Effect of tracheal smooth muscle tone on collapsibility of immature airways

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Abstract
To test the influence of smooth msucle tone on extremely immature airways, tracheal segments (n = 19) were excised from premature lambs at 114-121 days gestation and mounted in a chamber filled with Krebs solution. Inflation (Si) and collapsing (Sc) compliance were detemrined by altering transmural pressure from 30 to 0 torr and -30 to 0 torr, respectively, both during control (C) and after acetylcholine (ACh) administration (experimental, E). Flow (.ovrhdot.V 2-15 l/min) was then introduced through the tracheal lumen while chamber pressure (Pc) was increased from 0 to 30 Torr and driving pressure (Pd) was recorded for both C and E conditions. Tracheae were found to be extremely compliant; both Si and Sc were significantly (P < 0.005) lower after ACh administration. Resistance to airflow (R = Pd/.ovrhdot.V) was also significantly (P < 0.05) lower after ACh administration at each compressive pressure and each flow value. These results suggest that the highly compliant preterm trachea exhibits pressure-flow characteristics similar to a Starling resistor, and the effects of compressive pressures may be attenuated by ACh-induced smooth muscle contraction. Comparison of these results with data from adult and newborn animals suggests a developmental difference in tracheal mechanics and pressure-flow relationships, as well as in the way airway function is altered by smooth muscle stimulation.

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