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A B S T R A C T A new triple tracer indicatorIn control studies, VAxC was similar to VACo2, obtained with the steady-state CO2 method (r = 0.87), while in critically ill patients the xenon measurement was significantly lower, averaging 54% of VAco2. In theory, underestimates in VAxC and decrease in the ratio VAX,/VAco2 relate to nonuniformity in regional ventilation and perfusion. The effect is greatest for the slightly soluble gas, xenon. The significant inverse correlation between .VAxe/ViAco2 and the physiologic shunt is consistent with this postulate.FRCxe was similar to the predicted FRC in animals but was 76% of the helium measured FRC in patients. FRCxe 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%7o 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

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