Determination of the mean transit time for the transport of aerosolized99mTc-DTPA across the pulmonary epithelial membrane. A plasma sample method

Abstract

During the last decade it has been customary to estimate pulmonary epithelial permeability (P-P) as the pulmonary clearance of inhaled nebulized 99mTc-DTPA from a time-activity (t-a) curve registered externally by monitoring over the chest. The t-a curve, however, is not defined by the degree of P-P alone but also by factors such as the deposition of the 99mTc-DTPA in the lungs. To avoid these problems a plasma sample method was derived. It describes P-P by the mean transit time (.hivin.t) for the transport of 99mTc-DTPA across the pulmonary epithelial membrane, .hivin.t(L). The calculation of .ovrhdot.t(L) involves two steps. Following the inhalation of Tc-DTPA the plasma t-a curve is defined and used to calculate .hivin.t for the transport for 99mTc-DTPA across the pulmonary epithelial membrane, through the ECV and until elimination by the kidneys, .hivin.t(L + ECV). Subsequently, 99mTc-DTPA is injected as a bolus i.v. and the new t-a curve is used to calculate .hivin.t for the transport of 99mTc-DTPA through ECV to the kidneys, .hivin.tI(ECV). Finally .hivin.t(L) = -.hivin.t(L + ECV) .hivin.t(ECV). We applied the method in nine non-smoking individuals (median age 25.5 years, range 20-28) and compared the results oto .hivin.t as calculated from the initial slope of an externally derived t-a curve. The individual .hivin.t(L) values were systemically greater than those of the external detection method (P < 0.05). It is concluded that the initial slope method overestimates total PCl as measured by the plasma sample method. When choosing between the two methods, special interest should be paid to the inherent problems of the methods.