Validity of Empirical Estimates of the Ratio of Dead Space to Tidal Volume in ARDS.

Pubmed ID: 33082218

Journal: Respiratory care

Publication Date: April 1, 2021

MeSH Terms: Humans, Lung, Tidal Volume, Respiratory Dead Space, Respiratory Distress Syndrome, Capnography, Carbon Dioxide

Authors: Slutsky AS, Dianti J, Goligher EC

Cite As: Dianti J, Slutsky AS, Goligher EC. Validity of Empirical Estimates of the Ratio of Dead Space to Tidal Volume in ARDS. Respir Care 2021 Apr;66(4):559-565. Epub 2020 Oct 20.

Studies:

Abstract

BACKGROUND: The ratio of dead space to tidal volume (V<sub>D</sub>/V<sub>T</sub>) is a clinically relevant parameter in ARDS; it has been shown to predict mortality, and it determines the extent to which extracorporeal CO<sub>2</sub> removal reduces tidal volume (V<sub>T</sub>) and driving pressure (ΔP). V<sub>D</sub>/V<sub>T</sub> can be estimated with volumetric capnography, but empirical formulas using demographic and physiological information have been proposed to estimate V<sub>D</sub>/V<sub>T</sub> without the need of additional equipment. It is unknown whether estimated and measured V<sub>D</sub>/V<sub>T</sub> produce similar estimates of the predicted effect of extracorporeal CO<sub>2</sub> removal on ΔP. METHODS: We performed a secondary analysis of data from a previous clinical trial including subjects with ARDS in whom V<sub>D</sub>/V<sub>T</sub> and CO<sub>2</sub> production ([Formula: see text]) were measured with volumetric capnography. The estimated ratio of dead space to tidal volume (V<sub>D,est</sub>/V<sub>T</sub>) 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 CO<sub>2</sub> removal as computed using the measured ratio of alveolar dead space to tidal volume (V<sub>Dalv</sub>/V<sub>T</sub>) or estimated V<sub>Dalv</sub>/V<sub>T</sub> (V<sub>Dalv,est</sub>/V<sub>T</sub>) was also evaluated. RESULTS: V<sub>D,est</sub>/V<sub>T</sub> was higher than measured V<sub>D</sub>/V<sub>T</sub>, 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 V<sub>D,est</sub>/V<sub>T</sub>. The predicted reduction in ΔP with extracorporeal CO<sub>2</sub> removal computed using V<sub>Dalv,est</sub>/V<sub>T</sub> was in reasonable agreement with the expected reduction using V<sub>Dalv</sub>/V<sub>T</sub> (bias -0.7 cm H<sub>2</sub>O, limits of agreement -1.87 to 0.47 cm H<sub>2</sub>O). In multivariable regression, measured V<sub>D</sub>/V<sub>T</sub> was associated with mortality (odds ratio 1.9, 95% CI 1.2-3.1, <i>P</i> = .01), but V<sub>D,est</sub>/V<sub>T</sub> was not (odds ratio 1.2, 95% CI 0.8-1.8, <i>P</i> = .3). CONCLUSIONS: V<sub>D</sub>/V<sub>T</sub> and V<sub>D,est</sub>/V<sub>T</sub> showed low levels of agreement and cannot be used interchangeably in clinical practice. Nevertheless, the predicted decrease in ΔP due to extracorporeal CO<sub>2</sub> removal was similar when computed from either estimated or measured V<sub>Dalv</sub>/V<sub>T</sub>.