Abstract
The alveolar-arterial difference A-aD for Po2, Pco2, and Pn2 have been measured at a number of FiOO2. When FiOO2 was raised from .2 to .4 A-aDo2 and A-aDn2 increased and remained at the same level for any further rise of FiOO2. A-aDco2 did not seem to be affected by the FiOO2 change. In young healthy subjects, mean A-aDo2 reached 30 mm Hg and A-aDn2 8 mm Hg. In a group of older or diseased subjects these values were 55 and 18 mm Hg, respectively. In both groups the A-aDco2 was due to overventilated alveoli among the predominant type of alveolus. Alveoli of the predominant type seem to have a mean ventilation/perfusion ratio (Va/Q) close to the over-all mean value. The increases in A-aDo2 and A-aDn2 at high FiOO2 show the presence of a smaller group of alveoli having a very low, if not undeterminable, Va/Q ratio. The size of A-aDn2 as compared to the size of A-aDo2 proves that the real anatomical shunt is not as large as has been thought from measurement of the A-aDo2 alone. At least 38–45% of the total A-aDo2 is caused by very poorly ventilated alveoli. pulmonary shunts; gas partial pressures in blood Submitted on March 8, 1963
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