Calculation of physiologic dead space: Comparison of ventilator volumetric capnography to measurements by metabolic analyzer and volumetric CO2monitor

Abstract

Calculation of physiologic dead space (dead space divided by tidal volume [VDVT]) using the Enghoff modification of the Bohr equation requires measurement of the partial pressure of mean expired CO2 (PECO2) 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 (VCO We calculated PECO2 and VDVT from ventilator derived volumetric CO2 measurements of VCO2 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 PECO2 and VD/VT. The correlations, bias, and precision between the ventilator and metabolic analyzer measurements for PECO2 were r 0.97, r2 0.93 (P < 001), and the bias and precision were 0.02±0.04. The correlations between the ventilator and the volumetric CO2monitor for PECO2 were r 0.96 and r2 0.92 (P