Transcutaneous PCO monitoring in critically ill patients: update and perspectives.

The physiology of venous and tissue CO monitoring has a long and well-established physiological background, leading to the technological development of different tissue capnometric devices, such as transcutaneous capnometry monitoring (TCM). To outline briefly, measuring transcutaneous PCO (tcPCO) depends on at least three main phenomena: (I) the production of CO by tissues (VCO), (II) the removal of CO from the tissues by perfusion (wash-out phenomenon), and (III) the reference value of CO at tissue inlet represented by arterial CO content (approximated by arterial PCO, or artPCO). For this reason, there are, at present, roughly two clinical uses for tcPCO measurement: a respiratory approach where tcPCO is likely to estimate and non-invasively track artPCO; and a hemodynamic under-estimate use where tcPCO can reflect tissue perfusion, summarized by a so-called "tc-art PCO gap". Recent research shows that these two uses are not incompatible and could be combined. The spectrum of indications and validation studies in ICUs is summarized in this review to give a survey of the potential applications of TCM in critically ill patients, focusing mainly on its potential (micro)circulatory monitoring contribution. We strongly believe that the greatest benefit of measuring tcPCO is not to only to estimate artPCO, but also to quantify the gap between these two values, which can then help clinicians continuously and noninvasively assess both respiratory and hemodynamic failures in critically ill patients.