An infant on conventional mechanical ventilation has blood gas values: pH 7.0, PaCO2 55 mmHg, PaO2 38 mmHg, HCO3- 13 mEq/L. Which interpretation best fits this ABG?

• A. Uncompensated metabolic acidosis
• B. Uncompensated mixed acidosis
• C. Uncompensated respiratory acidosis
• D. Uncompensated metabolic alkalosis

Answer: (B)

Acidemia is present, as shown by the pH of 7.0. The low bicarbonate level (HCO3- = 13 mEq/L) points to metabolic acidosis. At the same time, the elevated PaCO2 (55 mmHg) indicates concurrent respiratory acidosis. The combination means both metabolic and respiratory factors are driving the acidemia, a mixed acid-base disorder. Moreover, the PaCO2 is higher than what would be expected if only metabolic acidosis were present (predicted PaCO2 would be around 25–30 mmHg for this HCO3-), so there is no respiratory compensation occurring. Hence, this is an uncompensated mixed acidosis. The very low PaO2 also reflects significant hypoxemia, consistent with severe lung dysfunction on ventilation.