what does it mean to physiologically ineffective in terms of dead space and shunted blood

Effective AaD O2 functions as the indicator for identifying regions with low V A/ Q values, including right-to-left shunt ( V A/ Q = zero), while physiological V D/ V T is useful for detecting regions with high V A/ Q values, including alveolar dead space ( V A/ Q = infinite). When effective P ACO2 (= P aCO2) is known, other indicators, including effective P AO2, effective AaD O2, and physiological V D/ V T, can be easily decided. If this assumption is not adopted, measurements of P O2 and P CO2 in mixed venous blood, which are only possible through right heart catheterization, are indispensable for determining representatives of P AO2 and P ACO2. Key assumption allowing classical gas exchange theory to be applied for clinical practice is that effective P ACO2 can be replaced with arterial P CO2 (P aCO2) that is measured routinely at bedside. In classical gas exchange theory, effective alveolar gas compositions (effective P AO2 and P ACO2), effective AaD O2, and physiological V D/ V T are clinically important while diagnosing gas exchange abnormalities concerning O 2 and CO 2 in a first approximation. On the other hand, effective alveolar–arterial P O2 difference (effective AaD O2) and physiological dead space ( V D/ V T), both of which are appraised as a base of the classical gas exchange theory, can be measured with ease. Although determination of ventilation–perfusion ( V A/ Q) distribution and/or simultaneous determination of V A/ Q and diffusing capacity–perfusion ( D/ Q) distribution are needed to precisely diagnose gas exchange abnormalities occurring in the lung periphery, these analyses cannot be straightforwardly performed in clinical situation.

What does it mean to physiologically ineffective in terms of dead space and shunted blood