In arterial blood gas analysis, what does a low pH with reduced HCO3- indicate, and what compensatory change would you expect?

• A. Metabolic alkalosis; expect increased PaCO2.
• B. Respiratory alkalosis; expect increased HCO3-.
• C. Metabolic acidosis; expect increased PaCO2.
• D. Metabolic acidosis; expect decreased PaCO2 due to respiratory compensation.

Answer: (D)

When the pH is low, the body is in acidemia. If the bicarbonate level is also reduced, the primary problem is metabolic acidosis. The body then compensates through the lungs by increasing ventilation to blow off carbon dioxide, which lowers the PaCO2. So the expected change is a decreased PaCO2 due to respiratory compensation. This aligns with metabolic acidosis plus an appropriate respiratory response (often described using Winter’s formula to estimate the expected PaCO2). If PaCO2 were not decreased, or fell beyond what’s predicted, it would suggest another mixed disturbance.