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

BACKGROUND: Calculation of physiologic dead space (dead space divided by tidal volume [VD/VT]) using the Enghoff modification of the Bohr equation requires measurement of the partial pressure of mean expired CO2 (PĒCO2) by exhaled gas collection and analysis, use of a metabolic analyzer, or use of a volumetric CO2 monitor. The Dräger XL ventilator is equipped with integrated volumetric CO2 monitoring and calculates minute CO2 production (V̄CO2). We calculated PĒCO2 and VD/VT from ventilator derived volumetric CO2 measurements of V̄CO2 and compared them to metabolic analyzer and volumetric CO2 monitor measurements. METHODS: A total of 67 measurements in 36 subjects recovering from acute lung injury or ARDS were compared. Thirty-one ventilator derived measurements were compared to measurements using 3 different metabolic analyzers, and 36 ventilator derived measurements were compared to measurements from a volumetric CO2 monitor. RESULTS: There was a strong agreement between ventilator derived measurements and metabolic analyzer or volumetric CO2 monitor measurements of PĒCO2 and VD/VT. The correlations, bias, and precision between the ventilator and metabolic analyzer measurements for PĒCO2 were r = 0.97, r2 = 0.93 (P < .001), bias −1.04 mm Hg, and precision ± 1.47 mm Hg. For VD/VT the correlations were r = 0.95 and r2 = 0.91 (P < .001), and the bias and precision were 0.02 ± 0.04. The correlations between the ventilator and the volumetric CO2 monitor for PĒCO2 were r = 0.96 and r2 = 0.92 (P < .001), and the bias and precision were −0.19 ± 1.58 mm Hg. The correlations between the ventilator and the volumetric CO2 monitor for VD/VT were r = 0.97 and r2 = 0.95 (P < .001), and the bias and precision were 0.01 ± 0.03. CONCLUSIONS: PĒCO2, and therefore VD/VT, can be accurately calculated directly from the Dräger XL ventilator volumetric capnography measurements without use of a metabolic analyzer or volumetric CO2 monitor.