Respiratory muscle compensation for unilateral or bilateral hemidiaphragm paralysis in awake canines

In humans and some animals, the surviving respiratory muscles are able to compensate fully for unilateral, and partially for bilateral, hemidiaphragm paralysis. To examine differential activity of individual respiratory muscles after unilateral or bilateral diaphragm paralysis, length and electromyogram (EMG) of left costal and crural diaphragm segments, parasternal intercostal, and transversus abdominis were measured directly in five awake canines after implantation with sonomicrometry transducers and bipolar EMG electrodes under three conditions: during normal breathing (NOFRZ), after infusion of local anesthetic (bupivacaine) through a cervical phrenic nerve cuff to induce reversible contralateral hemidiaphragm (CNFRZ), and after bilateral diaphragm (BIFRZ) paralysis. From NOFRZ to CNFRZ, costal, crural, parasternal, and transversus abdominis increased shortening and EMG activity to compensate for contralateral diaphragm paralysis, but the increase in activity was not equivalent for each muscle. With BIFRZ, parasternal and transversus abdominis showed further increases in activity, coordinated between both inspiration and expiration. Normalized intrabreath profiles revealed dynamic differences in development of muscle activity within each breath as paralysis worsened. Review of simultaneous muscle activities showed coordinated interactions among the compensating muscles: passive shortening of transversus, and lengthening of costal and crural, coincided with increased active inspiratory shortening of parasternal. We conclude that an integrated strategy of respiratory muscle compensation for unilateral or bilateral diaphragm paralysis occurs among chest wall, abdominal, and diaphragm segmental muscles, with relative contributions of individual muscles adjusted according to the degree of diaphragm dysfunction.