When analyzing ABG, how do you differentiate respiratory from metabolic causes?

• A. By measuring lactate levels
• B. By analyzing the pattern of pH, PaCO2, and HCO3- and the direction of compensation
• C. By checking only PaO2
• D. By patient age

Answer: (B)

Interpreting ABGs to tell respiratory from metabolic causes hinges on the pattern of pH, PaCO2, and HCO3- and how the body compensates. If the primary problem is respiratory, PaCO2 is abnormal and the pH shifts in the direction of the acid-base disturbance; the metabolic system then compensates by changing HCO3- in the opposite direction, with a slower, time-dependent trend (acute vs chronic). If the primary problem is metabolic, HCO3- is abnormal and the pH moves in the opposite direction; the respiratory system compensates by adjusting PaCO2 (hyperventilation to lower PaCO2 in metabolic acidosis, hypoventilation to raise PaCO2 in metabolic alkalosis). The degree and speed of this compensation help distinguish acute from chronic. Lactate, PaO2, or age don’t define the primary acid-base disorder on their own.