Gas volume corrections in intensive care unit: needed or pointless?

Abstract

The conditions of temperature, pressure, and saturation in which respiratory gas volumes are expressed [standard temperature and pressure, dry (STPD), ambient temperature and pressure, saturated (ATPS), or body temperature and pressure, saturated (BTPS)] are physiologically relevant, but often ignored or unknown in clinical practice. In this study, we aimed to investigate whether and at which extent the gas volume corrections, either in natural or artificial lung, may alter key respiratory and metabolic variables and the possible clinical consequences. We primarily referred to the effects of gas volume corrections on three physiological variables: physiological dead space, venous admixture, and total CO₂ production (V̇co₂) during extracorporeal support. We used three physiological models in which calculations of these variables have been performed with and without correction of gas volumes, both in a theoretical model and in 448 patients. The lack of gas volume correction leads to an error in the computation of physiological dead space fraction between 0.05 and 0.15, both in the theoretical model and in the patient population. The venous admixture was minimally affected by the absence of correction (0.01-0.04 error). During extracorporeal support, if the V̇co₂ of natural and membrane lung is expressed in different conditions, potentially large errors (0%-18.4%) may occur in the computation of total V̇co₂ (V̇co_2tot = V̇co_2ML + V̇co_2NL). This may lead to inappropriate settings of mechanical ventilation with higher plateau pressure. As the dead space and the CO₂ sharing between natural and artificial lung are relevant both as prognostic index and as a guide for appropriate mechanical ventilation, their inappropriate computation may lead to erroneous categorization of the patients and inappropriate mechanical treatment.NEW & NOTEWORTHY Gas volume conditions are often ignored or unknown in the clinical practice. However, they could have relevance for the calculation of some key variables in ICU setting. This study shows that gas volume corrections are mostly relevant when assessing CO₂ clearance, both in mechanical ventilation and during extracorporeal support, whereas irrelevant for oxygenation assessment of patients. Knowing when the appropriate corrections are needed allows to better understand patients' clinical conditions and to tailor the treatment.

Keywords: dead space; extracorporeal support; gas volumes; intensive care; venous admixture.