breathing system (35-70 L/min with a pressure of 45-60 pounds per square inch gauge, which becomes approximately 1 L/s into the breathing system). Excessive inflow can occur from improper use of the oxygen flush valve, aggressive ventilator settings (high airway pressures and tidal volumes), and/or inappropriate connection of oxygen tubing (meant for oxygen insufflation via open mask) to a cuffed endotracheal tube, laryngeal mask airway, or other airway device without the ability to allow excess gas to vent.
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