BACKGROUND: Recent findings suggest that using alveolar PCO2 (PACO2) estimated by volumetric capnography in the Bohr equation instead of PaCO2 (Enghoff modification) could be appropriate for the calculation of physiological dead space to tidal volume ratio (VD/VT Bohr and VD/VT Enghoff, respectively). We aimed to describe the relationship between these 2 measurements in mechanically ventilated children and their significance in cases of ARDS.METHODS: From June 2013 to December 2013, mechanically ventilated children with various respiratory conditions were included in this study. Demographic data, medical history, and ventilatory parameters were recorded. Volumetric capnography indices (NM3 monitor) were obtained over a period of 5 min preceding a blood sample. Bohr's and Enghoff's dead space, S2 and S3 slopes, and the S2/S3 ratio were calculated breath-by-breath using dedicated software (FlowTool). This study was approved by Ste-Justine research ethics review board.RESULTS: Thirty-four subjects were analyzed. Mean VD/VT Bohr was 0.39 ± 0.12, and VD/VT Enghoff was 0.47 ± 0.13 (P = .02). The difference between VD/VT Bohr and VD/VT Enghoff was correlated with PaO2/FIO2 and with S2/S3. In subjects without lung disease (PaO2/FIO2 ? 300), mean VD/VT Bohr was 0.36 ± 0.11, and VD/VT Enghoff was 0.39 ± 0.11 (P = .056). Two children with status asthmaticus had a major difference between VD/VT Bohr and VD/VT Enghoff in the absence of a low PaO2/FIO2.CONCLUSIONS: This study suggests that VD/VT Bohr and VD/VT Enghoff are not different when there is no hypoxemia (PaO2/FIO2 > 300) except in the case of status asthmaticus. In subjects with a low PaO2/FIO2, the method to measure VD/VT must be reported, and results cannot be easily compared if the measurement methods are not the same.