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 NICE NG115 Shapiro 1988.