Pulmonary blood flow, diffusing, capacity and tissue volume by rebreathing: theory

The determination of pulmonary capillary blood flow (Q), diffusing capacity (D) and tissue volume (Vtis) from rebreathing equilibration kinetics is studied on mathematical models. The validity of a continuously ventilated two-compartment model (model II) is analyzed with reference to a cyclically ventilated model (model III) and compared to a one-compartment model (model I). Use of model II leads to slight overestimation of Q and D, and to underestimation of Vtis whereas model I yields considerable underestimation of D, Q and Vtis. The following procedures are recommended: (1) Rebreathing data should be analyzed on the basis of model II. (2) Q and D should be calculated from the rate constant of fast (k1) and slow (k2) exponential component and extrapolated zero-time intercept of K2, based on end-expired gas fractions, of the gas under study (e.g. C2H2, CO). (3) Addition of an insoluble inert gas (e.g. He) is required only for Vtis. (4) Soluble inert gases with blood-gas partition coefficient (lambda) less than 1 (e.g. C2H2, CHC1F2) applied at low concentration should be used for determination of Q and Vtis. (5) A high rebreathing ventilation is recommended.