Comment on Tusman et al.: Validation of Bohr dead space measured by volumetric capnography

Abstract

Dear Editor, I read with great interest the paper by Tusman et al. on Bohr dead space (VDBohr) measurement from volumetric capnography. Their analysis based on an equation fitting the CO2 expirogram provides a substantial improvement for the objective assessment of its components [1]. Even though it is clear that the use of PaCO2 as a surrogate for ideal alveolar PCO2 in the Bohr equation— as suggested by Enghoff—may lead to overestimation of physiological dead space [2, 3], replacing it for mean alveolar PCO2 (PACO2)—as originally devised by Bohr and recently accomplished by Tusman et al. [1]—will solely account for airway dead space (VDaw) and not for alveolar dead space (VDalv). This is shown by the following set of equations. According to Fowler’s approach, tidal volume (VT) divides into VDaw and alveolar VT (VTalv); VT = VDaw ? VTalv. The mixed expired CO2 fraction (FeCO2) over one breath is the ratio between the expired CO2 volume (VCO2br) and VT; FeCO2 = VCO2br/VT. According to the alveolar gas equation, the mean alveolar CO2 fraction (FACO2) is the ratio between VCO2br and VTalv, or FACO2 = VCO2br/(VT– VDaw). Replacing the terms above in the Bohr equation as suggested by Tusman et al. [1] gives: