Bedside quantification of dead-space fraction using routine clinical data in patients with acute lung injury: secondary analysis of two prospective trials

Abstract

IntroductionDead-space fraction (Vd/Vt) has been shown to be a powerful predictor of mortality in acute lung injury (ALI) patients. The measurement of Vd/Vt is based on the analysis of expired CO2 which is not a part of standard practice thus limiting widespread clinical application of this method. The objective of this study was to determine prognostic value of Vd/Vt estimated from routinely collected pulmonary variables.MethodsSecondary analysis of the original data from two prospective studies of ALI patients. Estimated Vd/Vt was calculated using the rearranged alveolar gas equation: Vd/Vt=1−[(0.86×V˙CO2est)/(VE×PaCO2)] where V˙CO2est is the estimated CO2 production calculated from the Harris Benedict equation, minute ventilation (VE) is obtained from the ventilator rate and expired tidal volume and PaCO2 from arterial gas analysis. Logistic regression models were created to determine the prognostic value of estimated Vd/Vt.ResultsOne hundred and nine patients in Mayo Clinic validation cohort and 1896 patients in ARDS-net cohort demonstrated an increase in percent mortality for every 10% increase in Vd/Vt in a dose response fashion. After adjustment for non-pulmonary and pulmonary prognostic variables, both day 1 (adjusted odds ratio-OR = 1.07, 95%CI 1.03 to 1.13) and day 3 (OR = 1.12, 95% CI 1.06 to 1.18) estimated dead-space fraction predicted hospital mortality.ConclusionsElevated estimated Vd/Vt predicts mortality in ALI patients in a dose response manner. A modified alveolar gas equation may be of clinical value for a rapid bedside estimation of Vd/Vt, utilizing routinely collected clinical data.

The Physiology of Ventilation

“…In both stages, the dead-space fraction was higher in subjects who died than in those who survived and was independently associated with a greater risk of death. Similar results were reported by Lucangelo et al 25 regarding measuring the V AE /V T fraction at admission and after 48 h of mechanical ventilation in subjects with ALI or ARDS and by Siddiki et al 69 regarding estimating physiologic V D /V T from the calculation of V CO 2 using the Harris-Benedict equation. Finally, Kallet et al 70 tested the association between the V D /V T fraction and mortality in subjects with ARDS diagnosed using the Berlin Definition 34 who were enrolled in a clinical trial incorporating lung-protective ventilation and found that markedly elevated physiologic V D /V T (Ͼ 0.60) in early ARDS was associated with higher mortality.…”

Dead space: the physiology of wasted ventilation

“…CEPOVKA et al [33] performed a subsequent study of ARDS patients ventilated with 6 cm 3 ·kg −1 tidal volumes, demonstrating a nearly identical predictive power of physiological dead space measurements, but no significant correlation between pulmonary artery pressures and the dead space measurements. Further confirmation of the prognostic value of physiological dead space measurement in both acute lung injury (ALI) and ARDS was presented by SIDDIKI et al [34] utilising previously collected ARDS network data from 1896 patients. While arterial blood gas and ventilator measurements were available, the mixed expired CO 2 values were not measured, and hence mixed expired CO 2 had to be calculated based on predicted CO 2 production rates appropriate for acutely ill mechanically ventilated patients.…”

“…While arterial blood gas and ventilator measurements were available, the mixed expired CO 2 values were not measured, and hence mixed expired CO 2 had to be calculated based on predicted CO 2 production rates appropriate for acutely ill mechanically ventilated patients. Based on the arterial CO 2 measurements and predicted mixed expired CO 2 concentrations, SIDDIKI et al [34] found that at both day 1 and day 3 of ARDS diagnosis, patients with a dead space fraction in excess of 0.50 had a risk for death that increased with every additional 0.10 increment in dead space fraction, a risk prediction that almost exactly equalled the predictive power of the complete CO 2 measurements on ARDS patients described by NUCKTON et al [32] and CEPOVKA et al [33].…”

Calculation of Physiologic Dead Space: Comparison of Ventilator Volumetric Capnography to Measurements by Metabolic Analyzer and Volumetric CO2 Monitor

“…Methods for estimating V D /V T by predictive equations have been described using the arterial to end-tidal CO 2 difference 32,33 and estimation of V CO 2 . 34 The increasing availability of volumetric capnography make the use of predictive equations unnecessary.…”