Estimation of pulmonary blood flow from sinusoidal gas exchange during anaesthesia: a theoretical study

We simulated the use of simultaneous sinusoidal changes of inspired O-2 and N2O (Williams et al., I Appl Physiol, 1994; 76: 2130-9) at fractional conc entrations up to 0.3 and 0.7, respectively, to estimate FRC and pulmonary b lood flow (PBF) during anaesthesia, using O-2 as an insoluble indicator. Ha hn's approximate equations, which neglect the effect of pulmonary uptake an d excretion on expiratory flow, estimate dead space and alveolar volume (V- A) with systematic errors less than 10%, but yield systematic errors in PBF which are approximately proportional to FIN2O in magnitude. A correction f actor (1 - (P) over bar)(-1) for Hahn's equations for PBF (where (P) over b ar is the mean partial pressure of the soluble indicator) reduces the depen dence of PBF estimates on FIN2O, and the solution of equations describing t he simultaneous mass balance of both indicators yields accurate results for a wide range of mean FIN2O However, PBF estimates are sensitive to measure ment errors and a third gas must be present to ensure that the indicator ga ses behave independently.