What is the a/A ratio for a 26-week gestation-age neonate on mechanical ventilation with given PaO2 and PaCO2 levels?

The a/A ratio is a calculation used to assess the efficiency of gas exchange in the lungs, particularly in patients with conditions like respiratory distress syndrome, which is common in preterm neonates. To determine the a/A ratio, you'll need the values for arterial oxygen tension (PaO2) and the alveolar oxygen tension (A), which can be derived from the arterial carbon dioxide tension (PaCO2) and the fractional inspired oxygen (FiO2).

In the case of a 26-week gestation-age neonate on mechanical ventilation, clinicians typically expect the a/A ratio to provide insight into the degree of hypoxemia present in the infant. The key to ensuring a correct calculation lies in applying the formula:

a/A ratio = PaO2 / (FiO2 x (Pb - PH2O) - (PaCO2/R))

In this formula, "Pb" refers to barometric pressure, "PH2O" is the partial pressure of water vapor (usually around 47 mmHg at body temperature), and "R" is the respiratory exchange ratio (commonly assumed to be around 0.8 in clinical scenarios).

In evaluating the answer options, the calculated a/A ratio being 0.17 signifies a