A rebreathing method for estimating pulmonary D o2: Theory and measurements in dog lungs

The equilibration of a rebreathed gas volume with mixed venous blood with respect to p o 2 is investigated in theory on the basis of two simplified models of the system, which consists of a single, perfused alveolar space and a rebreathing container. In Model I (without ventilation limitation) the approach of rebreathing gas P o 2, to equilibrium turns out to be mono-exponential. Its rate constant is determined by the total gas volume, the blood flow, the slope of the blood dissociation curve, and by the diffusing capacity for O 2, Do o 2. In Model II (with ventilation limitation) the equilibration kinetics is shown to be bi-exponential. The rate constants are additionally influenced by the partitioning of the total gas volume to the lung and the rebreathing container, and by the ventilation between these. The D o 2, value can be obtained in either Model from the P o 2, equilibration kinetics, provided all other factors are known. The theory developed for the Models was applied to measurements performed in rebreathing experiments on excised blood-perfused dog lung lobes and on anesthetized dogs. The following mean D o 2, values, calculated according to Model II, were found: 0.049 mmol · min −1 · torr −1 in excised lower lung lobes of dogs (mean body weight 9.5 kg), 0.85 mmol · min −1 · torr −1 in anesthetized dogs (mean weight 23 kg). These values are in the range of the D o 2, values obtained by conventional steady state techniques.