vq physiology V/Q Ratio "Normal" V/Q = 0.8 V = alveolar ventilation Q = pulmonary blood flow (perfusion) "Normal" V/Q depends on "normal" respiratory rate, tidal volume, and cardiac output PaO2 = 100 mm Hg PaCO2 = 40 mm Hg Ventilation/perfusion matching is essential for ideal gas exchange of O2 and CO2 "ideal" V/Q = 1.0 exercise → ↑ cardiac output → vasodilation of apical arteries V/Q approaches 1.0 Distribution of V/Q in Lung Ventilation and perfusion are nonuniformly distributed in normal, upright lung Zone 1 apex of lung decreased ventilation and decreased perfusion V/Q is highest (= 3.0) because of relatively greater decrease in perfusion wasted ventilation PaO2 is highest and PaCO2 is lowest login to view 1 more bullet Zone 3 base of lung V/Q is lowest (= 0.6) wasted perfusion PaO2 is lowest and PaCO2 is highest V/Q Defects Dead Space (V/Q → ∞) e.g., pulmonary embolism (blood flow obstruction) ventilation of lung regions that are not perfused wasted ventilation physiologic dead space no gas exchange occurs login to view 2 more bullets 100% O2 improves PaO2 Shunt (V/Q = 0) e.g., airway obstruction, dependent portion of lung in ARDS can act as "shunt" perfusion of lung regions that are not ventilated wasted perfusion no gas exchange login to view 1 more bullet 100% O2 does not improve PaO2