Comparison of estimates of cardiac output by indicator dilution and freon 22 uptake during gas mixing in dogs

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Abstract
Study objective — The aim was to measure cardiac output while rebreathing tidal volumes, by correction of soluble gas uptake for gaseous mixing. Design — Simultaneous measurements of cardiac output by indocyanin green and freon 22 uptake during rebreathing were made. Mixing for a hypothetical gas of identical gaseous diffusivity to freon 22 was calculated by interpolation between concentrations of two insoluble gases, helium and sulphur hexafluoride. Mixing efficiency was estimated by the number of breaths for helium to become 99% equilibrated with lung gas (n99-He). Experimental material — Five anaesthetised dogs rebreathed at intervals with 300 ml of test gas. Measurements and main results — 63 comparisons of cardiac output using indocyanin green and freon 22 uptake (over breaths 7-13 using the mean mixed volume of distribution), gave a mean (95% confidence interval) underestimation of 0.345 (0.093-0.597) litre·min−1 (14%). Exclusion of 12 points in which n99-He was greater than 15 resulted in a mean underestimation of 0.052(−0.163·0.267) litre·min−1(2%). Without correction for gaseous mixing, freon 22 uptake for these data overestimated blood flow by a mean of 1.31 litre·min−1 (overestimation = 2.7 over breaths 5-11). Use of the equilibrium volume of distribution resulted in an overestimation of blood flow relative to green dye of 1.2 litre·min−1 (breaths 5-11) and 0.76 litre·min−1 (breaths 7-13). Conclusions — Estimates of cardiac output by soluble gas uptake are optimal when correction is made for mixing of gas of identical diffusivity. The mean mixed gas volume gives the best correlation with the reference method, implying a selective distribution of blood flow to the better ventilated areas.

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