Influence of smooth muscle tone and longitudinal tension on the collapsibility of immature airways.

Mechanical properties and pressure-flow relationships of tracheae excised from very premature lambs were studied in a plethysmograph. Control (Group I) data revealed the tracheae to be extremely compliant, collapsible airways, with an inflation compliance (Si) of 0.033 (+/- 0.004 SE) mmHg-1, collapsing compliance (Sc) of 0.026 (+/- 0.001 SE) mmHg-1, and pressure-flow relationships similar to those of a Starling resistor. Acetylcholine administration (Group II) lowered both Si, 0.026 (+/- 0.003 SE) mmHg-1 and Sc, 0.022 (+/- SE) mmHg-1, as did longitudinal stretch (Group III): Si, 0.021 (+/- .003 SE) mmHg-1, and Sc, 0.017 (+/- 0.002 SE) mmHg-1. Alterations in tracheal collapsibility were also evidenced by significant reductions in resistance to airflow when the tracheae were subjected to compressive forces. Altering both smooth muscle tone (acetylcholine administration) and longitudinal length simultaneously (Group IV) produced results similar to those obtained for Group III. These data help to define the functional characteristics of immature airways and may provide insight for more effective clinical management of the premature infant.