Gas Exchange: improves oxygenation by re-expanding collapsed alveoli (reduced shunt) and redistributing lung water from alveolar to interstitial space, both improve V/Q matching. If too high could hyperinflate impairing blood flow increasing deadspace lowering PaO2 raising PaCO2.
Compliance = tidal volume/(Plateau press – PEEP). CPAP/PEEP can increase, decrease, or not change compliance. Increase indicates alveolar recruitment. Decrease indicates overdistention. No change indicates ventilation occurring on the linear portion of volume/pressure curve.
Hemodynamics: generally increased intrathoracic pressure decreases venous return. It can also increase right heart afterload, ventricular compliance and contractility.