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Assessing the Adequacy of Gas Exchange

Suppose your patient has PaO2 of 85 that results in an SpO2 around 97-98%. Even though these values are adequate for sustaining the patient, they do not mean that gas exchange is normal. In fact, a patient can have a PaO2 and SpO2 in this range and still have markedly abnormal gas exchange. As a result, whenever you interpret the PaO2 on an arterial blood gas, you must look at that number in light of the amount of oxygen that is being delivered to the patient. There are several ways you can get a sense of this value:

The P/F Ratio

This is the ratio of the PaO2 (in mmHg) to the FiO2 (expressed as a decimal). The smaller the value, the worse the patientís gas exchange. For example, consider a patient with a PaO2 of 80. If they are on an FiO2 of 0.3, the P/F ratio is 266. If they are, instead, on FiO2 of 0.8, the P/F ratio is only 100, a much worse value that signifies greater impairment in gas exchange.  The  P/F ratio is commonly used in patients on mechanical ventilation as part of the assessment of whether they have the Acute Respiratory Distress Syndrome (ARDS). A P/F ratio below 300 is consistent with Acute Lung Injury, while a P/F ratio below 200 is consistent with ARDS (provided the patient meets the other criteria including a CXR with diffuse bilateral opacities and evidence that cardiac function is normal).

The AaO2 Difference

The same number that is used to help determine the cause of hypoxemia can also be used to determine the adequacy of gas exchange. In patients with an elevated AaO2 Difference, larger values imply a greater degree of either low V/Q areas and/or shunt. The size of the AaO2 Difference does not help you distinguish between low V/Q and shunt, however. The way to make that determination is to measure the response to supplemental oxygen administration. The PaO2 will rise and the AaO2 difference will shrink in patients with low V/Q as the primary cause of their hypoxemia (e.g., the COPD patient during an exacerbation) whereas in shunt, supplemental oxygen does not lead to improvements in the PaO2.

Both methods for assessing the adequacy of gas exchange require an accurate FiO2 measurement, which, as noted above, can only be assured if the patient is on room air, a high flow face mask, non-invasive mechanical ventilation or invasive mechanical ventilation.

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