Expression used to calculate physiologic dead space for gas and estimate number of alveoli with impaired perfusion contributing to dead space; equation is VD = (FEÐFA) _ VT(FIÐFA), where VD is dead space, FE is fractional concentration of gas in expired gas, FA is fractional concentration of gas in alveolar gas, FI is fractional concentration of gas in inspiratory gas, VT is tidal volume.