Abstract
BACKGROUND:
The ratio of dead space to tidal volume (VD/VT) is a clinically relevant parameter in ARDS; it has been shown to predict mortality, and it determines the extent to which extracorporeal CO2 removal reduces tidal volume (VT) and driving pressure (ΔP). VD/VT can be estimated with volumetric capnography, but empirical formulas using demographic and physiological information have been proposed to estimate VD/VT without the need of additional equipment. It is unknown whether estimated and measured VD/VT produce similar estimates of the predicted effect of extracorporeal CO2 removal on ΔP.
METHODS:
We performed a secondary analysis of data from a previous clinical trial including subjects with ARDS in whom VD/VT and CO2 production (
RESULTS:
VD,est/VT was higher than measured VD/VT, and agreement between them was low (bias 0.05, limits of agreement –0.21 to 0.31). Differences between measured and estimated
CONCLUSIONS:
VD/VT and VD,est/VT showed low levels of agreement and cannot be used interchangeably in clinical practice. Nevertheless, the predicted decrease in ΔP due to extracorporeal CO2 removal was similar when computed from either estimated or measured VDalv/VT.
Keywords
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