Which factor contributes to the PCO2 electrode requiring 60–120 seconds to reach equilibrium?

• A. Diffusion characteristics of the membrane
• B. Actual blood PO2
• C. Type of calibrating standard
• D. Potential of the polarizing mercury cell

Answer: (A)

The time it takes for a PCO2 electrode to reach equilibrium is governed by how quickly CO2 can diffuse through the membrane into the internal electrolyte and form the equilibrated chemical signal. The membrane’s diffusion characteristics—thickness, material, and permeability—set the rate at which CO2 crosses into the sensing solution. If the membrane is thicker or less permeable, diffusion is slower, so it takes longer to stabilize, often on the order of 60–120 seconds. The other factors don’t primarily control this equilibration rate: the oxygen tension of the blood (PO2) affects oxygen measurements more than CO2 diffusion, the type of calibrating standard doesn’t change how fast diffusion occurs, and the polarizing mercury cell’s potential maintains the electrochemical driving force but doesn’t determine the diffusion-limited equilibration time.