The preoperative nurse is caring for a client with chronic obstructive pulmonary disease (COPD). Which arterial blood gas (ABG) value, reflecting chronic (compensated) respiratory acidosis, does the nurse expect to observe?

A. A high pH (greater than 7.45) indicates a state of alkalosis, which is the opposite of the acidic state expected in a client with COPD. In chronic respiratory acidosis, the pH is typically on the lower end of the normal range (7.35 to 7.40) because the body has had time to compensate. A truly "high" pH would suggest a primary metabolic or respiratory alkalosis rather than the chronic retention of carbon dioxide. B. A low PaCO2 (less than 35 mmHg) is indicative of hyperventilation or respiratory alkalosis, where the lungs are blowing off too much carbon dioxide. Clients with COPD are "CO2 retainers" due to alveolar hypoventilation and air trapping, meaning their PaCO2 is chronically elevated (hypercapnia). A low PaCO2 would be an atypical and unexpected finding in a patient suffering from chronic obstructive lung disease and respiratory acidosis. C. A high PaO2 is rarely seen in clients with COPD, who typically experience chronic hypoxemia due to impaired gas exchange and ventilation-perfusion mismatch. These patients often have a "normal" PaO2 that is lower than that of the general population, frequently ranging from 60 to 80 mmHg. A high oxygen level would be more suggestive of excessive supplemental oxygen administration rather than a natural manifestation of the disease process. D. In chronic respiratory acidosis, the kidneys compensate for the high PaCO2 by retaining bicarbonate (HCO3) to help buffer the acidic pH. This metabolic compensation is a slow process that occurs over days, resulting in an HCO3 level that is elevated above the normal reference range (greater than 26 mEq/L). The presence of a high HCO3 alongside a near-normal pH and high PaCO2 confirms that the respiratory acidosis is in a compensated, chronic state.