Giving more fio2 and increasing PEEP are two ways one can increase oxygen content. Fio2- increases alveolar partial pressure of oxygen and PEEP by recruiting more alveoli. But I have seen a few times patient becoming more hypoxic with increasing PEEP, what do you think is the pathophysiology ?
I think it could be due to increased alveolar pressure which leads to collapse of the capillaries around alveoli and change in lung zones.
As @Deepa mentioned , hyperinflating the alveoli could collapse the vessels , as well as increasing the intra thoracic pressure that could compromise the venous return , leading to decrease cardiac output that may lead to decrease pulmonary circulation .
I believe the probable causes are nicely mentioned in the comments above. High PEEP increases the anatomical and physiological dead space as well.
Yes. Is necessary to measure the ideal PEEP. When you generate a PEEP higher then the preassure for a superior inflection point you no longer promote expansion, instead you are creating areas of overdistention wich can cause capillary compression and increasing intra alveolar dead space. Thus your patient can became not only hypoxemic but also hypercapnic.
Answers provided above by Mohamed and Bruno.
Something to add to the mix if you have the equipment capability is Volumetric End Tidal C02. Valuable tool in determining optimal peep and a helpful tool in determining that point of overdistension of the alveoli/impingement of alveolar blood flow
Sometimes a change of only a single CmH2O of PEEP can make a difference.
Also, alveoli which are more compliant can become overdistended compared to noncompliant alveoli. This reduces perfusion compared to the more diseased, less compliant alveoli. The noncompliant alveoli are not ventilated, and the compliant alveoli are not perfused.
Article Platypnea-orthodeoxia: Patent foramen ovale unmasked by pulm...
As mentioned above intracardiac shunts at the atrial level and pulmonary atrio-ventricular malformations can cause hypoxia. This may be worsened by increased pulmonary vascular resistance (e.g. due to hypoxic pulmonary vasoconstriction or increased PEEP). The most common cause of an intracardiac shunt is patent foramen ovale. This concept is discussed in the article above. This may be important to consider during the COVID-19 pandemic. I recommend consideration of the presence of intracardiac shunt and pulmonary AVM in any patient whose degree of hypoxia is more than expected for the severity of lung injury on imaging.
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