Oxygen diffusing capacity estimates derived from measured V̇A/Q̇ distributions in man

Data from eighteen subjects, studied in hypoxia (minimum PI_O2 = 80 Torr) both at rest and during exercise, were analyzed using computer models which estimate O₂ diffusing capacity from measured V̇A/Q̇ distributions (obtained using the multiple inert gas elimination technique 'MIGET') and measured O₂ exchange. Two of these models assigned the distribution of the diffusing capacity (D) in proportion to either the perfusion (DL_O2-Vwt) distributions from MIGET, and thus modeled the effects of V̇A/Q̇ and D/Q̇β(where Q̇β is the perfusive conductance inequalities respectively. The third model (DL_O2-3C) assigned all the diffusing capacity to a single homogeneous compartment. At rest DL_O2 was 41.1 ± 41.4 ± 5.4 and 30.2 ± 2.1 ml · min^-1 · Torr^-1 for the Qwt, Vwt and 3C models respectively. These rose to 93.7 ± 2.6, 109.3 ± 4.5 and 81.1 ± 1.9 ml · min^-1 · Torr^-1 respectively at maximal exercise, all significantly different from rest (P < 0.001 for each). The effects of measured V̇A/Q̇ and theoretical D/Q̇β inhomogeneities on diffusing capacity estimates were significant even in normal lungs. Both types of inequality caused an appreciable underestimation of DL_O2. These multi-compartment model estimates, using real data, are consistent with published theoretical predictions of the effects of V̇, Q̇ and D inequalities. These results during exercise come close to morphometric predictions of maximal oxygen diffusing capacity in man.