Which of the following blood gas parameters are measured directly by the blood gas analyzer electrochemically as opposed to being calculated by the instrument?

• A. pH, HCO-3, total CO2
• B. PCO2, HCO3, PO2
• C. pH, PCO2, PO2
• D. PO2, HCO3, total CO2

Answer: (C)

Blood gas analysis uses sensors that directly measure pH and the gas tensions, while bicarbonate and total CO2 are derived from those measured values. The pH is read with a glass electrode, PCO2 with a Severinghaus-type CO2 sensor, and PO2 with a Clark-type oxygen sensor. In contrast, HCO3- and total CO2 come from the bicarbonate buffering relationship in blood, described by Henderson-Hasselbalch: pH = pKa + log([HCO3-]/(0.03×PCO2)). From the measured pH and PCO2, the bicarbonate concentration can be calculated, and total CO2 is the sum of dissolved CO2 (0.03×PCO2) and HCO3-. Therefore, the parameters measured directly are pH, PCO2, and PO2.