What is the correct interpretation of these ABGs? pH 7.35, HCO3 18, PCO2 30?

What is the correct interpretation of these ABGs? pH 7.35, HCO3 18, PCO2 30?

• A. Acute metabolic acidosis with no compensation
• B. Respiratory alkalosis
• C. Metabolic acidosis with partial respiratory compensation
• D. Normal acid-base status

Answer: (C)

When interpreting ABGs, start by identifying the primary disorder from the bicarbonate and the carbon dioxide, then assess whether the respiratory system is compensating.

Here the bicarbonate is low (18 mEq/L), which points to metabolic acidosis. The pCO2 is also low (30 mmHg), showing the respiratory system is trying to compensate by blowing off CO2. The pH is at the lower limit of normal (7.35), not clearly acidemic or alkalemic, which suggests the compensation is not bringing the pH fully back to normal. That pattern fits metabolic acidosis with partial respiratory compensation.

Using Winter’s formula for metabolic acidosis (expected pCO2 ≈ 1.5 × HCO3 + 8 ± 2), with HCO3 of 18 the expected pCO2 is around 33–37 mmHg. The observed pCO2 of 30 is lower than that, indicating a more pronounced respiratory response, but because the pH remains at the edge of normal rather than clearly alkalemic, the overall interpretation is metabolic acidosis with partial respiratory compensation.