The Science Journal of the American Association for Respiratory Care

1995 OPEN FORUM Abstracts

DETERMINATION OF REGIONAL VENTILATION IN MECHANICALLY VENTILATED PATIENTS: USEFULNESS OF ^{99M}TC-DTPA AEROSOL

Corazon J Cabahug MD, Michael McPeck RRT, Lucy B Palmer MD, Ann Cuccia RRT, Harold L Atkins MD & Gerald C Smaldone MD PhD. Departments of Radiology, Respiratory Care & Medicine, SUNY at Stony Brook.

In spontaneously breathing patients, previous investigators have claimed that regional ventilation (RV) can be estimated by deposition images following inhalation of radiolabeled aerosols. The goal of this study was to determine the usefulness of radiolabeled aerosols in the assessment of RV in tracheotomized patients maintained on mechanical ventilation.

Methods: First, prior to clinical studies, 3 commercially available radioaerosol nebulizer kits were studied on the bench to determine nebulizer efficiency and particle distribution of ^{99m}Tc-DTPA aerosols. Then, using a gamma camera, we studied 5 ventilated human subjects and simultaneously measured RV with ^{81m}Kr gas and ^{99m}Tc-DTPA aerosol. Images were compared visually and by analysis of radioactivity distributions in computer-generated regions of interest.

Results: Although its Inhaled Mass % was not as great as the AeroTech I, we found that the UltraVent system produced the smallest particles with a mass median aerodynamic diameter of 0.9 µm compared to the AeroTech I and VentiScan II systems which both produced aerosols of 1.3 µm. In spite of the relatively

Nebulizer TestedMean Inhaled Mass % MMAD (µm)

Mallinckrodt "UltraVent" 21.15 ± 2.13 SD 0.9 ± 1.67 (\sigmag)

BioDex "VentiScan II" 15.50 ± 0.11 SD 1.3 ± 1.92 (\sigmag)

CIS-US "AeroTech I" 34.14 ± 0.76 SD 1.3 ± 1.92 (\sigmag)

small particles, ^{99m}Tc-DTPA deposition images with the UltraVent nebulizer did not accurately represent RV as measured by ^{81m}Kr equilibrium. Visual inspection of images revealed significant amounts of particle deposition in the region of the trachea which was diminished but not eliminated after replacing the tracheotomy tube inner cannula. Using regional analysis, correlations between radioactivity distributions of both isotopes were 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.

Conclusions: The lungs of patients receiving continuous mechanical ventilation can be imaged after the inhalation of ^{99m}Tc-DTPA aerosol from commercially available delivery kits, but the correlation between aerosol deposition and RV is poor. Better definition of ventilated lung segments is obtained when using a gas such as ^{81m}Kr because tracheal activity with the radiolabeled gas is minimized. RCPs should be familiar with these techniques in the event they must assist.

OF-95-194

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