This is a trueism of Bag Mask Ventilation: Mechanical ventilation - Exhalation in mechanical ventilation is almost always completely passive. The ventilator's expiratory valve is opened, the bag re-expands, the elastic recoil of the bag allows for passive exhalation of the lungs. The expiratory flow is allowed until the baseline pressure (PEEP) is reached on this closed circuit, especially if a PEEP valve has been placed on the BVM or already part of it's construction. Expiratory flow is determined by patient factors such as compliance and resistance.
What if the Bag were changed to a pump or something that looks like a bellow that blew air onto a fire as in a furnace? As you squeeze down, you push air out almost like when you squeeze on the bag of a BVM, but you can also expedite exhalation, pulling apart the handles of the same billow. Negative Breath, actively pulling air out of the tip of the nozzle. Negative Pressure Ventilation a.k.a. negative breaths.
Hence you can be more active on treating patients with hypercarbia close to respiratory failure or already in respiratory arrest. But to what dangers. The following dangers are with Positive pressure ventilation
1. You can over ventilate and overinflate the alveoli with too much peep via positive pressure ventilation
2. Give discoordinated breaths by bagging while the patient is blowing out air or actively taking a breath.
One can imagine the problems with over-exhaling in general and at the alveoli level.
1. Can be very uncomfortable that the device being used is exhaling more than you are ready to exhale for a conscious patient or patient who is paralyzed.
2. Can you overly collapse the recently recruited and opened alveoli from positive pressure ventilation and lead to microscopic damage of the lungs.
Likely a plateau pressure of negative and positive pressure ventilation with Bag Mask Ventilation or with Mechanical Ventilation would need to be measured to ensure patient safety.
But is a Bellow enough to control how much air you push forward into the lungs and then pull out of the lungs. Better idea may be the form of a pump.
You tell me.