Aerial blood gas of a 5-year-old child done at sees level gives the following results: pH 7.41, PaO2 100 mgHg, and PaCO2 40 mm Hg. The child is being ventilated with 80% oxygen. What is the (A-a) PO2?
**Core Concept**
The (A-a) PO2, or alveolar-arterial oxygen gradient, is a measure of the difference between the oxygen levels in the alveoli and the blood. It is an important indicator of lung function and can help diagnose conditions such as chronic obstructive pulmonary disease (COPD) or pulmonary embolism.
**Why the Correct Answer is Right**
To calculate the (A-a) PO2, we need to first calculate the alveolar oxygen partial pressure (PAO2). The formula for PAO2 is: PAO2 = FiO2 (barometric pressure - PH2O) - PaCO2/R. In this case, FiO2 is 0.8 (80% oxygen), barometric pressure is approximately 760 mmHg, PH2O is 47 mmHg (at sea level), and PaCO2 is 40 mmHg. Using the given values, we can calculate PAO2 as follows: PAO2 = 0.8 (760 - 47) - 40/0.8 = 523 mmHg. The (A-a) PO2 is then calculated as PAO2 - PaO2, which is 523 - 100 = 423 mmHg.
**Why Each Wrong Option is Incorrect**
**Option A:** This option would require a calculation error, such as using a different value for barometric pressure or FiO2. However, the given values are correct, and the calculation is straightforward.
**Option B:** This option would require an error in the calculation of PAO2, such as using the wrong value for PH2O or PaCO2. However, the values are given and used correctly.
**Option C:** This option would require an error in the calculation of (A-a) PO2, such as subtracting PaO2 from the wrong value or using the wrong units. However, the calculation is correct, and the units are consistent.
**Option D:** This option is not provided, but it would likely be an incorrect value or calculation.
**Clinical Pearl / High-Yield Fact**
A normal (A-a) PO2 is typically less than 20 mmHg. An elevated (A-a) PO2 can indicate conditions such as COPD, pulmonary embolism, or pneumonia.
**Correct Answer: C. 423 mmHg**