Corrections of Enghoff's dead space formula for shunt effects still overestimate Bohr's dead space

Abstract

Dead space ratio is determined using Enghoff's modification (VdB-E/Vt) of Bohr's formula (VdBohr/Vt) in which arterial is used as a surrogate of alveolar PCO2. In presence of intrapulmonary shunt Enghoff's approach overestimates dead space. In 40 lung-lavaged pigs we evaluated the Kuwabara's and Niklason's algorithms to correct for shunt effects and hypothesized that corrected VdB-E/Vt should provide similar values as VdBohr/Vt.We analyzed 396 volumetric capnograms and arterial and mixed-venous blood samples to calculate VdBohr/Vt and VdB-E/Vt. Thereafter, we corrected the latter for shunt effects using Kuwabara's (K) VdB-E/Vt and Niklason's (N) VdB-E/Vt algorithms. Uncorrected VdB-E/Vt (mean±SD of 0.70±0.10) overestimated VdBohr/Vt (0.59±0.12) (p<0.05), over the entire range of shunts. Mean (K) VdB-E/Vt was significantly higher than VdBohr/Vt (0.67±0.08, bias −0.085, limits of agreement −0.232 to 0.085; p<0.05) whereas (N)VdB-E/Vt showed a better correction for shunt effects (0.64±0.09, bias 0.048, limits of agreement −0.168 to 0.072; p<0.05).Neither Kuwabara's nor Niklason's algorithms were able to correct Enghoff's dead space formula for shunt effects.