Arterial blood gas (ABG) analysis can reflect respiratory acidosis. Which statement describes a process that causes respiratory acidosis?

• A. COPD
• B. Pulmonary fibrosis with no gas exchange issues
• C. High altitude exposure with hyperventilation
• D. Metabolic alkalosis

Answer: (A)

Respiratory acidosis happens when ventilation is insufficient to remove CO2, so PaCO2 rises and shifts the acid-base balance toward acidosis. In COPD, chronic airflow limitation and air trapping lead to reduced alveolar ventilation, especially during expiration, causing CO2 retention. Over time the kidneys compensate by increasing bicarbonate, so the ABG can show elevated PaCO2 with a higher HCO3− and a near-normal or only modestly reduced pH in chronic disease. The result is a pattern of respiratory acidosis due to hypoventilation from obstructive lung disease.

The other scenarios do not fit this mechanism. Hyperventilation at high altitude lowers CO2, causing respiratory alkalosis. Metabolic alkalosis raises bicarbonate with a primary non-respiratory cause, not CO2 retention. Pulmonary fibrosis can impair gas exchange but doesn’t inherently produce CO2 retention unless ventilation is depressed; with no gas exchange issues, respiratory acidosis isn’t the expected pattern.