![]() ![]() After all, physiological dead space is simply the difference between arterial and mixed expired pCO 2 divided by the arterial pCO 2. While this is not an entirely novel discovery (West performed very similar calculations in 1969 as did Mecikalski and colleagues in 1984), it is well worth having Niklason and colleagues remind us that physiological dead space not only can be caused by the development of regions with a high ventilation/perfusion ratio ( V ˙ A / Q ˙ but also can come from areas of low V ˙ A / Q ˙ and a shunt. They also show that the increase in pCO 2 can be avoided by even modest increases in total alveolar ventilation (although the calculated dead space will remain elevated). This will increase the calculated physiological dead space accordingly (above that normally present due to the volume of air in the conducting airways). Depending on the disease condition, additional mechanisms that can contribute to an elevated physiological dead space measurement include shunt, a substantial increase in overall V'A/Q' ratio, diffusion impairment, and ventilation delivered to unperfused alveolar spaces.In the previous issue of Critical Care, Niklason and colleagues use computer modeling to point out that blood flowing through unventilated regions of the lung (a shunt) will increase arterial partial pressure of carbon dioxide (pCO 2) if ventilation remains constant. For the range of physiological abnormalities associated with an increased physiological dead space measurement, increased alveolar ventilation/perfusion ratio (V'A/Q') heterogeneity has been the most important pathophysiological mechanism. Although a frequently cited explanation for an elevated dead space measurement has been the development of alveolar regions receiving no perfusion, evidence for this mechanism is lacking in both of these disease settings. An elevated physiological dead space, calculated from measurements of arterial CO2 and mixed expired CO2, has proven to be a useful clinical marker of prognosis both for patients with acute respiratory distress syndrome and for patients with severe heart failure. ![]()
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