A comparison of preterm and adult airway smooth muscle mechanics.

To determine whether airway smooth muscle undergoes a maturational change regarding force generation, length-tension relationships were determined in isolated trachealis strips from adult and preterm sheep. At the length of maximum force generation, passive active and total tensions of the adult muscle were 2.5 times greater than preterm values (P less than 0.001). KCl stimulation yielded a greater peak tension in the adult strips than in the preterm strips (P less than 0.01). Preterm strips required higher concentrations of KCl to initiate contractions and higher concentrations to reach peak tension. Acetylcholine- (ACh) induced contraction resulted in greater force development at each dose in the adult strips compared with preterm strips (P less than 0.001). The dose of ACh required to reach a half-maximal response was significantly less for the adult strips than for the preterm strips (P less than 0.005). These data demonstrate that both force generation and receptor sensitivity increase with age. This inability of immature smooth muscle to generate as much force as adult smooth muscle may help explain why very preterm neonates requiring intermittent positive-pressure ventilation are at risk for developing structural airway problems.