Mechanical action of the interosseous intercostal muscles as a function of lung volume

On the basis of local stimulation of individual muscles, it has been suggested that both the external (El) and internal interosseous intercostal muscles have an inspiratory action at low lung volumes and an expiratory action at high lung volumes. In this study, we assessed the action of the interosseous intercostal muscles at different lung volumes in 19 anesthetized dogs by synchronously activating the intercostal muscles via ventral root stimulation (VRS). An electrode was positioned on the upper thoracic spinal cord according to previously described techniques (11). The cervical phrenic rootlets were sectioned bilaterally, the accessory muscles were sectioned from the rib cage, and the origins of the abdominal muscles were sectioned from the middle region of the rib cage. Changes in airway pressure (ΔP) were monitored during the application of supramaximal stimuli after hyperventilation-induced apnea and during airway occlusion. Animals were passively inflated or deflated with a volume syringe. Precontractile airway pressure was used as an index of lung volume. External and parasternal intercostal muscle (PA) lengths were monitored by sonomicrometry in the third intercostal space. Thoracoabdominal motion was monitored by Respitrace® bands. During VRS, both PA and EI shortened at all lung volumes. Mean ΔP progressively decreased with increasing lung volume. At precontractile airway pressures of -10, 0, and +30 cm H₂O, ΔP were -25 ± 1, -16 ± 1, and -5 ± 1 cm H₂O, respectively. After section of the internal intercostal nerves lateral to the costochondral junctions from the first through the seventh intercostal spaces to eliminate PA action, EI shortened, whereas PA usually lengthened. Mean ΔP at precontractile airway pressures of -10, 0, and +30 cm H₂O were -15 ± 1 cm H₂O (p < 0.05), -9 ± 0.5 cm H₂O (p < 0.05), and -3 ± 0.5 cm H₂O(NS). VRS was associated with rib cage expansion at all lung volumes. We conclude that the interosseous intercostal muscles of the upper rib cage maintain an inspiratory action over the entire vital capacity range.