Comparison of Bohr and Enghoff Methods for Assessing Ventilatory Inefficiency in Mechanically Ventilated Intensive Care Unit Patients

Abstract Background and Objective: Dead space fraction (VD/VT) is widely used to assess ventilatory efficiency in mechanically ventilated patients. The Enghoff modification of the Bohr equation incorporates arterial CO 2 and reflects not only alveolar dead space but also ventilation–perfusion mismatch and shunt, serving as an index of global gas exchange inefficiency. This study compared VD/VT calculated using Bohr’s equation and Enghoff’s modification in patients with relatively normal and diseased lungs. Methods: This cross-sectional study was conducted in a multidisciplinary intensive care unit over 18 months. Adult patients (≥18 years) requiring invasive mechanical ventilation were enrolled. Patients categorized as having “normal lungs” had no clinical or radiological evidence of primary pulmonary pathology on examination, chest radiography, or lung ultrasound; however, this cohort represents relative non-pulmonary disease rather than physiologically normal lungs. Volumetric capnography measured PACO 2 and PECO 2 and arterial blood gas analysis provided Paco 2 . VD/VT was calculated using both equations. Diseased lungs ( n = 23) were compared with the relatively normal group ( n = 27). Results: Enghoff-derived VD/VT was significantly higher than Bohr-derived VD/VT in both the relative normal group (mean difference 0.091 ± 0.098; P < 0.001) and the diseased lung group (mean difference 0.156 ± 0.199; P < 0.01). Conclusion: Enghoff-derived VD/VT consistently exceeded Bohr-derived VD/VT, indicating a broader measure of gas-exchange inefficiency rather than true physiological dead space. Bohr-derived VD/VT more specifically reflects alveolar dead space; however, findings should be interpreted cautiously given the exploratory design and potential ventilator-related confounding.