[CO2 pressure used in the diagnosis of ischemia]

Under ischemic conditions O2 delivery is insufficient, and the cells convert to anaerobic metabolism with production of lactic acid. The protons formed in this process are to a large extent rapidly buffered inside the cell by proteins and HCO3-. Protons buffered by HCO3- form CO2 in the tissue. Since the blood supply during ischemia is minimal, CO2 is not transported away from the tissue and will reach tensions far above pCO2 found under aerobic conditions. Thus, measuring pCO2 can be used to detect ischemia in an organ. Gastrointestinal tonometry is based on the concept of CO2 accumulation during anaerobiosis. It has been customary to calculate so-called interstitial pH (pHi) by incorporating the measured pCO2 in the tonometer and the HCO3- in an arterial blood gas in the Henderson-Hasselbalch equation. However, this method has several weaknesses, and we recommend using the measured pCO2, or rather the difference between gastrointestinal and arterial pCO2. Experimental studies have shown that pCO2 electrodes sensitively detect the onset of ischemia also in solid organs. The accumulation of pCO2 coincides with the shift from supply-independent to supply-dependent oxygen consumption and correlates with other markers of ischemia. pCO2 measurement at organ level is a promising tool in the monitoring of organ ischemia.