Inspiratory muscle interaction in the generation of changes in airway pressure

The mechanical interaction of the inspiratory muscles in the generation of changes in airway pressure is unclear. Using upper thoracic spinal cord stimulation to activate the intercostal muscles (IC) and bilateral supramaximal phrenic nerve stimulation to activate the diaphragm (D), we measured the changes in airway pressure produced by separate and combined IC and D activation over a wide range of lung volumes. Changes in parasternal IC and D length were assessed by sonomicrometry. With increasing lung volume, activation of the IC and D resulted in progressive decrements in generated airway pressure. Combined IC and D contraction produced greater negative swings in airway pressure than the arithmetic sum of separate IC and D contraction alone, indicating a synergistic effect. Moreover, synergism increased progressively with increasing lung volume. During combined muscle contraction, both the IC and D shortened less than during contraction of either muscle group alone. The tendency for the parasternal muscle to lengthen for a given change in airway pressure during D contraction alone increased with increasing lung volume, suggesting that the tendency for the rib cage to recoil inward increased progressively with increasing lung volume. Likewise, the tendency of the D to lengthen for a given change in airway pressure during IC contraction alone also increased progressively with increasing lung volume, suggesting that the tendency for the abdomen-D compartment to recoil inward also increased with increasing lung volume. We conclude that the IC and D interact synergistically to produce changes in airway pressure. Our results suggest that this effect may be secondary to 1) the reduction in muscle shortening that occurs during combined muscle contraction and/or 2) the importance of combined IC and D activation on chest wall stability.