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 patient’s
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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