Evaluation of a method for determination of mean transit time of xenon-133 in the lungs.

The purpose of the study was to determine the mean transit time, t, for Xe-133 in the lungs and to compare the results with t for helium determined simultaneously by the helium-dilution technique. Thirteen normal subjects were studied, and four patients with pulmonary disease. No significant difference was observed between the mean transit times for Xe-133 and helium obtained in normal subjects during equilibrium as well as during desaturation. The mean washout time for Xe-133 during desaturation, rho a/h (calculated as the area under the desaturation curve divided by activity at equilibrium), was significantly longer than the mean transit time for He. Similar results were obtained in the patients. Thus it is possible to determine total ventilation per unit volume correctly when the initial washout rate is used, whereas calculations based on rho a/h underestimate V/V. Accordingly, rho a/h should not be used as equivalent to the mean transit time. However, rho a/h might give information of clinical value, especially in patients with chronic obstructive pulmonary disease.