Airway and parenchymal tissue resistance and elastance in ex vivo sheep lungs: effects of bronchochallenge and deep inspiration.

Lung resistance ( R L ) is determined by airway and parenchymal tissue resistance, as well as the degree of heterogeneity in airway constriction. Deep inspirations (DIs) are known to reverse experimentally induced increase in R L , but the mechanism is not entirely clear. The first step toward understanding the effect of DI is to determine how each of the resistance components is affected by DI. In the present study, we measured R L and apparent airway resistance ( R AW , which combines the effects of airway resistance and airway heterogeneity) simultaneously before and after a DI in acetylcholine (ACh)-challenged ex vivo sheep lungs. We found that at normal breathing frequency (0.25 Hz) ACh-challenge led to a doubling of R L , 80.3% of that increase was caused by an increase in R AW ; the increase in apparent tissue resistance ( R T ) was insignificant. 57.7% of the increase in R AW was abolished by a single DI. After subtracting R AW from R L , the remaining R T was mostly independent of ACh-challenge and its reduction after a DI came mostly from the change in the mechanical properties of lung parenchyma. We conclude that at normal breathing frequency, R L in an unchallenged lung is mostly composed of R T , and the increase in R L due to ACh-challenge stems mostly from the increase in R AW and that both R AW and R T can be greatly reduced by a DI, likely due to a reduction in true airway resistance and heterogeneity, as well as parenchymal tissue hysteresis post DI.