Assessment of Bohr and Enghoff Dead Space Equations in Mechanically Ventilated Children.

Recent findings suggest that using alveolar P_CO2 (P_ACO2 ) estimated by volumetric capnography in the Bohr equation instead of P_aCO2 (Enghoff modification) could be appropriate for the calculation of physiological dead space to tidal volume ratio (V_D/V_{T Bohr}and V_D/V_{T Enghoff}, respectively). We aimed to describe the relationship between these 2 measurements in mechanically ventilated children and their significance in cases of ARDS.

The authors of the study enrolled children from June 2013 to December 2013, who were mechanically ventilated for various respiratory conditions. 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.

Thirty-four subjects were analyzed. Mean V_D/V_{T Bohr} was 0.39 ± 0.12, and V_D/V_{T Enghoff} was 0.47 ± 0.13 (P = .02). The difference between V_D/V_{T Bohr} and V_D/V_{T Enghoff} was correlated with P_aO2 /F_IO2 and with S2/S3. In subjects without lung disease (P_aO2 /F_IO2 ≥ 300), mean V_D/V_{T Bohr} was 0.36 ± 0.11, and V_D/V_{T Enghoff} was 0.39 ± 0.11 (P = .056). Two children with status asthmaticus had a major difference between V_D/V_{T Bohr} and V_D/V_{T Enghoff} in the absence of a low P_aO2 /F_IO2 .

The authors concluded that this study suggests that V_D/V_{T Bohr} and V_D/V_{T Enghoff} are not different when there is no hypoxemia (P_aO2 /F_IO2 > 300) except in the case of status asthmaticus. In subjects with a low P_aO2 /F_IO2 , the method to measure V_D/V_T must be reported, and results cannot be easily compared if the measurement methods are not the same.

Comment by Bhavani Shankar Kodali MD

In future, the application of determining physiological dead space non invasively will increase in intensive care patients. There are many studies that have assessed this relationship under various circumstances.