The A-VO2 Diff was not measured by direct oximetry. Although desirable, this would have involved invasive catheterization of pulmonary and systemic arteries during exhaustive upright exercise at baseline and again at follow-up of elderly outpatients
and would have involved significant participant burden and risk. Instead, A-VO2 Diff was estimated using the Fick equation where VO2 measured by expired gas analysis is divided by measured CO.