![]() ![]() Venous admixture is that amount of mixed venous blood which would have to be added to ideal pulmonary end-capillary blood to explain the observed difference between pulmonary end-capillary PO2 and arterial PO2. 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. However, pathology reports from COVID-19 infected lungs also frequently demonstrate pulmonary vasculature involvement, including severe endothelial injury. Shunt is the volume of blood which enters the systemic arterial circulation without participating in gas exchange. ![]() Physiologic dead space is ventilation of poor perfused alveoli. Shunt is perfusion of poorly ventilated alveoli. 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. There are 2 types of mismatch: dead space and shunt. 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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