Validity Of Empirical Estimates Of The Ratio Of Dead Space To Tidal Volume In Ards

BACKGROUND: The ratio of dead space to tidal volume (V-D/V-T) 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 (V-T) and driving pressure (Delta P). V-D/V-T can be estimated with volumetric capnography, but empirical formulas using demographic and physiological information have been proposed to estimate V-D/V-T without the need of additional equipment. It is unknown whether estimated and measured V-D/V-T produce similar estimates of the predicted effect of extracorporeal CO2 removal on Delta P. METHODS: We performed a secondary analysis of data from a previous clinical trial including subjects with ARDS in whom V-D/V-T and CO2 production (V-CO2) were measured with volumetric capnography. The estimated ratio of dead space to tidal volume (V-D,V-est/V-T) was calculated using standard empiric formulas. Agreement between measured and estimated values was evaluated with Bland-Altman analysis. Agreement between the predicted change in Delta P with extracorporeal CO2 removal as computed using the measured ratio of alveolar dead space to tidal volume (V-Dalv/V-T) or estimated V-Dalv/V-T (V-Dalv,V-est/V-T) was also evaluated. RESULTS: V-D,V-est/V-T was higher than measured V-D/V-T, and agreement between them was low (bias 0.05, limits of agreement -0.21 to 0.31). Differences between measured and estimated V-CO2 accounted for 57% of the error in V-D,V-est/V-T. The predicted reduction in Delta P with extracorporeal CO2 removal computed using V-Dalv,V-est/V-T was in reasonable agreement with the expected reduction using V-Dalv/V-T (bias -0.7 cm H2O, limits of agreement -1.87 to 0.47 cm H2O). In multivariable regression, measured V-D/V-T was associated with mortality (odds ratio 1.9, 95% CI 1.2-3.1, P = .01), but V-D,V-est/V-T was not (odds ratio 1.2, 95% CI 0.8-1.8, P = .3). CONCLUSIONS: V-D/V-T and V-D,V-est/V-T showed low levels of agreement and cannot be used interchangeably in clinical practice. Nevertheless, the predicted decrease in Delta P due to extracorporeal CO2 removal was similar when computed from either estimated or measured V-Dalv/V-T.