Indicator Dilution Measurements of Lung Volumes and Alveolar Air Exchange During Breathing

Herbert B. Hechtman; Michael H. Reid; Barry C. Dorn; Richard D. Weisel

doi:10.1172/JCI107289

Indicator Dilution Measurements of Lung Volumes and Alveolar Air Exchange During Breathing

Herbert B. Hechtman, Michael H. Reid, Barry C. Dorn, and Richard D. Weisel

Published May 1, 1973 - More info

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Abstract

A new triple tracer indicator dilution technique has been used to measure alveolar ventilation as well as air and tissue volumes in the lungs of experimental animals and man. The tracers indocyanine green, [¹²¹I]antipyrine and xenon-133 were rapidly injected into the right atrium, while sampling was carried out from a peripheral artery.

Blood flow and tissue volumes were obtained by classical analysis of the indocyanine green and antipyrine concentration-time curves. A double exit-port, constant air flow model was used to analyze the xenon curves, because ventilatory loss led to incomplete recovery of the gas tracer in effluent blood. Uniform ventilation and perfusion were assumed. This analysis permitted calculation of alveolar ventilation (V̇A_{X_e}) and functional residual capacity (FRC_Xe) during normal breathing.

In control studies, V̇A_{X_e} was similar to V̇A_co₂, obtained with the steady-state CO₂ method (r = 0.87), while in critically ill patients the xenon measurement was significantly lower, averaging 54% of V̇A_co₂. In theory, underestimates in V̇A_{X_e} and decrease in the ratio V̇A_{X_e}/V̇A_co₂ relate to nonuniformity in regional ventilation and perfusion. The effect is greatest for the slightly soluble gas, xenon. The significant inverse correlation between V̇A_{X_e}/V̇A_co₂ and the physiologic shunt is consistent with this postulate.

FRC_Xe was similar to the predicted FRC in animals but was 76% of the helium measured FRC in patients. FRC_Xe was significantly lower than the xenon measured air volumes during breath-holding when nonuniformity of ventilation was not operative. Lung tissue volumes in animals were 83% of gravimetric lung weights, while in patients the volumes were much lower than predicted. Nonhomogeneous lung function, including failure to perfuse the entire capillary bed, with resultant incomplete penetration of tracers into all segments of lung air and tissue, may explain these findings. The resultant errors can be significant in sick patients, and may themselves be used to study nonhomogeneities in the distribution of ventilation and volume.

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