A patient’s arterial blood gas shows pH (potential of hydrogen) 7.30, pCO2 (partial pressure of carbon dioxide) 30 mm Hg, HCO3 (bicarbonate) 19 mEq/L, and pO2 (partial pressure of oxygen) 80 mm Hg. What is the acid–base disorder?

Metabolic Acidosis, Partially Compensated

This ABG demonstrates metabolic acidosis with partial respiratory compensation. The pH of 7.30 is acidemic (below the normal range of 7.35-7.45), the bicarbonate of 19 mEq/L is low (normal 22-26 mEq/L), and the pCO2 of 30 mmHg is reduced below the normal range of 35-45 mmHg, indicating the lungs are attempting to compensate by hyperventilating to blow off CO2 1, 2, 3.

Systematic ABG Interpretation

Step 1: Assess the pH

• pH 7.30 is below 7.35, confirming acidemia 1, 2
• This immediately tells us the patient has an acidotic process

Step 2: Identify the Primary Disorder

• HCO3- of 19 mEq/L is below the normal range of 22-26 mEq/L, indicating a metabolic problem 2, 3
• pCO2 of 30 mmHg is below the normal range of 35-45 mmHg, which would cause alkalosis if it were the primary problem 1, 2
• Since the pH is acidotic and the bicarbonate is low, the primary disorder is metabolic acidosis 3, 4

Step 3: Determine Compensation Status

• The pCO2 of 30 mmHg shows respiratory compensation is occurring—the patient is hyperventilating to reduce CO2 and partially correct the acidosis 3, 4
• However, the pH remains abnormal at 7.30 (not normalized to 7.35-7.45), indicating partial compensation 2, 3
• If compensation were complete, the pH would have normalized to the 7.35-7.45 range while both pCO2 and HCO3- remained abnormal 3

Step 4: Assess Oxygenation

• pO2 of 80 mmHg is below the normal threshold of >90 mmHg but above the critical hypoxemia threshold of 60 mmHg 2, 5
• This indicates mild hypoxemia that should be addressed but is not immediately life-threatening 2

Clinical Implications

Calculate the anion gap to determine the etiology: [Na+] - ([Cl-] + [HCO3-]) 6, 2. An anion gap >12 mEq/L suggests lactic acidosis, ketoacidosis, renal failure, or toxin ingestion, while a normal anion gap (8-12 mEq/L) points toward bicarbonate loss from diarrhea or renal tubular acidosis 2, 3.

Common pitfall: Do not confuse this with respiratory alkalosis simply because the pCO2 is low 4, 7. The key is that the pH moves in the same direction as the primary disturbance—here, the low bicarbonate is causing the acidosis, and the low pCO2 is the body's compensatory response 3, 4.

Answer: A. Metabolic acidosis, partially compensated