Measurement of ventilation/perfusion distribution with alveolar-arterial differences

Abstract

The alveolar-arterial difference A-aD for Po₂, Pco₂, and Pn₂ have been measured at a number of Fi_OO2. When Fi_OO2 was raised from .2 to .4 A-aDo₂ and A-aDn₂ increased and remained at the same level for any further rise of Fi_OO2. A-aDco₂ did not seem to be affected by the Fi_OO2 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 Fi_OO2 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