How Can We Use Tissue Carbon Dioxide Measurement as an Index of Perfusion?

Abstract

Adequate tissue perfusion is an essential component of oxygenation in critically ill and high-risk surgical patients. Tissue oxygenation is satisfied when tissue oxygen demand corresponds to oxygen consumption (VO2), which is defined as the product of oxygen delivery (DO2) by the oxygen extraction ratio (O2ER) []. Under conditions where DO2 does not meet oxygen demand tissue dysoxia occurs leading to organ failure. Early identification and correction of tissue hypoperfusion, focusing on cardiac output and tissue oxygen supply, is of particular importance and may improve outcomes []. However, the presence of apparent normal macro circulatory parameters does not ensure adequate oxygen supply and the absence of compromised tissue perfusion [], []; and oxygen-derived variables are poorly correlated with anaerobic metabolism [] and may, therefore, be normal when tissue dysoxia is present due to microcirculatory deficit []. This leads to difficulties in interpreting oxygen-derived variables for detecting tissue hypoxia: A low VO2 may be due either to tissue hypoxia whatever its mechanism (e.g., sepsis, hypovolemia) or to reduced oxygen demand without hypoxia []. In addition, monitoring tissue dysoxia and the adequacy of tissue oxygenation is difficult to achieve, while measuring VO2 in intensive care unit (ICU) and surgical patients requires the insertion of invasive pulmonary arterial catheters.