BACKGROUND: Calculation of physiologic dead space (dead space divided by tidal volume [V D / V T ]) using the Enghoff modification of the Bohr equation requires measurement of the partial pressure of mean expired CO 2 (P E CO 2 ) by exhaled gas collection and analysis, use of a metabolic analyzer, or use of a volumetric CO 2 monitor. The Dräger XL ventilator is equipped with integrated volumetric CO 2 monitoring and calculates minute CO 2 production (V CO 2 ). We calculated P E CO 2 and V D /V T from ventilator derived volumetric CO 2 measurements of V CO 2 and compared them to metabolic analyzer and volumetric CO 2 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 CO 2 monitor. RESULTS: There was a strong agreement between ventilator derived measurements and metabolic analyzer or volumetric CO 2 monitor measurements of P E CO 2 and V D /V T. The correlations, bias, and precision between the ventilator and metabolic analyzer measurements for P E CO 2 were r ؍ 0.97, r 2 ؍ 0.93 (P < .001), bias ؊1.04 mm Hg, and precision ؎ 1.47 mm Hg. For V D /V T the correlations were r ؍ 0.95 and r 2 ؍ 0.91 (P < .001), and the bias and precision were 0.02 ؎ 0.04. The correlations between the ventilator and the volumetric CO 2 monitor for P E CO 2 were r ؍ 0.96 and r 2 ؍ 0.92 (P < .001), and the bias and precision were ؊0.19 ؎ 1.58 mm Hg. The correlations between the ventilator and the volumetric CO 2 monitor for V D /V T were r ؍ 0.97 and r 2 ؍ 0.95 (P < .001), and the bias and precision were 0.01 ؎ 0.03. CONCLUSIONS: P E CO 2 , and therefore V D /V T , can be accurately calculated directly from the Dräger XL ventilator volumetric capnography measurements without use of a metabolic analyzer or volumetric CO 2 monitor.
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