• Mark S Siobal, Hannah Ong, and Josephine Valdes.
• Respiratory Care Services, Department of Anesthesia and Perioperative Care, San Francisco General Hospital, University of California-San Francisco, CA 94110, USA. msiobal@sfghsom.ucsf.edu
• Respir Care. 2013 Jul 1;58(7):1143-51.

BackgroundCalculation 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 (VCO2). We calculated PĒCO2 and VD/VT from ventilator derived volumetric CO2 measurements of VCO2 and compared them to metabolic analyzer and volumetric CO2 monitor measurements.MethodsA 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.ResultsThere 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, r(2) = 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 r(2) = 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 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 CO2 monitor for VD/VT were r = 0.97 and r(2) = 0.95 (P < .001), and the bias and precision were 0.01 ± 0.03.ConclusionsPĒCO2, 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.

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