Currently, dead space is measured at the bedside by volumetric capnography, which reports expired CO 2 elimination as a function of expired VT, and VD/VTphys is calculated using the Enghoff’s modification of Bohr’s original equation: VD/VTphys = (PaCO 2 – PECO 2)/PaCO 2, where PaCO 2 is the arterial partial pressure of CO 2 obtained by arterial blood sampling and PECO 2 is an estimate of mixed expired partial pressure of CO 2 obtained from the mid-portion of phase III of the volumetric capnogram. The physiologic ventilatory dead space fraction (VD/VTphys) is usually defined as the fraction of tidal volume (VT) that does not participate in gas exchange. Dead space comprises two separate components: airway dead space (the volume of areas that do not contribute to gas exchange) and alveolar dead space (the volume of well-ventilated alveoli that receive minimal blood flow). Dead space refers to lung areas that are ventilated but not perfused.
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