Effect of transpulmonary pressure on airway diameter and responsiveness ofimmature and mature rabbits

We previously demonstrated that airway responsiveness is greater in immatur e than in mature rabbits; however, it is not known whether there are matura tional differences in the effect of transpulmonary pressure (Ptp) on airway size and airway responsiveness. The relationship between Ptp and airway di ameter was assessed in excised lungs insufflated with tantalum powder. Diam eters of comparable intraparenchymal airway segments were measured from rad iographs obtained at Ptp between 0 and 20 cmH(2)O. At Ptp > 8 cmH(2)O, the diameters were near maximal in both groups. With diameter normalized to its maximal value, changing Ptp between 8 and 0 cmH(2)O resulted in a greater decline of airway caliber in immature than mature airways. The increases in lung resistance (RL) in vivo at Ptp of 8, 5, and 2 cmH(2)O were measured d uring challenge with intravenous methacholine (MCh: 0.001-0.5 mg/kg). At Pt p of 8 cmH(2)O, both groups had very small responses to MCh and the maximal fold increases in RL did not differ (1.93 +/- 0.29 vs. 2.23 +/- 0.19). At Ptp of 5 and 2 cmH(2)O, the fold increases in RL were greater for immature than mature animals (13.19 +/- 1.81 vs. 3.89 +/- 0.37) and (17.74 +/- 2.15 vs. 4.6 +/- 0.52), respectively. We conclude that immature rabbits have gre ater airway distensibility and this difference may contribute to greater ai rway narrowing in immature compared with mature rabbits.