Situation: A 66-year-old man with chronic obstructive pulmonary disease (COPD) is admitted to the medical ward with increasing breathlessness and purulent sputum for 3 days. He has smoked for 40 years. His clinic record contains an arterial blood gas (ABG) result taken 3 months ago when he was stable, and the physician orders a repeat ABG on admission. His ABG from the clinic, taken on room air when he was stable, shows the following (PaCO2 = partial pressure of carbon dioxide in arterial blood; HCO3− = bicarbonate): pH 7.37 PaCO2 56 mmHg HCO3− 31 mEq/L (Reference ranges: pH 7.35–7.45; PaCO2 35–45 mmHg; HCO3− 22–26 mEq/L) How should the nurse interpret this result?

The pH is within the normal range, so any compensation is full, but it lies on the acid side of 7.40. The PaCO2 of 56 mmHg is high (an acid change) and the HCO3− of 31 mEq/L is high (a base change); the value that matches the acid-side pH is the PaCO2, so the primary disorder is respiratory acidosis. The kidneys have retained enough bicarbonate over time to return the pH to normal, a pattern typical of stable chronic carbon dioxide retention in COPD.

Step 1: Anchor the interpretation to the pH

The first value to examine in any arterial blood gas (ABG) analysis is the pH. Here the pH is 7.37, which falls within the reference range of 7.35–7.45. However, it sits on the acid side of the absolute midpoint of 7.40. A pH that is normal but shifted toward the acidic side indicates that the primary disorder has been fully compensated, because the body has had time to bring the pH back into the normal range while still leaning in the direction of the original disturbance. Full compensation does not mean the acid–base problem has resolved; it means the opposing system has adjusted enough to normalize the pH.

Step 2: Identify the primary disorder by matching the acid-side pH

The PaCO2 is 56 mmHg, which is elevated above the normal range of 35–45 mmHg. Carbon dioxide is an acid; a high PaCO2 pushes the pH downward. The HCO3− is 31 mEq/L, which is also elevated above the normal range of 22–26 mEq/L. Bicarbonate is a base; a high HCO3− pushes the pH upward. Since the pH is on the acid side of 7.40, the value that matches the direction of the pH shift is the elevated PaCO2. The primary disorder is therefore respiratory acidosis, and the elevated bicarbonate represents the metabolic compensation.

Step 3: Why full compensation fits the COPD context

This ABG was drawn when the patient was clinically stable, not during the current acute exacerbation. In chronic obstructive pulmonary disease (COPD), persistent airflow limitation leads to chronic carbon dioxide retention. Over weeks to months, the kidneys respond by retaining bicarbonate to buffer the chronic respiratory acidosis. This renal compensation is slow but powerful, and it can eventually return the pH to the normal range. A stable COPD patient with chronic CO2 retention commonly shows a normal or near-normal pH, an elevated PaCO2, and an elevated HCO3−, which is the classic fully compensated respiratory acidosis pattern. The fact that the pH is 7.37 rather than 7.40 or higher is a subtle but important clue that the underlying process is acidotic, not alkalotic.

Watch out! Do not mistake the elevated HCO3− for a primary metabolic alkalosis. The pH is on the acid side, not the alkaline side, so the high bicarbonate is a compensatory response, not the primary event.

Key point! When both PaCO2 and HCO3− are abnormal in the same direction as each other, determine the primary disorder by looking at which value matches the direction of the pH shift. If the pH is acidotic, the acid parameter (PaCO2) is primary; if alkalotic, the base parameter (HCO3−) is primary.

Step 4: Clinical correlation with acute exacerbation

The patient now presents with increasing breathlessness and purulent sputum, which suggests an acute exacerbation of COPD. During an acute exacerbation, ventilation worsens and PaCO2 may rise further before the kidneys have time to compensate. This can shift the picture from fully compensated to partially compensated respiratory acidosis, where the pH drops below 7.35 despite an already elevated HCO3−. The repeat ABG ordered on admission will help determine whether the patient has moved from a chronic compensated state into an acute-on-chronic respiratory acidosis. The baseline ABG serves as the patient’s personal reference point, and comparing the admission ABG against it reveals how much acute worsening has occurred.

Step 5: Why the other options are incorrect

Option	Why it fails
2. Metabolic alkalosis, partially compensated	The pH is not alkalotic; it is on the acid side of 7.40. A primary metabolic alkalosis would require a pH above 7.40 and a primary elevation of HCO3−, which is not the case here.
3. Respiratory acidosis, partially compensated	Partial compensation means the pH remains outside the normal range. Here the pH is 7.37, which is within normal limits, indicating full compensation.
4. Metabolic alkalosis, fully compensated	The primary disorder cannot be metabolic alkalosis because the pH is not shifted toward alkalosis. The elevated HCO3− is a compensatory response to chronic respiratory acidosis.

Step 6: ABG interpretation framework for the licensure exam

A systematic four-step approach reduces errors on ABG questions. First, evaluate the pH: acidotic, alkalotic, or normal. Second, determine the primary disorder by matching the pH direction with either PaCO2 or HCO3−. Third, assess compensation: if the pH is normal, compensation is full; if the pH is still abnormal, compensation is partial or absent. Fourth, apply the clinical context, because chronic conditions such as COPD produce predictable ABG patterns that differ from acute conditions. In COPD, chronic CO2 retention with renal bicarbonate retention produces a fully compensated respiratory acidosis when the patient is stable, and this baseline must be distinguished from the acute worsening seen during an exacerbation.