How can I differentiate between metabolic acidosis and respiratory acidosis using ABG values?

To differentiate between metabolic acidosis and respiratory acidosis using arterial blood gas (ABG) values, focus primarily on the pH, partial pressure of carbon dioxide (PaCO2), and bicarbonate (HCO3-) levels. In metabolic acidosis, the pH is low (<7.35), HCO3- is decreased due to a primary metabolic cause, and PaCO2 is typically decreased as a compensatory respiratory alkalosis to blow off CO2. In respiratory acidosis, the pH is also low (<7.35), but the primary disturbance is an elevated PaCO2 due to hypoventilation, while HCO3- is normal or elevated if compensation has occurred over time.

Specifically, metabolic acidosis is characterised by a low pH, low HCO3-, and a low PaCO2 (compensatory), whereas respiratory acidosis shows a low pH, high PaCO2, and normal or high HCO3- (if chronic compensation is present). The key differentiator is the primary abnormality: low HCO3- in metabolic acidosis versus high PaCO2 in respiratory acidosis. Additionally, the compensation patterns differ: metabolic acidosis leads to respiratory compensation (reduced PaCO2), while respiratory acidosis leads to renal compensation (increased HCO3-) over time.

Therefore, by analysing the ABG values, if the primary disturbance is a low bicarbonate with a compensatory low PaCO2, it indicates metabolic acidosis; if the primary disturbance is a raised PaCO2 with a compensatory rise in bicarbonate, it indicates respiratory acidosis ¹ (Shapiro, 1988).