COMPARATIVE-STUDY OF MECHANICAL INTERDEPENDENCE - EFFECT OF LUNG-VOLUME ON RAW DURING INDUCED CONSTRICTION

The mechanical load imposed by the parenchyma can decrease the magnitu de of agonist-induced airway constriction. To examine whether variabil ity in mechanical interdependence could account for differences in air way narrowing among different species, we compared the effects of chan ging lung volume on airway resistance (Raw) under baseline conditions and during methacholine (MCh)-induced constriction in rats and guinea pigs. Direct measurements of Raw were made under baseline conditions a t different levels of end-expiratory transpulmonary pressure (PL = 3-1 1 cmH(2)O). Then aerosolized MCh was delivered (128 mg/ml in rats, 8 m g/ml in guinea pigs), and measurements were obtained at different leve ls of PL (3-11 cmH(2)O). We measured tracheal flow and tracheal and al veolar pressure (using alveolar capsules) in open-chest animals during mechanical ventilation (tidal volume 6 ml/kg, frequency 1 Hz) and cal culated Raw by subtracting tissue from total lung resistance. For inte rspecies comparisons, data were standardized using percent change in R aw (Delta%Raw) at PL of 3 cmH2O during baseline and, after MCh, log-sc aled Delta%Raw at PL of 3 cmH(2)O. We found that increasing lung volum e decreased Raw both at baseline and after MCh in both species. Howeve r, changes were far smaller in guinea pigs. During baseline, the slope of the Delta%Raw/Delta PL was -3.37 and -12.48 %/cmH,O for guinea pig s and rats, respectively (P < 0.001). After MCh, the slope of Delta lo g %Raw/Delta PL was -0.065 and -0.154 cmH(2)O(-1) for guinea pigs and rats, respectively (P < 0.01). These observations suggest that the bro nchodilating effect of lung volume is less in the guinea pig and may r eflect either differences in the mechanical properties of the airway o r in airway-parenchymal interdependence.