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![]() Figure 15.1 shows the lungs with blood passing through as normal (Q C), while some bypasses the heart (Q S) and is shunted back into the systemic circulation. Because even a small shunt can have a large effect on arterial PO 2, it is critical to determine the size of a shunt should one be suspected. 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. Calculating the Size of a Pulmonary Shunt. 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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