Coupling between the ribs and the lung in dogs

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Abstract
In contrast to the conventional theory, the external and internal intercostal muscles show marked rostrocaudal gradients in their actions on the lung. We hypothesized that these gradients are the result of a non‐uniform coupling between the ribs and the lung. Rib displacements (Xr) and the changes in airway opening pressure (Pa,o) were thus measured in anaesthetized, pancuronium‐treated, supine dogs while loads were applied in the cranial direction to individual pairs of odd‐numbered ribs and in the caudal direction to individual pairs of even‐numbered ribs. During cranial loading, Xr induced by a given load increased gradually with increasing rib number. The decrease in Pa,o also increased from the third to the fifth rib pair but then decreased markedly to the eleventh pair. A similar pattern was observed during caudal loading, although Xr and ΔPa,o were smaller. These results were then combined to calculate the net Xr and the net ΔPa,o that a hypothetical intercostal muscle lying parallel to the longitudinal body axis would produce in different interspaces. The net Xr was cranial in all interspaces. However, whereas the net ΔPa,o was negative in the cranial interspaces, it was positive in the caudal interspaces. These observations confirm that the coupling between the ribs and the lung varies from the top to the base of the ribcage. This coupling confers to both the external and the internal intercostal muscles an inspiratory action on the lung in the cranial interspaces and an expiratory action in the caudal interspaces.

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