Common clinical situations that lead to shock include hemorrhage, myocardial infarction, heart failure, trauma, sepsis, and cardiac arrest. Regardless of the cause, clinicians are better able to treat shock if they understand the underlying mechanisms, shared mechanisms, and physiologic events. It is the relationship between VO2 and carbon dioxide (CO2) production (VOCO2) that forms the general foundation for the utility of VOCO2 and end-tidal PCO2 (PETCO2) monitoring in shock states. The ability of the measurement of the partial pressure of expired carbon dioxide (PETCO2) monitoring to reflect tissue perfusion lies in its ability to closely reflect alveolar CO2. Several options to monitor tissue CO2 in various shock states have been studied, and include transcutaneous CO2(PtcCO2) skin monitoring, interstitial fiberoptic PCO2, gastric mucosal CO2 via gastric tonometry (PgCO2), and sublingual tonometry (PslCO2). These measurements detect changes in tissue CO2 as a reflection of changes in DO2.