Airway narrowing in porcine bronchi with and without lung parenchyma

Abstract

During bronchoconstriction elastic after-loads arise due to distortion of lung parenchyma by the narrowing airway. In the present study, the functional effect of parenchymal elastic after-load on airway narrowing was determined.Airway narrowing was measured in vivo over a range of transpulmonary pressures and compared with in vitro narrowing measured at corresponding transmural pressures. Bronchi were generation 10 with internal diameters of ,4 mm. In vivo luminal narrowing was measured by videobronchoscopy in anaesthetised and ventilated pigs. In vitro luminal narrowing was measured by videoendoscopy in isolated bronchial segments. Airways were activated by maximum vagal nerve stimulation and maximum electrical field stimulation in vivo and in vitro, respectively.At 5 cmH 2 O, stimulation produced a 35.9¡3.2% (n56) and a 36.5¡2.4% (n511) decrease in lumen diameter in vivo and in vitro, respectively. At 30 cmH 2 O, luminal narrowing fell to 23.7¡2.0% in vivo and 23.4¡2.5% in vitro. There was no difference between luminal narrowing in vivo and in vitro at any pressure.In conclusion, these findings suggest that in mid-sized, cartilaginous bronchi, parenchymal elastic after-loads do not restrict airway narrowing.

Airway smooth muscle dynamics: a common pathway of airway obstruction in asthma

“…This is somewhat surprising and contradictory to findings suggesting that parenchymal tethering is an important determinant of airway reactivity [8,9,[134][135][136]200]. Although the reason for the discrepancy is not clear, the mid-sized (10th generation) airways studied by NOBLE et al [289] are more cartilaginous than those of comparable size and generation in humans. In fact, the seventh generation human bronchi were found to be cartilage-free [290], but the pig airways appear not to be stiffer than human bronchi [291,292].…”

“…Under conditions following a deep inspiration, if lung hysteresis exceeds airway hysteresis, it is predicted that parenchymal tethering would have a minimal effect on airway calibre. In the study by NOBLE et al [289], pressure-strain curves showed minimal hysteresis in the airways in contrast to the much greater hysteresis in the lung lobe, which is consistent with the reported lack of tethering effect reported. However, in that study, airway narrowing was recorded under pressurecontrolled conditions, over a range of inspiratory lung pressures and therefore without the complicating effects of hysteresis caused by a deep inspiration.…”

“…As mentioned previously, there appears to be a lack of parenchymal-airway interdependence in the pig lung, at least as far as mid-sized airways are concerned [289]. If this is also found in the human lung, it will drastically change current interpretation of many previous studies, and indeed, it will change the view of how the lung works.…”

“…For this to happen in vivo implies an accompanying fluid flux within the airway wall or shortening of the airways longitudinally. The elastic properties of the different stress-bearing structures that lead to coupling of the parenchyma to the ASM are relatively unexplored, and, as mentioned elsewhere in the current study, these structures could allow uncoupling of ASM from the adventitia and account for the reported dissociation of the parenchyma and ASM described above [289]. It is also possible that differences in airway and parenchymal hysteresis could reduce the effect of parenchymal tethering on airway diameter.…”

“…The mechanical properties of these anatomical linkages, however, have never been directly examined in human subjects or human lung tissue. In a recent study, NOBLE et al [289] examined pig airway narrowing either with or without parenchyma and concluded that under some physiological conditions, parenchymal tethering played no role in altering airway calibre. Their findings showed that while lung inflation did increase the diameter of relaxed airways and reduce the extent of narrowing produced by ASM stimulation, expected effects of parenchymal tethering from shear forces on airway diameter or airway narrowing were absent.…”