What will be the PA O2 when FiO2 is 0.5?
## Core Concept
The question relates to the relationship between the fraction of inspired oxygen (FiO2) and the partial pressure of arterial oxygen (PaO2), which is a critical concept in respiratory physiology and critical care medicine. The arterial blood gas (ABG) analysis is used to assess the respiratory function, and the relationship between FiO2 and PaO2 can help in evaluating the severity of respiratory failure and the need for oxygen therapy.
## Why the Correct Answer is Right
The correct answer can be deduced using the alveolar gas equation, which simplifies to: PaO2 = FiO2 * (760 - 47) - PaCO2 / R, where 760 is the atmospheric pressure at sea level, 47 is the partial pressure of water vapor, and R is the respiratory quotient (approximately 0.8). However, a more straightforward and commonly used approximation in clinical settings to estimate PaO2 from FiO2 is the P/F ratio (PaO2/FiO2 ratio). A normal P/F ratio is > 400 mmHg. For a FiO2 of 0.5, if we aim for a P/F ratio of approximately 400, the PaO2 would be around 200 mmHg (0.5 * 400 = 200).
## Why Each Wrong Option is Incorrect
**Option A:** This option suggests a very low PaO2, which would indicate severe hypoxemia but does not align with the expected value for a FiO2 of 0.5 in a person with normal lungs or mild to moderate respiratory distress.
**Option B:** This option might seem plausible but does not accurately reflect the expected PaO2 for a FiO2 of 0.5 based on standard physiological expectations and the P/F ratio.
**Option D:** This option suggests a PaO2 that is higher than what would be expected for a FiO2 of 0.5, considering the usual P/F ratios and the goal of oxygen therapy to achieve adequate oxygenation without excessive oxygen administration.
## Clinical Pearl / High-Yield Fact
A key clinical pearl is that the P/F ratio (PaO2/FiO2) is a useful index to classify the severity of acute respiratory distress syndrome (ARDS). For example, mild ARDS is defined by a P/F ratio between 201 and 300 mmHg, moderate ARDS between 101 and 200 mmHg, and severe ARDS ≤ 100 mmHg. This ratio helps guide the level of respiratory support needed.
## Correct Answer Line
**Correct Answer: B. 250**