We conducted a prospective observational study to compare Bohr's and Enghoff's measurements of V D/V T in mechanically ventilated children, and we hypothesized that a difference between these 2 measurements may be observed in cases of lung injury. Especially in the case of lung injury, comparison of V D/V T Bohr and V D/V T Enghoff may have complementary physiological meaning, as recently suggested 9, 10: Bohr's equation estimates the true dead space (ie, high ventilation/perfusion units plus anatomical and mechanical dead space), whereas Enghoff's estimates not only the dead space but also the shunting and low V̇/Q̇ regions of the lungs ( Fig. P aCO 2 (Enghoff modification) can be used as well (V D/V T Enghoff). Recent findings suggest that using P ACO 2 estimated by volumetric capnography can be appropriate to calculate the Bohr physiological dead space/tidal volume ratio (V D/V T Bohr). 2 The dead space on V T (V D/V T) ratio informs caregivers as to the effect of therapeutic procedures such as prone positioning, 3 surfactant administration, 4 or lung recruitment maneuvers 5, 6 and provides information useful in prognostication, depending on the severity of lung disease in adults and children. 1 Currently, physiologic dead space measurement is used by clinicians in the management of mechanical ventilation because CO 2 removal is inversely proportional to V D/V T, and V D fluctuates considerably, depending upon the severity of lung disease. 1 Later, Enghoff proposed a simplification of Bohr's formulae to calculate the physiologic dead space ratio at the bedside using arterial P CO 2 (P aCO 2) instead of P ACO 2. Employing the law of mass conservation, Bohr proposed a formula using alveolar P CO 2 (P ACO 2) to estimate physiologic dead space, expressed as a ratio of dead space volume (V D) to tidal volume (V T). Lung physiologic dead space (V D) is defined as the wasted tidal volume during respiration (ie, the volume remaining in the conducting airways and in poorly perfused and non-perfused alveoli that are not participating in gas exchange).
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