Respiratory mechanical advantage of the canine external and internal intercostal muscles

The current conventional view of intercostal muscle actions is based on the theory of Hamberger (1749) and maintains that as a result of the orientation of the muscle fibres, the external intercostals have an inspiratory action on the lung and the internal interosseous intercostals have an expiratory action. This notion, however, remains unproved. In the present studies, the respiratory actions of the canine external and internal intercostal muscles were evaluated by applying the Maxwell reciprocity theorem. Thus the effects of passive inflation on the changes in length of the muscles throughout the rib cage were assessed, and the distributions of muscle mass were determined. The fractional changes in muscle length during inflation were then multiplied by muscle mass and maximum active stress (3·0 kg cm−2) to evaluate the potential effects of the muscles on the lung. The external intercostals in the dorsal third of the rostral interspaces were found to have a large inspiratory effect. However, this effect decreases rapidly both toward the costochondral junctions and toward the base of the rib cage. As a result, it is reversed to an expiratory effect in the most caudal interspaces. The internal intercostals in the caudal interspaces have a large expiratory effect, but this effect decreases ventrally and rostrally, such that it is reversed to an inspiratory effect in the most rostral interspaces. These observations indicate that the canine external and internal intercostal muscles do not have distinct inspiratory and expiratory actions as conventionally thought. Therefore, their effects on the lung during breathing will be determined by the topographic distribution of neural drive. Although it is now well established that the interchondral portion of the internal intercostal muscles (the so-called parasternal intercostals) elevates the ribs and inflates the lung when it contracts (De Troyer & Kelly, 1982; De Troyer et al. 1996), the actions of the external intercostals and the interosseous portion of the internal intercostals remain uncertain. The current conventional view is based on the theory proposed 250 years ago by Hamberger (1749). According to this theory, the fibres of the external intercostals slope obliquely caudad and ventrally from the rib above to the rib below, and so their lower insertion is further from the centre of rotation of the ribs (i.e. the costo-vertebral articulations) than their upper insertion. Consequently, when this muscle contracts with its force equal and opposite at both insertions, the torque acting on the lower rib, which tends to raise it, is greater than that acting on the upper rib, which tends to lower it. The net effect of the muscle, therefore, would be to raise the ribs and to inflate the lung. In contrast, the fibres of the internal interosseous intercostals slope obliquely caudad and dorsally from the rib above to the one below so that their lower insertion is closer to the centre of rotation of the ribs than the upper one. As a result, the net effect of their contraction would be to lower the ribs and to deflate the lung. This theory, however, has not been verified, and computations based on the orientation of the muscle fibres and on descriptions of rib displacement in dogs (Margulies et al. 1989) have recently suggested that the actions of the external and internal intercostal muscles on the lung might vary between the dorsal and the ventral aspects of the rib cage as well as between the rostral and caudal interspaces (Wilson & De Troyer, 1993). In the present studies, we have examined the effects of the canine external and internal interosseous intercostals on the lung by using a standard theorem of mechanics, the Maxwell reciprocity theorem. When applied to the respiratory system (Wilson & De Troyer, 1992, 1993), this theorem predicts that the respiratory effect of a particular muscle (that is, the potential change in airway pressure - ΔPao - produced by the muscle contracting alone against a closed airway) is related to the mass (m) of the muscle, the maximal active muscle tension per unit cross-sectional area (σ), and the fractional change in muscle length (ΔL/L) per unit volume increase of the relaxed chest wall (ΔVL)Rel, such that: (1) For a machine, such as a lever, mechanical advantage is defined as the ratio of the force delivered at the load to the force applied at the handle. By analogy, the mechanical advantage of a respiratory muscle may therefore be defined as ΔPao/mσ and, according to eqn (1), could be evaluated by measuring (ΔL/(L ΔVL))Rel. In other words, a muscle that shortens during passive inflation would have an inspiratory mechanical advantage and would cause a fall in Pao when it contracts. Conversely, a muscle that lengthens during passive inflation would have an expiratory mechanical advantage and would cause a rise in Pao during contraction.