Removal of the HME from the circuit significantly decreased V (D)/V (T) (by approximately 6) and P (aCO2) (by approximately 5 mm Hg). The volume of gas in the airways only (ie, gas proximal to the respiratory bronchiole including mouth, nose, and ventilator tubing) represents anatomic dead space (no gas exchange is possible). 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. Physiologic dead space (V (D)/V (T)) was calculated using the Enghoff modification of the Bohr equation. Physiologic dead space The figure depicts dead space. 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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