Estimation of pulmonary blood flow fromsinusoidal gas exchange during anaesthesia: a theoreticalstudy

We simulated the use of simultaneoussinusoidal changes of inspired O 2 and N 2 O(Williams et al ., J Appl Physiol , 1994; 76 : 2130–9) at fractional concentrations up to 0.3 and 0.7,respectively, to estimate FRC and pulmonary blood flow (PBF) duringanaesthesia, using O 2 as an insoluble indicator. Hahn'sapproximate equations, which neglect the effect of pulmonary uptake andexcretion on expiratory flow, estimate dead space and alveolar volume( V A ) with systematic errors less than 10%, butyield systematic errors in PBF which are approximately proportional to F i N 2 O in magnitude. Acorrection factor(1 –  P ¯ ) −1 for Hahn's equations for PBF (where P ¯ is the mean partial pressure ofthe soluble indicator) reduces the dependence of PBF estimates on F i N 2 O , and thesolution of equations describing the simultaneous mass balance of bothindicators yields accurate results for a wide range of mean F i N 2 O . However, PBFestimates are sensitive to measurement errors and a third gas must bepresent to ensure that the indicator gases behaveindependently.