USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

How does respiratory compensation for metabolic alkalosis get limited?

1. Increased CO2 levels

2. Decreased pH levels

3. Hypoxemia

4. Elevated bicarbonate levels

The correct answer is: Hypoxemia

In the context of metabolic alkalosis, respiratory compensation occurs as the body attempts to correct the elevated pH by reducing ventilation, which leads to an increase in carbon dioxide levels (CO2) and subsequently lowers the pH back towards normal. However, this compensatory mechanism can be limited in the presence of hypoxemia. When oxygen levels in the blood are low, the body's primary drive to increase respiration is the need for oxygen, rather than the need to balance CO2 levels and pH. As a result, the respiratory system may prioritize oxygenation over the compensation of acid-base status, thus impeding the ability to effectively compensate for metabolic alkalosis. While elevated bicarbonate levels are characteristic of metabolic alkalosis, they do not directly limit the physiological compensation. Similarly, increased CO2 levels are a result of diminished ventilation, and decreased pH levels would be an expected outcome of successful compensation, rather than a limiting factor in itself. Therefore, hypoxemia is key in limiting the respiratory response in this scenario.