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 (ΔP). 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 ΔP.

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 () were measured with volumetric capnography. The estimated ratio of dead space to tidal volume (V_D,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 ΔP 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,est/V_T) was also evaluated.

RESULTS: V_D,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 accounted for 57% of the error in V_D,est/V_T. The predicted reduction in ΔP with extracorporeal CO₂ removal computed using V_Dalv,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,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,est/V_T 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 V_Dalv/V_T.