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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).
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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).
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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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