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    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. We performed a secondary analysis of data from a previous clinical trial including subjects with ARDS in whom VD/VT and CO2 production ([Formula: see text]) were measured with volumetric capnography. The estimated ratio of dead space to tidal volume (VD,est/VT) was calculated using standard empiric formulas. Agreement between measured and estimated values was evaluated with Bland-Altman analysis. Agreement between the predicted change in ΔP with extracorporeal CO2 removal as computed using the measured ratio of alveolar dead space to tidal volume (VDalv/VT) or estimated VDalv/VT (VDalv,est/VT) was also evaluated. 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 [Formula: see text] accounted for 57% of the error in VD,est/VT. The predicted reduction in ΔP with extracorporeal CO2 removal computed using VDalv,est/VT was in reasonable agreement with the expected reduction using VDalv/VT (bias -0.7 cm H2O, limits of agreement -1.87 to 0.47 cm H2O). In multivariable regression, measured VD/VT was associated with mortality (odds ratio 1.9, 95% CI 1.2-3.1, P = .01), but VD,est/VT was not (odds ratio 1.2, 95% CI 0.8-1.8, P = .3). 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. Copyright © 2021 by Daedalus Enterprises.


    Jose Dianti, Arthur S Slutsky, Ewan C Goligher. Validity of Empirical Estimates of the Ratio of Dead Space to Tidal Volume in ARDS. Respiratory care. 2021 Apr;66(4):559-565

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    PMID: 33082218

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