Positive pressure ventilation

Well with an intrapulmonary shunt, blood flows to the lungs as it should, but it doesn't pick up Oxygen because your aveoli aren't functioning properly(maybe you got some atelectasis). Have you guys covered V/Q ratio and mismatch? The normal V/Q ratio is actually 0.8, but in the case of shunting your Q(blood flowing to the lungs) will be normal, and your V(oxygen that can come in and mix with the blood) will be low, so you'll have a LOW V/Q ratio. If the blood is flowing to the lungs normally, but the aveoli aren't functioning...you can give them 100 liters of 02 via facemask, nasal cannula, whatever you want, and it won't help because the aveoli are collapsed in the case of atelectasis, or just aren't functioning. So what are you to do? Use pressure to keep them open and prevent them from collapsing. CPAP, even though you aren't necessarily giving them 100% or some high concentration of oxygen, will keep those aveoli from collapsing and recruit others so that your V will move closer to normal. Once you have the pressure keeping them open, then the oxygen can mix with the blood like it should.

don't forget about PEEP (positive end expiratory pressure)...but in short, PEEP/CPAP/BiPAP helps the keep those alveoli open to facilitate gas exchange.