effective ventilation determines alveolar CO2
when balanced,
CO2 production rate=CO2 exhalation rate=alveolar CO2 x effective alveolar minute ventilation
alveolar CO2 = (CO2 production rate) / (effective alveolar minute ventilation)
effective perfusion & effective ventilation determines venous CO2
(venous CO2-alveolar CO2)x effective perfusion=CO2 production rate
venous CO2= alveolar CO2 + (CO2 production rate)/(effective perfusion Q)
=(CO2 production rate)[1/(effective Q)+1/(effective alveolar minute V)]
effective perfusion,ventilation & shunt determines arterial CO2
arterial CO2=shunt fraction x venous CO2 + (1-shunt fraction)x alveolar CO2
=(CO2 production rate)[ (shunt fraction)/(effective Q) + 1/(effective alveolar minute V) ]
When shunt fraction is small, cardiovascular system may compensate by increase cardiac output to keep the same effective lung perfusion. When shunt fraction is large to some extent, effective perfusion would decrease and leads to elevated venous and arterial CO2. In such condition, however, oxygen desaturation would develop before CO2 retention happens.
