Utility of technetium-99m-DTPA in determining regional ventilation

The goal of this study was to determine the usefulness of radiolabeled aerosols in the assessment of regional ventilation in tracheotomized patients maintained on mechanical ventilation.

Three commercially available radioaerosol nebulizer kits were studied on the bench to determine nebulizer efficiency and particle distribution of 99mTc-DTPA aerosols. We studied ventilated tracheotomized human subjects with a gamma camera and simultaneously measured regional ventilation with 81mKr gas and 99mTc-DTPA aerosol. Images were compared by analysis of radioactivity distributions in computer-generated regions of interest.

The UltraVent nebulizing system produced the smallest particles with a mass median aerodynamic diameter of 0.9 micron compared to the AeroTech I and Venti-Scan II systems, which both produced aerosols of 1.3 microns. Despite relatively small particle sizes, 99mTc-DTPA deposition images with the UltraVent nebulizer did not accurately represent regional ventilation as measured by 81mKr equilibrium. Visual inspection of images revealed significant amounts of particle deposition in the region of the trachea which was diminished but not eliminated following replacement of the tracheotomy tube inner cannula. Based on regional analysis, correlation between radioactivity distributions of both isotopes was poor (r = 0.262, p = 0.162) with segmental analysis suggesting that the upper and middle lung regions were significantly affected by residual tracheal activity.

The lungs of patients maintained on mechanical ventilation can be imaged after the inhalation of 99mTc-DTPA from commercially available delivery kits, but the correlation between aerosol deposition and regional ventilation is poor. Better definition of ventilated lung segments is obtained when using a gas such as 81mKr because tracheal activity with the radiolabeled gas is minimized.