![]() ![]() 14 The Enghoff index, however, does not reflect lung stress very well in the presence of a high shunt. 12, 13 We hypothesized that the Bohr dead space is a surrogate for lung stress, which is one of the major determinants of ventilator-induced lung injury, because it is derived exclusively from gas sampled from all parts of the lungs. 10, 11 These effects depend mainly on the ventilatory settings and on how the pressures and volumes delivered by the ventilator are distributed within the lungs. It is accepted that positive-pressure ventilation increases (V D aw and (V D alv by dilating the conducting airways and by decreasing pulmonary capillary perfusion at the alveolar–capillary membrane, respectively. 8 Therefore, Bohr dead space ((V D Bohr) and its subcomponents of airway dead space ((V D aw) and alveolar dead space ((V D alv) have become available at the bedside on a breath-by-breath basis. ![]() 7 Today, true dead space according to Bohr's formula can be estimated because modern volumetric capnography can measure mean alveolar P CO 2 in a noninvasive way. Because this P aCO 2-based apparent dead space does not represents any real volume, it is also called fictitious dead space. 4 In the clinical field, V d is commonly estimated applying the Enghoff modification of Bohr's original formula, replacing the alveolar P CO 2 value with the arterial value (P aCO 2) 5, 6 This approach, however, contaminates the true dead space in which alveoli are ventilated but not perfused by the effect of shunting blood through non-ventilated lung areas. 1– 3 Using expired CO 2 as a tracer, volumetric capnography estimates dead space (V d) based on the mass balance equation originally described by Bohr. ![]() Volumetric capnography is a bedside tool to assess lung function and to adjust ventilatory settings in mechanically ventilated patients. ![]()
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