PEEP is often titrated up along with FiO2 to increase oxygen saturation. Although the potential negative hemodynamic effect of high PEEP is often recognized, it is important to also note that high PEEP can also paradoxically worsen oxygen saturation.
The primary physiologic explanation for this phenomenon in a patient with pulmonary disease is due to the varying impact of PEEP on the intra- vs. extra-alveolar blood vessels. PEEP preferentially distends more normal/compliant lung which causes compression of intra-alveolar vessel at excessively high levels of PEEP. This causes pulmonary blood to be diverted to areas of lower vascular resistance (e.g. consolidated lung which is less distended due to its worsened compliance) and lower VQ matching. Essentially, blood flow to normal/healthy lung is decreased and is instead increased to diseased lung, worsening hypoxemia.
Bottom line:
High PEEP can potentially worsen hypoxemia and should be considered as an etiology for worsening oxygen saturation, particularly when the hypoxemia is out of proportion to the patient’s radiographic findings.
References
Çoruh B, Luks AM. Positive end-expiratory pressure. When more may not be better. Annals ATS. 2014;11(8):1327-1331.
