- The most common acid-base disturbance observed in patients with chronic obstructive pulmonary disease (COPD) is respiratory acidosis.
- This occurs due to the impaired ability of the lungs to excrete carbon dioxide (CO2) effectively.
- In COPD, airflow obstruction leads to difficulty in ventilation, causing CO2 retention.
- The accumulation of CO2 in the blood increases the partial pressure of carbon dioxide (PaCO2), resulting in a decrease in blood pH and hence respiratory acidosis.

- Over time, a patient with chronic respiratory acidosis often develops a compensatory metabolic alkalosis through renal retention of bicarbonate (HCO3-) to buffer the elevated CO2 levels. However, the primary and underlying disturbance remains respiratory acidosis.

- In contrast, respiratory alkalosis is more characteristic of conditions causing hyperventilation and excessive CO2 elimination, such as anxiety or early asthma attacks. Metabolic acidosis and metabolic alkalosis are typically caused by systemic conditions unrelated directly to lung function.

In summary:
- COPD causes airflow obstruction leading to CO2 retention.

- Elevated PaCO2 results in respiratory acidosis.

- The kidneys compensate by increasing bicarbonate reabsorption, but the primary disturbance remains respiratory in origin.

Reference: Robbins Basic Pathology, 10th Edition, Chapter 9: Diseases of the Respiratory System