Tracheal blood flow and luminal clearance of 99mTc-DTPA in sheep

Tracheal blood flow and 99mTc-labeled diethylenetriamine pentaacetic acid (DTPA) clearance were measured in the sheep trachea in vivo. The tracheal arteries were isolated and perfused. An isolated segment of tracheal lumen was filled with Krebs-Henseleit solution containing 99mTc-DTPA, and radioactivity was measured in blood from a catheterized tracheal vein. Infusions at constant pressure of methacholine (n = 5), albuterol (n = 6), and histamine (n = 5) increased arterial inflow [+250 ± 73.0, +74.2 ± 22.9, +68.9 ± 39.2% (SE), respectively] and venous outflow (+49.5 ± 13.8, +11.6 ± 4.5, +6.2 ± 13.9%) but decreased 99mTc-DTPA output (-36.8 ± 8.4, - 20.4 ± 6.2, -58.1 ± 11.7%) and concentration (-53.9 ± 10.1, -27.3 ± 7.5, -49.3 ± 14.4%). Phenylephrine (n = 9) decreased arterial inflow (-49.4 ± 10.0%) and venous outflow (-4.1 ± 5.9%) but increased 99mTc-DTPA output (+74.6 ± 44.2%) and concentration (+94.4 ± 56.6%). When the tracheal arteries were initially perfused at constant flow and the flow rate was then changed, 50% increases in flow (n = 5) increased perfusion pressure (+35.9 ± 2.2%) and venous outflow (+10.5 ± 3.8%) but decreased 99mTc-DTPA output (-24.4 ± 7.8%) and concentration (-30.4 ± 8.8%). Decreases in flow of 50% (n = 3) and 100% (n = 10) decreased perfusion pressure (-34.2 ± 4.2, -80.1 ± 3.5%, respectively) and venous outflow (-11.0 ± 4.8, -29.7 ± 7.2%) but increased 99mTc-DTPA output (+45.9 ± 27.5, +167.4 ± 70.4%) and concentration (+64.7 ± 26.7, +305.7 ± 110.2%). Infusions of the same drugs at constant flow produced significant changes in perfusion pressure but no significant changes in venous outflow or, except for histamine, in 99mTc- DTPA output. Thus 99mTc-DTPA output was inversely related to both mechanically and drug-induced changes in tracheal blood flow but, except for histamine, was not directly affected by the drugs. The results may be due to redistribution of blood between collateral circuits and altered interstitial fluid volume affecting 99mTc-DTPA transport across the interstitium.