The contribution of lymphatic drainage to the clearance of inhaled 99mTc-DTPA from the lungs.

When inhaled as an aerosol, 99mTc labelled diethylene triamine pentacetate (99mTc-DTPA) moves rapidly from the airspace to the vascular space. The rate at which it leaves the lungs is being used to measure the integrity of the pulmonary epithelium. In order to determine what part the lymphatic system plays in the clearance of 99mTc-DTPA from the lungs, we measured the rate of appearance in plasma and lymph of inhaled 99mTc-DTPA and intravenously injected Indium-113m labelled (113mIn-DTPA) in 5 sheep with chronic lung lymph fistalae. Inhaled 99mTc-DTPA was detected in the plasma and lymph after 1 minute. This suggests that the inhaled sub-micronic aerosol of 99mTc-DTPA was deposited predominately in a region of the lung with a large vascular surface area, ie. the terminal lung units. The lymph/plasma concentration (1/p) ratio for injected 113mIn-DTPA became greater than 1 by 4 minutes whereas the 1/p ratio for inhaled 99mTc-DTPA did not reach 1 until 25 minutes. This suggests that lymph drainage has very little part to play in the clearance of inhaled 99mTc-DTPA from the lungs.