Mechanical interdependence in relation to age: effects of lung volume on airway resistance in rats

Elastic recoil has been reported to decrease with increasing age in human subjects. In the current study, we hypothesized that aging influences mechanical interdependence, which affects the magnitude of agonist-induced airway constriction. To examine this hypothesis, we compared the effects of changing lung volume on airway resistance (Raw) under baseline conditions and during methacholine-induced constriction in adult (14 mo old, 624 +/- 14 g; n = 11) and aged (29 mo old, 629 +/- 9 g; n = 11) Sprague-Dawley rats. With use of alveolar capsules, Raw was directly measured under baseline conditions at different levels of end-expiratory transpulmonary pressure (PL; 3-11 cmH2O). Then aerosolized methacholine was delivered (125 mg/ml), and measurements were performed at different levels of PL (3 and 11 cmH2O). From measured tracheal flow and tracheal and alveolar pressures in open-chest animals during mechanical ventilation (6 ml/kg tidal volume, 1 Hz frequency), we calculated dynamic lung elastance and resistance of lung, tissue, and airway. In the baseline conditions, we found that increasing lung volume decreased Raw similarly in both groups but that lung elastance was reduced in the aged group at PL > or = 7 cmH2O. The shape constant obtained from the pressure-volume curve in the aged rats was significantly greater than that in the adult rats. During induced constriction, higher lung volume significantly lowered Raw in the adult group, whereas Raw was not significantly reduced by increasing PL in the aged group. These observations suggest that increasing age may affect the mechanical properties of the airway and/or airway-parenchymal interdependence.