Validation of Bohr dead space measured by volumetric capnography

Bohr's dead space (VD(Bohr)) is commonly calculated using end-tidal CO(2) instead of the true alveolar partial pressure of CO(2) (PACO(2)). The aim of this work was to validate VD(Bohr) using PACO(2) derived from volumetric capnography (VC) against VD(Bohr) with PACO(2) values obtained from the standard alveolar air formula.

Expired gases of seven lung-lavaged pigs were analyzed at different lung conditions using main-stream VC and multiple inert gas elimination technique (MIGET). PACO(2) was determined by VC as the midpoint of the slope of phase III of the capnogram, while mean expired partial pressure of CO(2) (PeCO(2)) was calculated as the mean expired fraction of CO(2) times the barometric minus the water vapor pressure. MIGET estimated expired CO(2) output (VCO(2)) and PeCO(2) by its V/Q algorithms. Then, PACO(2) was obtained applying the alveolar air formula (PACO(2) = VCO(2)/alveolar ventilation).

We found close linear correlations between the two methods for calculating both PACO(2) (r = 0.99) and VD(Bohr) (r = 0.96), respectively (both p < 0.0001). Mean PACO(2) from VC was very similar to the one obtained by MIGET with a mean bias of -0.10 mmHg and limits of agreement between -2.18 and 1.98 mmHg. Mean VD(Bohr) from VC was close to the value obtained by MIGET with a mean bias of 0.010 ml and limits of agreement between -0.044 and 0.064 ml.

VD(Bohr) can be calculated with accuracy using volumetric capnography.