If you have poor profusion in the place where you are measuring, it can definitely cause poor pulse-ox reading. When you are looking at a pulse-ox in a hospital setting, you can see something called the pleth wave that shows the stregnth of the pulse in the measures area. If you have a weak pleth, we know the O2 reading is off and we need to move it to another area.

Thats true in cpap (continuous positive airway pressure). In the hospital, we usually use BiPap (biphasic positive airway pressure). The pressure is significantly decreased during expiration, to the leak of exhaled air is pretty low.

Great question. When you intubate a patient, the tube that goes into the trachea has a balloon that inflates, which prevents air leak and creates a closed system. So the volume that the vent delivers is the volume that the lung gets. An individual taking a deep breath usually has a lung capacity of about 12ml/kg of ideal body weight. Prior to year 2000, that was the typical goal of adults on a vent. The vent does measure both volume and required pressure and getting to that volume took a peak pressure of about 50mmHg. In 2000 the ARDSNet trial showed that using much smaller volumes (about 6ml/kg) allowed for lower pressures (around 25 mmHG) and caused much less lung damage. To make up for the lower respiratory volumes, the respiratory rate in increased so that the total volume/min stays the same. Bi-pap, by comparison, is much less precise. There is a lot of leak around the mask, a lot of the volume you are delivering goes into the “dead-space” of the mouth and nasopharynx, and the delivery pressure at the mouth is often very different from what the lung sees. That being said, most of the time we try to use bi-pap first, since intubation required deep sedation, and can lead to a lot of other complications

Initially, we were being told to intubate early and avoid non-invasive ventilation like CPAP and BIPAP because it increased aerosolization of the virus, and because the volume of air pushed into the lung could be more precisely controlled, attempting to prevent lung damage. By late March, many of us in the EM community were pushing back on guidance asking us to intubate hypoxic but otherwise healthy looking patients. In the interim, folks figured out ways to rig a viral filter on to the outflow of a bibap mask, decreasing the hospitals concerns about letting us use non-invasive.