NCLEX Oxygenation Practice Test

If pH is low and PaCO2 is high on ABG, which acid-base disturbance is most likely?

Metabolic acidosis Respiratory acidosis Metabolic alkalosis Respiratory alkalosis

Correct Answer:

Respiratory acidosis

Explanation:

Interpreting ABG begins with identifying whether the pH indicates acidemia or alkalemia. In this scenario, the pH is low, so the patient has acidemia. To find the primary disturbance, look at the PaCO2. A high PaCO2 means carbon dioxide is accumulating, which forms carbonic acid and lowers pH—this points to a respiratory origin. Therefore, the most likely acid-base disturbance is respiratory acidosis. If it were metabolic acidosis, the PaCO2 would typically be low due to respiratory compensation (hyperventilation). If it were respiratory alkalosis, the PaCO2 would be low, not high. If it were metabolic alkalosis, the pH would be high. So the combination of acidemia with a high PaCO2 fits respiratory acidosis best.

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