The effect of aging and cardiorespiratory fitness on the lung diffusing capacity response to exercise in healthy humans

Aging is associated with deterioration in the structure and function of the pulmonary circulation. We characterized the lung diffusing capacity for carbon monoxide (DL_CO), alveolar-capillary membrane conductance (Dm_CO), and pulmonary-capillary blood volume (Vc) response to discontinuous incremental exercise at 25, 50, 75, and 90% of peak work (W_peak) in four groups: 1) Young [27 ± 3 yr, maximal oxygen consumption (VO_2max):110 ± 18% age predicted]; 2) Young Highly Fit (27 ± 3 yr, VO_2max: 147 ± 8% age predicted); 3) Old (69 ± 5 yr, VO_2max: 116 ± 13% age predicted); and 4) Old Highly Fit (65 ± 5 yr,VO_2max: 162 ± 18% age predicted). At rest and at 90% W_peak, DL_CO, Dm_CO, and Vc were decreased with age. At 90% W_peak, DL_CO, Dm_CO, and Vc were greater in Old Highly Fit vs. Old adults. The slope of the DL_CO-cardiac output (Q) relationship from rest to end exercise at 90% W_peak was not different between Young, Young Highly Fit, Old, and Old Highly Fit (1.35 vs. 1.44 vs. 1.10 vs. 1.35 ml_CO·mmHg^-1·liter blood^-1, P = 0.388), with no evidence of a plateau in this relationship during exercise; this was also true for Dm_CO-Q and Vc-Q. VO_2max was positively correlated with 1) DL_CO, Dm_CO, and Vc at rest; and 2) the rest to end exercise change in DL_CO, Dm_CO, and Vc. In conclusion, these data suggest that despite the age-associated deterioration in the structure and function of the pulmonary circulation, expansion of the pulmonary capillary network does not become limited during exercise in healthy individuals regardless of age or cardiorespiratory fitness level.