The practice of ‘bleed-in’ (or adding in) oxygen directly into a ventilator circuit is considered ‘off-label’ and not endorsed by OCC or by most ventilator manufacturers due to significant potential safety concerns. Anecdotally, the practice is commonly reported in some settings. Safety concerns and considerations include:
Adding flow to the circuit in a volume control mode may cause increased PEEP or inspiratory pressures, alter delivered tidal volume and/or reported tidal volume, and impact sensing/control of pressures (For example at 10 Lpm = 10,000 ml/min –>  167 ml/sec –> so inspiratory time of 1 sec adds 150mL + to set VT). Without precise control of tidal volumes and pressure, there is risk for barotrauma and volutrauma, and delivery of lung protective ventilation will be challenging. Some of these challenges may be mitigated by using only pressure modes of ventilation when using bleed in oxygen supply
Connections to facilitate this setup are often makeshift and may not be secure; inadvertent disconnect from the circuit or from the flowmeter could result in ventilation/oxygenation failure (all ventilator setups should be tested on test lungs prior to use on a patient)
Bleed-in may impact ability to sense the patient’s breath triggering
Bleed-in gas lacks humidity and may dry out secretions further depending on where bleed-in occurs – i.e. bleed-in between the ventilator and the HME or HMEF would in theory mitigate this.
Bleed-in at the outlet of the heated humidifier or outlet of the ventilator may be advantageous over bleed in at the Wye, because during expiration the inspiratory limb can then act like a reservoir filling with gas and helping increase FiO2
Bleeding into the inspiratory limb rather than between the HMEF and ETT also mitigates risk to HCWs should the bleed-in O2 tubing become disconnected
Depending on bias flow and flow rate, during expiration, the gas is flowing out the exhalation valve into the room, and may contribute to wasted gas supply
Introducing a gas source into the circuit may also introduce a new source for infectious particle exposure to the ventilator or the patient (if not passing first through an adequate filter)