Mask CPAP: Physiologic effects

Physics, Monitoring, & Devices

Gas Exchange

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.

Respiratory Mechanics

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.


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