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. 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.

It’s often referred to as wasted ventilation, as the inhaled air is not involved in the exchange of oxygen and carbon dioxide. 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. by John Landry, BS, RRT Dead space ventilation is defined as the volume of ventilated air that does not participate in gas exchange.

Dead space ventilation perfusion ratio