However, in states of ventilation perfusion mismatch, such as pulmonary embolism or right-to-left shunt, oxygen is not effectively transferred from the alveoli to the blood which results in an elevated A-a gradient.
In its expanded form, the A–a gradient can be calculated by: On room air ( FiO2 = 0.21, or 21% ), at sea level ( Patm = 760 mmHg ) assuming 100% humidity in the alveoli (PH2O = 47 mmHg), a simplified version of the equation is: The A–a gradient is useful in determining the source of hypoxemia.
The measurement helps isolate the location of the problem as either intrapulmonary (within the lungs) or extrapulmonary (elsewhere in the body).
[2] The value calculated for a patient's A-a gradient can assess if their hypoxia is due to the dysfunction of the alveolar-capillary unit, for which it will elevate, or due to another reason, in which the A-a gradient will be at or lower than the calculated value using the above equation.
[2] An abnormally increased A–a gradient suggests a defect in diffusion, V/Q mismatch, or right-to-left shunt.
Patients with pneumonia have a physical barrier within the alveoli, which limits the diffusion of oxygen into the capillaries.
The obstruction, in this case, would occur at the waterfall in our example, limiting the flow of water only through the second part of the river.
[2] Applying this analogy to different causes of hypoxemia should help reason out whether to expect an elevated or normal A-a gradient.
As a general rule of thumb, any pathology of the alveolar-capillary unit will result in a high A-a gradient.
[2] Because A–a gradient is approximated as: (150 − 5/4(PCO2)) – PaO2 at sea level and on room air (0.21x(760-47) = 149.7 mmHg for the alveolar oxygen partial pressure, after accounting for the water vapor), the direct mathematical cause of a large value is that the blood has a low PaO2, a low PaCO2, or both.
A low PaO2 indicates that the patient's current minute ventilation (whether high or normal) is not enough to allow adequate oxygen diffusion into the blood.