Once the normal value and the A-a gradient are calculated, the cause of the hypoxemia can be determined.
If the A-a gradient is within normal range, the cause of the hypoxemia is due to hypoventilation (elevated PaCO2) or a low barometric pressure (high elevations). Remember, since the A-a gradient uses PaCO2 as a part of the Alveolar air equation, if PaCO2 is high, the result can be a reduction in the A-a gradient. In contrast, if the PaCO2 is below the normal range, the A-a gradient can become elevated.
If the A-a is above the normal range, then the hypoxemia is due to a V/Q mismatch, shunt or impaired diffusion across the alveolar-capillary membrane.
To estimate of the percentage of physiologic shunt using the A-a gradient one can use the following rule: for every 50 mmHg difference in A-a gradient, a 2% shunt is approximated. For example, a 300 mmHg A-a gradient would be estimated as a 12% shunt (200/50 = 4 4 x 2% = 8%).
Experienced clinicians utilizing calculations at the bedside can provide better care for patients who are experiencing difficulty with gas exchange. Knowing how to calculate and use the A-a gradient to determine what the source of the patient’s hypoxemia is vital in providing the best care and is an integral part of monitoring the patient in the intensive care.
Breathe easy,
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